**8. Light Stress**

Sunlight is one the major factors of photosynthesis that provides the necessary energy for plant growth and development. Despite this, intense light, especially its ultraviolet (UV) part, causes serious damage to DNA, proteins, and other cellular components of plants [251]. Sunlight damages photosynthetic machinery, primarily photosystem II (PSII), increases ROS production, and causes photo-inhibition that can hinder plant photosynthetic activity, growth, and productivity [252]. Excess light accelerates ROS production in PSI and PSII of chloroplasts, which may balance photo-inhibition and the repair of plant cells [252]. Light-triggered plant responses depend on the fluency, exposure time, and acclimation of plants before light exposure [251]. Reductions in the quantity and quality of light could signal plants to activate defensive systems by enhancing adaptive alterations in stem morphology [252]. The signaling pathways of light can balance the constructive and destructive impact of light on plant defense and growth mechanisms.

Microbes are less studied in the mitigation of light stress compared to other abiotic conditions. Some PGPBs have shown great potential by enhancing photosynthesis, chlorophyll content, and photosynthetic pigments that can reduce light damage [253]. The impact of light on the composition of rhizosphere communities, such as prokaryotes and fungi, can be increased or decreased under climatic extremes. There are several bacterial species, including *Pseudomonas* sp., *Massilia* sp., *Burkholderia* sp., and Acidobacteria, that are classified as beneficial microorganisms in the context of high light intensity. In addition, some fungal species, including *Geminibasidium* sp. and *Oidiodendron* sp., were also described as the most abundant species under intense light. The microorganism communities derived from soil under the influence of high light intensity are different in taxonomy and physiological characterizations. The impact of light on the soil rhizosphere includes the detection of *Pseudomonas* sp. that could consequently increase photosynthesis and carbon and nutrient assimilation [254]. Stefan et al. [255] verified that seed inoculation with *Bacillus pumilus* and *Bacillus mycoides* increased photosynthetic activity, water use efficiency, and chlorophyll content in runner bean (*Phaseolus coccineus* L.). Suzuki et al. [256] reported that *Acinetobacter calcoaceticus* could increase the chlorophyll content of lettuce (*Lactuca sativa* L.).

#### **9. Conclusions**

This review elaborated the importance of plant growth-promoting microorganisms (especially bacteria and fungi) that can mitigate the damage caused by environmentally induced stresses (drought, salinity, heavy metals, flooding, extreme temperatures, and intense light). This review determined the potential, prospective, and biotechnological approaches of plant growth-promoting bacteria and fungi for the alleviation of plants in response to environmental stresses. Some bacteria and fungi under abiotic stress conditions can improve physiological and biochemical processes, such as nutrient uptake, photosynthesis, source–sink relationships, metabolism and the regulation of homeostasis, osmotic potential, protein function, phytohormone production (indole-3-acetic acid and 1-aminocyclopropane-1-carboxylic acid deaminase), enzymatic activity, nutrient solubilization, and plant nutrition. Therefore, the use of plant growth-promoting bacteria (PGPBs) and fungi contributes positively to agricultural production in abiotic stress conditions.

Despite several studies demonstrating the benefits of beneficial microorganisms, there are still research gaps and restrictions on the molecular mechanisms of crops. A mechanistic understanding of the interactions of plants and microorganisms under abiotic stress should be developed to address agricultural difficulties and resolve the nutritional and production concerns that are brought by climatic extremes. Therefore, further studies involving microorganisms are recommended to enhance sustainable crop production and food security in the light of potentially unstable climatic conditions.

**Author Contributions:** Conceptualization, A.J. and M.C.M.T.F.; methodology, A.J. and C.E.d.S.O.; validation, A.J., F.S.G. and C.E.d.S.O.; formal analysis: A.J.; resources, M.C.M.T.F.; data curation, A.J., C.E.d.S.O. and P.A.L.R.; writing—original draft preparation, A.J., F.S.G. and C.E.d.S.O.; writing—review and editing, M.C.M.T.F., A.J. and F.S.G.; supervision, M.C.M.T.F.; project administration, A.J. and M.C.M.T.F.; funding acquisition, A.J. and M.C.M.T.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This review received funding from The World Academy of Science (TWAS) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the first author's doctoral fellowship (CNPq/TWAS grant number: 166331/2018-0) and the productivity research grant (award number 311308/2020-1) of the corresponding author.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank São Paulo State University (UNESP) for providing technology and support as well as CNPq for financial support.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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