New Progresses in Plant Secretory Structures

Dear Colleagues

Secretory structures range from isolated cells to complex multicellular structures specialized in the synthesis, accumulation and/or release of specific substances, such as nectar, mucilage, essential oils, resins, odor and latex. Several functions have been attributed to plant secretions, such as protecting plants against unfavorable environmental conditions, attracting pollinating agents and dispersers of fruits and seeds, or even protecting against herbivores and infection by pathogens. In addition, many of the plant secretions are sources of raw materials for food products, cosmetics, medicines, insecticides, among others. The growing search for natural products as raw material for an increasingly diversified industry has stimulated research into all aspects of secretory structures.

Secretory structures have aroused great interest in botanists since the advent of light microscopy in the 17th century, and their presence or absence, as well as their morphology, are used as characters with taxonomic value. The first studies on secretory structures were carried out with nectaries, hydathodes, salt glands, digestive enzymes and laticifers focused on the physiological processes of transport and elimination of exudate. The great advance in the knowledge of the biology of secretory cells came, without a doubt, from the invention of the transmission electron microscope in 1931, which made it possible to visualize cell organelles in detail. From the second half of the 20th century onwards, secretory structures were the object of numerous anatomical and cytological studies that contributed to the knowledge of their ontogenesis and development, to the elucidation of the main synthesis and compartmentalization pathways of the produced metabolites, as well as to the clarification of its functions. During this period, studies also began to focus on the chemical profile of plant secretions, especially using high-sensitivity gas chromatography combined with mass spectroscopy. Advances in this area have stimulated research in allelochemistry, pharmacology, phytopathology and ecology particularly involving aspects of insect–plant, microbe–plant and plant–pollinator interactions. At the beginning of the 21st century, there was an increase in publications on the biochemical potential of terpene-secreting trichomes aimed at manipulating the secretion. During this period, there was a significant advance in the methods of isolation of secretory trichomes, allowing for more precise analysis of the metabolism, enzymology and genetic basis of the development of secretory cells.

Despite the incredible amount of information obtained in the last decades, for a long time, all lines of research dealing with plant secretions developed independently of each other, and there are still questions and challenges open in this area of knowledge.

One of the important challenges is related to the cellular dynamics of the secretory process, where many aspects have not yet been adequately elucidated. For example, vesicular trafficking (exocytosis and endocytosis), the interaction between membranes and organelles, secretion autotoxicity and cytoprotective mechanisms, as well as autophagic processes that precede secretory cell autolysis, are still research targets. Another aspect that is still the object of research concerns the architecture of the cell wall and cuticle and its consequences in the exteriorization of secretion. The integration of cellular, physiological, biochemical and molecular mechanisms linked to the secretory process and how they interact with plant regulators and nutrients are among the topics that are still research objects.

Research that integrates morphological diversity, spatial and temporal variations in the functioning of glands both between plants and within the same plant, as well as their consequences on mutualisms, are essential to support studies on evolutionary ecology.

Although some studies have carried out assays on the evolution of morphological characters related to secretory structures, the literature still lacks information on the reconstruction of the ancestral character states of these structures, both in smaller taxonomic groups (e.g., genera, tribes) as well as in angiosperms and other groups of plants. This is a huge gap that lacks rigorous investigations and integration with systems and evolutionary biologists.

Another interesting point that is still an object of investigation concerns the functional versatility of the secretory cell and the regulation of gene expression in responses to cellular stress. There is evidence of interference of climatic factors, especially sunlight and temperature in the development of secretory structures and in the modulation of the performance of the secretory function. The interference of seasonality in the physiological and biochemical mechanisms of essential oil secretion in agricultural, aromatic and medicinal plants is also a target of research.

In this scenario, integrated approaches from botany, cell biology, ecology, chemistry and genetics are vital to access the factors that interfere in the production and composition of plant secretions. Such studies are urgent, especially when considering the richness of species that produce active substances and the threats to biodiversity loss that we have faced in recent decades, whether due to the destruction of their habitats or the unrestrained exploitation of natural products.

Therefore, in this Special Issue of articles (original, observational and/or manipulative research articles, perspectives, hypotheses, opinions, reviews, approaches and modeling methods) on secretory structures and plant secretions and their regulation, including cytology, biochemistry, physiology, ecology and evolution, proteins, specialized metabolites, genes at all levels, comprising transcriptome, proteome, metabolome and epigenome studies, in cultivated and native model plants, are welcome.

Prof. Dr. Silvia Rodrigues Machado
Guest Editor

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• autotoxicity and cytoprotective mechanisms
• cellular bases of the secretory process
• ecology and evolution of plant glands
• functional versatility of the secretory cell
• regulation of plant gland metabolism
• regulation of secretory cell gene expression
• secretory structures and biotechnology
• secretory structures interactions