Advances in Crop Genetic Improvement to Overcome Drought Stress: Bibliometric and Meta-Analysis

Abstract

Plant resistance to drought stress is a parameter that should be studied with more emphasis in the search for higher agricultural yields. In this scenario, research within breeding programs should be directed toward specific mechanisms of action and important agricultural crops in worldwide agribusiness. From this perspective, this study carried out a bibliographic investigation regarding the advances in genetic improvement aimed at drought stress in crops using a hybrid model of analysis of scientific articles. The analysis employed bibliometric parameters for qualitative and quantitative discussion of scientific production and the methodological process of systematic review for the synthesis of the results obtained. The work was divided into four stages: the search for articles in databases, meta-analysis, bibliometric analysis, and systematic analysis. Scientific articles were searched for on the Scopus, Scielo, and Web of Science databases within a 20-year timeframe. Most authors and institutions were from Asian countries, demonstrating the need for global expansion of research on the subject. With regard to the co-occurrence networks between the keywords used in the search, a focus was observed on the following terms: drought resistance, drought stress; drought, and drought tolerance. Evidently, the primary mechanism of tolerance or even resistance studied in breeding programs is associated with the expression of genes and genetically modified organisms that confer resistance to plants. Also, the crops addressed in the research retrieved are highly diverse.

1. Introduction

Drought-tolerant crops are of paramount importance in a worldwide scenario of increased food demand caused by the growth of the human population [1]. Drought is one of the most frequent stresses in several parts of the world, affecting many agricultural species, reducing photosynthesis, and favoring the accumulation of reactive oxygen species (ROS) and the occurrence of cell membrane damage, significantly reducing the yield and quality of agricultural products [2,3].

In this scenario, the theoretical and practical search to identify drought-resistant materials in crops of economic importance are essential to identify drought tolerance alleles, an important mechanism in breeding programs [4].

Selecting plants or genotypes under abiotic stress conditions requires breeding programs and techniques from related areas in order to understand the various genetic and physiological mechanisms involved in tolerance-related traits [5].

The ideal genotype should present satisfactory productivity under non-stress and drought stress conditions. Therefore, genetic improvement research should aim at the maximum potential for productivity while reducing the gap between the production potential and yield under drought stress conditions [6].

From this perspective, the search for advances in breeding programs aimed at resistance to drought stress in agricultural crops is a path that should be pursued with greater emphasis. Breeding programs have been working with different crops and mechanisms to investigate resistance to drought stress, including gene expression, genetically modified organisms, plant hormones, biostimulants, and the identification of genes that confer resistance in crops such as corn, soybean, cotton, rice, wheat, common bean, sugarcane, and tomato, among others [1,4,7,8,9,10,11,12,13,14,15,16,17].

In this scenario, the hypothesis to be elucidated is that there is a gap in information about the most studied crops and regions with more outstanding advances with regard to genetic improvement studies aiming at plant resistance/tolerance to drought stress. Because of this, there is a need to analyze the current level of knowledge on plant breeding in order to identify drought stress mechanisms in crops and gather the results of the research developed so far, aiming to assist breeding programs in developing strategic research for different crops and regions.

Literature reviews, e.g., bibliometric and systematic studies, are defined by [18] as methodologies capable of clarifying scientific advances and identifying existing controversies. These reviews represent a helpful tool in the search to elucidate gaps in different areas of scientific research.

From this perspective, bibliometric analysis is a field of information science that involves the application of quantitative, statistical, and mathematical techniques to analyze and construct indicators referring to the dynamics and evolution of scientific and technological information in bibliographical data, e.g., publications and citations [19] in several areas, organizations, and/or countries. In this regard, bibliometric studies are characterized as being more objective and extensive in their scope compared to other types of reviews, i.e., it is possible to analyze hundreds or thousands of papers about a given subject in different regions [20,21]. Authors have reported that, when focused on a well-defined question, this type of research enables the identification, selection, evaluation, and synthesis of relevant evidence available in papers published in national and international journals.

In this scenario, the present study aimed to carry out bibliographic research regarding the advances in genetic improvement involving drought stress in crops based on a hybrid model of analysis of scientific papers using bibliometric parameters for the qualitative and quantitative discussion of scientific production and the methodological process of systematic review for the synthesis of the results obtained.

2. Materials and Methods

2.1. Study Phases

A hybrid model was applied in this research by systematizing a set of phases and stages, seeking to investigate the panorama, applications, and results obtained according to the methodology proposed by [22]. The work was divided into four distinct phases, which helped in the delimitation of the systematic research method: (1) a database search, (2) meta-analysis, (3) bibliometric Analysis, and (4) systematic analysis (Figure 1).

