Biotic stress nexus: Integrating various physiological processes in medicinal and aromatic plants

Abstract

Medicinal and aromatic plants (MAPs) from natural habitat have been integral to both traditional and modern medicine systems for their healing properties in curing various diseases and disorders of mankind. At the same time, cultivation of these crops has also become alternative source of income for farmers by providing raw materials to pharmaceuticals. Like other crops, MAPs also have to face various abiotic and biotic stresses leading to compromised yield and quality. Biotic stresses, encompassing diseases, nematodes, pests and parasitic flowering plants can directly or indirectly impact MAPs by altering their fundamental physiological processes leading to symptoms like leaf necrosis, chlorosis, tissue rotting, leaf spot, wilting, foliage defoliation, root damage, etc. In this review article, plausible explanations regarding physiology of stressed plants and its defense mechanisms including primary and secondary metabolism are presented. The research on source-stressor interactions in food crops is well-established as well as compiled and reviewed but there is a lack of comprehensive compilation in the context of MAPs. Therefore, this review is vital for exploring the mechanisms and physiological alterations in MAPs under biotic stress conditions. Plants, under attack of stressors respond with hypersensitive reactions (HRs), modifying cell wall properties, such as altered cell walls, chloroplast damage, changed chlorophyll content, altered stomatal behaviour causing change in gaseous exchange capacities, reformed enzyme activity and gene expression, leading to reduced photosynthesis and increased respiration. Along with this, nutrient assimilation and its translocation are adversely affected leading to compromised biomass production of the host plant. Additionally, the disruption in the balanced distribution of photosynthates between source-sink tissues leads to increased sugar utilization for defense, making the primary metabolism more complex in plant’s responses to these stressors. The compromised carbon fixation through primary metabolism diverts light energy towards secondary metabolism and results in increased secondary metabolites (SMs) production. SMs also get elevated as a part of host defense mechanism under mild stress conditions without significantly impacting the plant growth. However, in severe stress conditions, these improvement in quality cannot compensate the deviation in biological yield. Notably, in case of MAPs, the simulated application of biotic elicitors can also amplify SMs production, ultimately enhancing the plant's inherent qualities. Therefore, the secondary metabolism is of utmost importance in MAPs. In this review, we have compiled the scattered information and simplified the understanding of alterations in various physiological processes including primary and secondary metabolism, changes in biochemical attributes and gene expression profiles in MAPs during biotic stress situations. An extensive categorization of these physiological changes in MAPs discussed in this review will help in planning research strategies focused on resistance plant breeding programs for MAPs affected by various biotic stressors.