2.2. Search Bases and Strings

The following databases were used in the search phase: Scopus, Scielo, and Web of Science, corresponding to platforms of broad scientific scope with worldwide publications in the area of plant genetic improvement. The platforms were accessed through the online portal of the Coordination for the Improvement of Higher Education Personnel (CAPES) to find published studies on advances in genetic improvement aimed at resistance to drought stress in plants. The selection of scientific manuscripts occurred by systematizing the Principal Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA), which uses a four-stage classification that consists of: (1st stage) identification, (2nd stage) selection, (3rd stage) eligibility, and (4th stage) inclusion [24].

The survey of manuscripts in the databases was carried out from December 2022 to March 2023. In the first stage (identification), the keywords combined with the Boolean operator (OR and END) were searched for simultaneously in the databases, as follows: (“advances in genetic” OR “improvement”) AND “drought resistance in crops”); (“genetic improvement” AND “drought resistance of crops”); (“drought resistance of crops” AND “genetic improvement”); (“advance improvement” AND “drought resistance”);(“drought resistance”); AND “advance improvement”); (“genetic improvement” OR “crop drought resistance”); (“drought resistance of crops” OR “genetic improvement”); (“advance improvement” OR “drought resistance”); (“drought resistance”); OR “advance improvement”). This search resulted in 1140 publications from 3 March 2003 to 3 March 2023.

The second stage consisted of the selection, in which the exclusion criteria were applied to the initial sample through filters and only the papers published in the final version in open access journals were selected. As a result, the sample was reduced to 670 papers. At this stage, the manuscripts from the different databases were incorporated into the RStudio bibliometrix package (.csv and .txt), converted, combined to eliminate duplicates (200), and analyzed using the software Biblioshiny version 4.0.0., resulting in 470 manuscripts at the end of this stage.

In the third stage, called eligibility, a more critical analysis was carried out, in which the articles with the highest impact factor and order of relevance on the topic were selected, resulting in 50 papers. Then, the verification began by reading the titles and abstracts of the papers selected in the previous stage. In the fourth stage, inclusion, the full articles that showed the highest impact factors and were the most relevant for genetic improvement aimed at plant resistance/tolerance to drought were read.

The inclusion and exclusion criteria that supported eligibility with bias control were organized as follows: of the 470 articles initially selected and tabulated, each was evaluated by 16 researchers, receiving the classification of “Accepted/inclusion” or “Rejected/exclusion” from each evaluator. If an article received the approval of 50% + 1 of the researchers, it was assigned a Boolean logical operator of True, i.e., the classification given was absolute (“Accepted/included”). However, if the article received an evaluation below 50%, the logical operator was assigned was False, and the article automatically received the classification “Rejected/exclusion”. In order for this to be possible, all researchers received the selected articles and a spreadsheet with their title, abstracts, and a tab in which it was possible to record the “Accepted/inclusion” or “Rejected/exclusion” classification.

This programming system was carried out with the Microsoft Office Excel Professional Plus 2016 software 64-bit version, in which it was possible to compile the responses and thus attain greater sensitivity in the “Approved/inclusion” or “Rejected/exclusion” in order to establish a logic parameter that significantly reduced the bias in the selection of manuscripts. Of the total, 50 manuscripts were “Approved/included” and 420 “Rejected/excluded”, as their title and summary did not contain terms and development related to the theme proposed.

Finally, the 50 manuscripts were read in full by the 16 researchers using the same inclusion and exclusion logic as before, among which 10 papers were included for analysis of scientific production since they had greater impact factors and fully addressed the theme proposed. These articles were selected because they dealt in a well-defined way with water stress/tolerance in agricultural crops, contained a definition of the mechanisms of action used in genetic improvement, showed a clear response of these mechanisms in resistance to stress in crops, and had a high impact factor. The bibliometric data of these articles were exported to formats (.xls) read by bibliometric software.

The difference between the number of documents located and the number of articles included in the analysis highlights the relevance of the PRISMA methodology in the production of systematic reviews and analyses using bibliometric indicators, as it allows for minimizing biases.

2.3. Meta-Analysis

The bibliometric data exported from the selected scientific articles were organized and extracted according to year, country, authors, journal, institution, keywords, mechanism aiming at water resistance/tolerance, objectives, contributions, crops studied, the impact of the publication, and the type, among other parameters.

2.4. Bibliometric Analysis

Manuscript analysis with the advent of bibliometrics offers a comprehensive scenario and advances in the literature [25]. The indicators used in the bibliometric analysis were qualitative and quantitative. The computer programs Microsoft Office Excel and RStudio (Bibliometrix package) were employed in this analysis, which was used for the quantitative investigation of the sample through descriptive statistics to construct graphical elements that showed the information extracted.

2.5. Systematic Analysis

Based on the initial problem, which dealt with advances in studies on genetic improvement with a focus on plant resistance/tolerance to drought stress, the articles were analyzed using the data collected. For this purpose, tables and summaries were prepared to synthesize information such as the type of mechanism investigated, the main crops studied, the technologies employed, the regions with the greatest impact, the most relevant universities in research on the subject, the co-occurrence networks, and the most prominent authors. This approach will contribute to the formulation of future studies, since the systematic analysis of the manuscripts allows the construction of a broad and consistent overview of the state of the art, facilitating the identification of trends in scientific production in the studied area by applying a method that reduces the occurrence of biases and can be reproduced consistently [26,27].

3. Results

3.1. Bibliometric Analysis

Scientific Production per Country and Affiliation

The scientific production of the countries addressing the theme “advances in genetic improvement aiming at crop drought stress” has an uneven distribution (Figure 2A). The 10 countries that most study the topic are China, the USA, Pakistan, India, Japan, Brazil, South Korea, and Iran, with a frequency corresponding to 454, 62, 43, 37, 31, 29, 29, and 27 articles over the last 20 years, respectively.

Over the years, there has been a significant increase in the number of publications dealing with genetic improvement to develop plants more adapted to drought stress/resistance or tolerance conditions (Figure 2B). It is possible to observe that this increase started in 2011, achieving the highest number in 2022, with 102 publications, followed by 2021, with 54, and 2020, with 40 articles published in that year (Figure 2B). It is possible to show that, regarding the frequency of scientific production in the countries (Figure 2A), there was greater dispersion in relation to the overall mean, with a standard deviation above 90 and a standard error above 19. However, this fact is justified by the differences observed in the frequencies, since there are countries with more than 400 manuscripts and others with just over 20 manuscripts in the last 20 years. When reliability was analyzed using the standard error for scientific production per year (Figure 2B), less variation was noted compared to frequency, revealing that the smaller the standard error, the smaller the dispersion, with a higher likelihood that, over the course of the years, scientific production in relation to the advances in genetic improvement targeting water stress showed an actual increase.

According to data published by the Science and Engineering Indicators 2020 from the National Science Foundation (NSF, Alexandria, VA, USA), the number of scientific articles per country from 2000 to 2018 reaffirms this increase in publications over the years [28].

When analyzing the primary publication affiliations in global terms (Figure 3), it appears that the institutions with the most publications on the studied topic are: Northwest A&F University, Henan Agricultural University, Northeast Agricultural University, Shanxi Agricultural University, Gansu Agricultural University, Huazhong Agricultural University, and Huazhong Agricultural University. These institutions have more than 20 publications added over the years, all located in China and focused on advances in genetic improvement aimed at drought stress/water resistance tolerance. This information highlights the impact of China in the global context regarding the search for solutions involving plant breeding.

It should also be noted that when the data were analyzed using descriptive statistics regarding the main affiliations of the authors, the overall mean number of manuscripts was 16, with a standard deviation of 7.42 and a standard error of 1.37, implying a low degree of dispersion and high data reliability, since the sample mean accurately represents the total mean number of manuscripts per affiliations.

The countries with the highest number of citations and the mean number of citations per article are shown in Figure 4. Notably, the highest number of citations comes from studies conducted in China, corresponding to 3638 citations. However, the highest mean number of citations of articles occurred in the Czech Republic, with 130 per article. Brazil achieved 81 citations, with a mean value of 16.20 citations per article (Figure 4), occupying the 15th position in the ranking of studies on genetic improvement aimed at resistance/drought tolerance of crops.

It was also observed that, among the countries that are part of Nema, a region with severe problems regarding water deficit, Iran is among the 10 countries with the highest frequency of scientific production, a number of articles greater than 100, and a mean of 10.10 citations per article (Figure 4).

The high dispersion and variability observed in the descriptive statistics (mean, standard deviation, and standard error) for the number of citations per country are due to China, Spain, Colombia, the USA, and the United Kingdom having a number of cited manuscripts ranging from 3638 to 214, whereas the other countries have less than lower than 170 citations per manuscript on advances in genetic improvement targeting water stress, and some have less than 10 citations per manuscript.

The most relevant journals with the highest impact factors in publications on genetic improvement for crop drought stress resistance/tolerance are shown in Figure 5. The five most relevant journals worldwide in terms of number of publications on the subject are the International Journal of Molecular Sciences, Frontiers in Plant Science, BMC Plant Biology, Plants-Basel, and Agronomy-Basel corresponding to 76, 69, 24, 21, and 18 publications, respectively (Figure 5A).

In terms of the impact factor of journals based on the H-index, the five journals with the highest H-index values in the area of breeding for drought stress resistance in plants are Frontiers in Plant Science, International Journal of Molecular Sciences, BMC Plant Biology, Plant Physiology, and the Journal of Experimental Botany, with H-index values of 22, 16, 11, 10 and 9, respectively (Figure 5B).

The journals with a greater weight of publications (Figure 5A) and a higher impact factor (Figure 5B) in relation to advances in genetic improvement aimed at water stress have low dispersion and consequently less variation, which affects the overall mean reliability; therefore, the smaller the dispersion, the smaller the standard error. It should be noted that the mean, standard deviation, and standard error are best used when the data have a normal or symmetrical distribution [29].

The countries of the corresponding authors and collaboration rate within the country (SCP) and between countries (MCP) are shown in Figure 6. China has a percentage of collaboration between countries of 11.11%, Pakistan has a collaboration percentage of 40%, and Colombia has a collaboration percentage of 66.67%. On the other hand, the other countries only have publications with authors from their respective countries (Figure 6).

It should also be noted that authors from China mainly collaborate with authors from Korea, Pakistan, the Philippines, Saudi Arabia, Spain, and the USA; researchers from Pakistan mainly collaborate with authors from Saudi Arabia, Serbia, Spain, United Arab Emirates, and the United Kingdom; and researchers from Colombia mainly collaborate with Sweden and Japan.

3.2. Systematic Review

Keywords of Highest Occurrence and Co-Occurrence Network

Among the selected articles, the keywords with the highest occurrence are drought resistance, drought, physiological stress, drought stress, and drought tolerance (Figure 7A). It should be noted that, based on the descriptive statistics data, it is possible to establish that the standard deviation is related to the population mean, whereas the standard error quantifies the certainty with which the mean calculated from a sample estimates the true mean of the population [29]. Thus, it can be stated that the most relevant keywords were precise and showed the certainty of being related to advances in genetic improvement aimed at water stress (Figure 7A).

The network of keyword co-occurrence shows keywords’ interconnections for each subject discussed in the studies retrieved (Figure 7B). The keyword ‘drought’ is interconnected with ‘gene expression’, ‘regulation’, ‘metabolism’, ‘physiological stress’, ‘genotype’, and ‘chlorophyll’, which are resistance mechanisms used in studies to identify an induced adaptation to drought stress conditions in plants (Figure 7B).

‘Drought tolerance’ and ‘resistance’ are closely related to ‘genetic analysis’, ‘abiotic stress’, ‘abscisic acid’, ‘adaptation’, ‘maize’, ‘rice’, ‘Arabidopsis’, ‘productivity’, ‘water use efficiency’, ‘selection’, and ‘genetic improvement’ (Figure 7B).

The mechanisms of action used to identify drought resistance in agricultural crops found in papers published worldwide mainly include: the regulation of gene expression, abscisic acid, proline, hydrogen peroxide, transgenic plant, metabolism, molecular marker, biosynthesis, SLCIPK gene family, Tae miRNA, Zm gene family, and chromatography (Figure 8A).

In global terms, the most studied crops in terms of advances in genetic improvement seeking resistant/tolerant plants to drought stress are Arabidopsis thaliana, Oryza sativa, Zea mays, Triticum, Glycine max, Sorghum bicolor, Nicotiana tabacum, Solanum lycopersicum, Gossypium hirsutum L., Solanum tuberosum, Phaseolus-vulgaris, and Saccharum officinarum (Figure 8B). These species represent the most significant plants to agribusiness in global terms, as well as the most studied species due to their rapid cycle and easy identification of physiological, morphological, and biochemical changes. Arabidopsis thaliana, for example, is present in more than 40% of studies that seek resistance/tolerance to drought stress.

The standard errors of the mechanisms of action identified (Figure 8A) and the most studied crops (Figure 8B) in relation to the advances in genetic improvement aimed at water stress highlight the high dispersion in the mechanisms of action identified in the studies as well as in the crops studied, since 12 mechanisms of action and 12 crops can be clearly distinguished, in addition to a high variation in the percentages, ranging from 0.49 to 32.34% and from 0.6 to 43%, respectively.

Subsequently, the 10 papers with the highest impact factors on the topic were gathered by excluding literature reviews, book chapters, and non-full articles (

Table 1. Authors with the highest numbers of citations and H-index values, mechanism of action targeting drought stress resistance in breeding programs, and their response.

	Reference	Number of Times Cited	Mechanism	Response
1.	[1]	214	Gene expression	Identification of the CYP450 gene family, by highlighting the sensitivity and broadly targeted metabolites during drought stress.
2.	[30]	158	Molecular markers	Mepiquat chloride (MCD) increases water resistance in plants and inhibits growth. Molecular markers contribute to identifying some genes and metabolites and expressing differences in the biosynthesis pathways of isoflavonoids and flavonoids in soybean crops.
3.	[4]	133	Gene expression: Zm gene family	Drought resistance is significantly improved when plants overexpress genes from the ZmNF-YB16 family since these regulate photosynthetic and enzymatic activities.
4.	[31]	131	Molecular markers	QTL alleles introduced through marker-assisted selection can increase drought resistance, productivity, and yield stability across multiple environments in wheat.
5.	[10]	119	Abscisic acid (ABA)	The drought resistance of plants can be significantly improved with the application of phytohormones, especially abscisic acid (ABA), due to the proliferation of beneficial bacteria and fungi near the root zone of plants.
6.	[7]	117	Gene expression: Zm gene family and ABA	Overexpression of genes from the ZmbZIP33 family contributes to positive increases in chlorophyll levels and the roots of plants when subjected to water stress. The presence of ZmbZIP33 genes promotes a defense mechanism that helps in the production and accumulation of abscisic acid (ABA).
7.	[8]	114	Brassinosteroids	Plants synthesizing plant hormones such as brassinosteroids (BRs) are highly resistant to water stress.
8.	[9]	108	Genetic combination: GMOs	Genetically modified organisms (GMOs) have made a clear and objective contribution to developing species resistant to water stress. In these cases, the genetic combination seeks to optimize the morphological and physiological structure of the plants so that they can withstand the most frequent adverse climatic conditions in the future without reducing productivity.
9.	[32]	107	Gene expression	In drought-tolerant cultivars, nine transcription factors corresponded to overexpressed GNAC genes, predominant in drought-resistant cultivars.
10.	[33]	87	Gene expression: CodA gene family and GMOs	Genetically modified potato plants show overexpression of the CodA gene, which confers enhanced drought resistance and recovery capacity.

).

Among the selected studies, five describe mechanisms of action that consist of gene expression to identify plants with resistance to drought stress, with two using molecular markers, two using phytohormones, and one using gene combinations and genetically modified organisms.

The mechanisms of action using gene expression have contributed to the identification of genes of the families CYP450, ZmNF-YB16, ZmbZIP33, GNAC, and CodA (Table 1). These genes, in turn, are responsible for a large proportion of metabolic changes in photosynthesis, enzymatic activities, accumulation of plant hormones, and in the resistance/tolerance of plants to drought stress.

When it comes to molecular marker mechanisms, the most significant studies aimed at resistance to drought stress in crops assess mepiquat chloride (MCD) and QTL alleles introduced through assisted selection (Table 1). The use of growth phytohormones is one of the mechanisms studied with great emphasis in breeding that seeks plants resistant to drought stress worldwide, with abscisic acid (aba) and Brassinosteroids standing out by increasing Actinobacteria and reducing Chloroflexi and Acidobacteria, whereas Proteobacteria, Firmicutes, Ascomycota, and the concentration of Brassinosteroids increase, a fact that characterizes greater plant resistance to drought stress.

One of the most promising mechanisms in recent years has been genetic combinations using genetically modified organisms (Table 1), e.g., the genetic combination of lpa1 with the semi-dwarf mutant (dep1-ko or d2), which offers optimal water supply and drought resistance without affecting grain filling rates.

4. Discussion

4.1. Bibliometric Analysis

Scientific Production per Country and Affiliation

Due to drought stress, several agricultural plants in all parts of the world have suffered, causing growth and yield reductions. In this scenario, research seeking advances in breeding studies aimed at plant resistance/tolerance to drought stress worldwide is essential. Therefore, tools such as systematic reviews and bibliometric/systematic analysis of articles published worldwide using criteria such as PRISMA and computer programs such as RStudio integrated with bibliometrix are interesting alternatives to identify gaps and encourage new studies.

The data from this research indicate that the highest concentration of studies dealing with genetic improvement as a tool for research involving crop resistance to drought stress/resistance or drought tolerance is found in the Asian continent, with China standing out as the main research exponent in the area, largely focusing on the improvement of Oriza sativa and Solanum lycopersicum, a fact that corroborates the data from research carried out by [34] when studying the use of the CRISPR/CAS-9 system in plant breeding in a systematic literature review. Brazil occupies the seven th position regarding the frequency of scientific production on advances in crop genetic improvement.

Plant breeding programs of research institutions have invested in searching resistant or tolerant crops to drought or even in studies on the mechanisms that decisively influence the tolerance of plants to water deficit [35]. It should be noted that the vast majority of studies refer to crops such as soybean, corn, cotton, tomato, and common bean, aiming to identify the physiological responses of different clones in several growth phases associated with the application of phytohormones and abscisic acid, the use of molecular markers, and genetically modified organisms in studies on water resistance and drought tolerance.

Although China had the highest number of citations, and the mean number of citations per article was 16.60, similar to the top 10 countries with the highest number of research citations. This fact may be associated with the high rate of publications, which contributes to a reduced mean number of citations per article in China, whereas Brazil shows fewer studies on the subject but with a mean number of citations equivalent to China.

When studying the number of citations, ref. [36] stated that the number of total citations and the mean number of citations of Brazilian studies could be higher if Brazilian researchers valued national journals, increasing the reference to Brazil in relevant studies on various subjects.

It should also be noted that the quality of Brazilian research has grown in the international scenario, but the same does not occur regarding national studies [3,16,17,37]. It is common sense that valuable scientific data are published outside the country and should be cited; however, ref. [36] stated that it is unacceptable that 31.9% of the articles published in the journals analyzed in their study do not cite at least one article published in Brazil.

The journals with the highest number of publications also have the highest H-indices, implying publications with more impact and global coverage. The number of publications associated with the H-index constitutes a metric used to evaluate the quality of scientific productions [38]. The H-index is considered a robust and reliable bibliometric indicator, used as a reference to validate scientific findings, as it is not influenced by a set of articles with few citations or highly cited articles [39].

It is clear from the results obtained that some regions have more studies on the subject than others, highlighting the need for more research with clear objectives while listing the main mechanisms of action that confer resistance/tolerance of agricultural crops to water stress. However, there are still several gaps, especially in countries/regions that suffer the most from water scarcity problems. These regions show that research on this topic is incipient and requires a significant effort to study the mechanisms of resistance of agricultural crops and their responses.

4.2. Systematic Review

Knowing the most common keywords and the co-occurrence networks in studies conducted worldwide on resistance/tolerance to drought stress in plants serves to improve the understanding of the physiological mechanisms of response to water deficit in plants, in addition to serving as support for new research aimed at directing genetic improvement programs [3,40,41].

Crop response to drought or water stress/resistance/tolerance induces a set of physiological, biochemical, and molecular responses or mechanisms in plants, which, in order to overcome water restrictions, develop adaptations to withstand environmental circumstances, depending on the intensity and duration of drought stress, the interactive effects of other types of stress, the stage of development, and the genotype [42,43].

These plant response mechanisms are the basis of breeding research that seeks resistance/tolerance to drought stress in crops, providing plants with changes in physiological, biochemical, and adaptive parameters [1,5].

These responses range from changes related to stomatal opening and closure, energy dissipation in the first phase of photosynthesis (photochemical), enzymatic inhibition in the second phase of photosynthesis (Calvin cycle), e.g., Rubisco, the production of enzymes that confer resistance, reductions in the accumulation of reactive oxygen species (ROS), and osmotic adjustment, thus mitigating damage to the cell membrane [2,44,45].

Another strategy that has been widely used to promote plant resistance/tolerance to drought stress is the application of biostimulants (in the soil or via foliar spraying), followed by the assessment of differences in the plant subjected to stress [9,10,11,46,47,48]. Other studies argue that one of the best plant breeding strategies to identify resistance/tolerance to drought stress in crops is based on transgenic techniques, e.g., obtaining transgenic maize, soybean, tomato, rice, and potato with better drought tolerance. This technique is advocated as it reduces the time of classical breeding [5,9,49].

One of the biggest challenges in world agriculture is associated with water stress, so understanding how plants adjust to overcome this stress is one of the challenges for genetic improvement programs worldwide [12,13,14,50].

Arabidopsis thaliana is the most studied plant in breeding research, mainly due to this plant’s characteristics, which include small size, allowing the plant to grown in small spaces, rapid sexual reproduction, autogamy, short life cycle, and high seed production. Furthermore, an essential characteristic for Arabidopsis to be considered a model species is its possibility of being genetically transformed by Agrobacterium tumefaciens [51,52].

Among the studies dealing with gene expression, ref. [1] reported finding five genes related to resistance to water stress, being directly associated with proteins responsible for oxidizing a large number of substances CYP450; HORVU2Hr1G092360; HORVU2Hr1G092370; HORVU2Hr1G092390; HORVU2Hr1G092380; HORVU2Hr1G092400.

In addition, genome resequencing and SNP calling were detected in the CYP450 gene promoter. The genes of the CYP450 family contribute to improving resistance to water stress in plants since they actively participate in the production of various natural compounds produced by plants to protect them from abiotic and biotic stresses [1,53].

The genes of the HORVU2Hr1G09236 family contribute to assessing confidence in a molecular phylogeny, while those of the CYP family are essential for encoding the enzyme ferula-the-5-hydroxylase (F5H) in both Arabidopsis thaliana and Oryza sativa. Both genes are key elements in the pathways that synthesize lignin in various plants.

The genes of the LOC100808546, LOC100794180, and LOC100801187 families, when studied by [30], were positively regulated with the application of mepiquat chloride (DPC). However, this inhibited the pathways related to plant production, such as the synthesis of photoassimilates, contributing to limited plant growth.

Furthermore, when screening and analyzing differential metabolites (DAMs), it was found that in soybean varieties, phenolic compounds called flavonoids accounted for more than 60% of the total differential metabolite analysis, and most of the substances were positively conditioned, indicating that the process of spraying DPC contributes to increasing plant resistance to stress. The mepiquat chloride molecule decreases internode prolongation by repressing cell division and development in cotton, and several genes related to the cell cycle and cell division were altered by treatment with DPC; however, it increases resistance to water stress, corroborating the data obtained by [30].

Drought resistance, photosynthesis and the antioxidative capacity of maize are significantly better in plants showing overexpression of the ZmNF-YB16 gene, ref. [5] observed significant differences between wild plants and GMOs. Transcription is superior in transgenic plants to wild ones when subjected to water stress.

It is also worth noting that the genes responsible for positively regulating GMOs compared to wild ones are those associated with the coding of PsbH and PsbH-type proteins directly involved in the photosynthetic process [5].

The PsbZ protein is the linker between the light-harvesting complex II (LHCII) and the PSII core, where PsbH is essential for PSII stabilization and assembly. Given this, ref. [5] stated that the ZmNF-YB16 gene regulates the expression of several genes involved in photosynthesis, antioxidant capacity, and endoplasmic reticulum stress response, conferring resistance to drought stress.

Working with the maize gene ZmbZIP33, refs. [6,54] observed that this gene is linked to the promotion of plant tolerance to drought and also to the abscisic acid-dependent signaling pathway, which contributes to recovery from stress damage, making it clear that the ZmbZIP33, ZmbZIP15, and ZmbZIP19 gene family contributes to reducing the damage caused by water stress, specifically ZmbZIP33, which actively responds to exogenous treatment with abscisic acid (ABA).

Thus, the ZmbZIP33 gene may also have similar activity to the homologous genes, being responsive to water stress using the ABA metabolic pathways. Ref. [6] further pointed out that root system growth is positively related to the ability of maize plants to absorb and transport water. Therefore, the overexpression of ZmbZIP33 in species such as Arabidopsis obtained by GMOs tends to increase the plant’s tolerance to drought conditions as well as contribute to its ability to recover [55].

When studying altered transcript expression levels of GNAC TFs during drought stress in susceptible and tolerant peanut cultivars, ref. [24] observed that nine genes are responsible for increasing peanut tolerance to drought stress (GNAC 2, GNAC 16, GNAC 20, GNAC 38, GNAC 42, GNAC 53, GNAC 70, GNAC 71, and GNAC 77). This increase in the tolerance of peanut plants stimulates changes in the relative water content, osmotic potential, electrolyte leakage proline content, and chlorophyll content of the plants, making them better adapted to stress conditions.

Chlorophyll stability can be employed as an indicator of drought stress in peanut plants. It can be said that raising the level of proline in plants tends to increase the plant’s tolerance to stress. This was evidenced in [3,24], which found high levels of proline synthesized by wild plants exposed to water stress, although this was not observed in the cultivated clone, GJG22. Ref. [56] further stated that the proline content in the drought-tolerant variety was considerably higher than in the drought-susceptible type.

Genetically modified potato plants have an overexpressed CodA gene that confers greater drought resistance and recovery capacity, in addition to the accumulation of glycine betaine (GB), which is an extremely efficient compatible solute, and its presence is strongly associated with greater plant tolerance in stress environments [33].

With regard to the use of molecular markers to identify drought stress resistance in wheat and rice plants, refs. [31,57,58,59] used quantitative trait loci (QTL) identification via marker-assisted selection to increase drought stress resistance in wheat. The authors concluded that the introgression of ancestral QTL alleles from wild wheat can increase the yield and drought resistance in domesticated wheat. Furthermore, several near-isogenic lines (NILs) showed significant advantages over their recurrent parents in the test environment.

In recent decades, a large number of studies on quantitative trait loci (QTL) involving various crop plants have been discovered, including wheat. However, the number of research studies involving the validation of QTLs with the aim of analyzing resistance/tolerance to water stress is still incipient, and the little research that has been completed on the subject deals with so-called complex polygenic traits associated with productive responses and abiotic stresses [60].

A research work studying forage plants under water stress and diverse microbial activity using abscisic acid–polyacrylamide (ABA–PAM) [10] reported that combined ABA–PAM treatment potentiates the antioxidant activity of catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD), reducing the content of toxic substances such as malondialdehyde and H₂O₂, chlorophyll degradation, and damage caused by drought stress, thus providing adequate nutrition for plant growth and development, providing resistance to drought stress. This fact is closely related to the relative increase in Actinobacteria, whereas Chloroflexi and Acidobacteria decreased and Proteobacteria, Firmicutes, and Ascomycota increased.

Exogenously applied abscisic acid (ABA) increases ABA synthesis and redistribution within the plant under drought stress, promoting stomatal closure, reducing water loss [61], and significantly increasing the crop yield. When agricultural crops are subjected to water stress as a defense mechanism, they start producing various reactive oxygen species (ROS), which contribute to a series of chemical, biochemical, and physiological reactions. Therefore, plants with the capacity to increase production of the plant hormone abscisic acid (ABA) have the ability to produce antioxins as a defense, eliminating ROS, which helps to reduce damage to plant tissue and improve plant tolerance when subjected to stressful situations [62,63].

When studying drought resistance induced by brassinosteroids (BRs) of experiments using reactive maize genotypes in terms of physiological metabolism and gene expression analysis, refs. [3,8] identified an increase in enzyme activity, which may contribute to removing peroxides produced by stress, preventing plasma membrane oxidation and protecting cells from injury, which confers drought stress resistance to plants. Pathway enrichment analysis (KEGG) coupled with DEGs for graphical or visual representation of RT-PCR resulted in a total 20 plant products enhanced with BRs from genotypes H21 and 478, including 15 in the “organic cycle” cosmology, 9 in the “subatomic capacity” philosophy, and 7 in the “cell component” metaphysics. The differentially expressed genes present in the H21 and 478 genotypes were conclusive in the osmoregulation processes, covering eight terms associated with the organic cycle that influence the response to water stress [11,64,65].

These results show that maize genotypes were correlated with their degree of drought tolerance, since highly tolerant genotypes show higher brassinosteroid contents, as identified via pathway enrichment analysis (KEGG) with DEGs for graphical or visual RT-PCR representation of plants under drought stress.

When analyzing the narrow metaxylem of lpa1 to study increased drought tolerance and optimized water utilization for grain filling in dwarf rice (dep1-ko or d2), the authors found that the genetic combination of lpa1 with the semi-dwarf mutant (dep1-ko or d2) contribute to maintaining an adequate water supply when subjected to water restriction without compromising grain filling. This is associated with a reduction in the transpiration processes of the lpa1-3 gene on sunny days.

It is also important to note that, in lpa1 mutants, enlarged metaxylem vessels were absent, and protoxylem vessels were normally developed. This helps to optimize the flow of water according to the number of vessels and genetically controls the size of the conducting vessels as well as the height of the plant, increasing the plant’s ability to withstand adverse conditions in the water supply and climate change.

5. Conclusions

The systematic review of scientific articles associated with bibliometric analysis is an effective method to understand the technical–scientific panorama and the worldwide application of the advances in genetic improvement aimed at drought stress in agricultural crops, serving as a tool to map/guide future research in breeding programs.

The scarce number of existing publications in the adopted databases and in the timeframe analyzed was proven. However, the number of scientific studies has increased over the years, especially since 2018.

China has the greatest impact on research on drought stress resistance in crops in terms of publications, research, affiliations, and H-index.

The most studied mechanisms are gene expression, molecular markers, genes from specific families, abscisic acid (ABA), brassinosteroids, gene combination, and genetically modified organisms.