Medicinal plants contain numerous bioactive secondary metabolites (SMs) that can be used for the treatment and prevention of diseases. SM concentration is a crucial standard for evaluating the quality of medicinal plants. SM accumulation is affected by multiple factors, including genetic background, climate, soil physical and chemical properties, and environmental changes. In recent years, increasing studies have indicated that rhizospheric and endophytic microorganisms, play an essential role in regulating the accumulation of SMs in medicinal plants. While some microorganisms establish symbiotic relationships with medicinal plants to promote plant growth. Other microorganisms can directly synthesize SMs or promote plant SM biosynthesis through multiple strategies, such as activating plant immune signaling pathways and secreting plant hormones into host cells to manipulate hormone-mediated pathways. In contrast, SMs may improve plant resistance to environmental stresses, thereby affecting the composition of rhizospheric and endophytic microorganisms. In this review, we summarized the recent progress in understanding the role of microorganisms in regulating SM accumulation in medicinal plants. Further studies should focus on the application of utilizing microorganisms to enhance the accumulation of bioactive SMs in medicinal plants.

Plant cell-surface-localized pattern recognition receptors (PRRs) recognize microbial patterns and activate pattern-triggered immunity (PTI). Typical PTI responses include reactive oxygen species (ROS) burst controlled by the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOGUE D (RbohD) and activation of the mitogen-activated protein kinase (MAPK) cascade composed of MAPKKK3/5-MKK4/5-MPK3/6. However, the mechanisms through which PRRs regulate and coordinate these immune responses are not fully understood. Here, we showed that Arabidopsis thaliana OXIDATIVE SIGNAL-INDUCIBLE1 (OXI1), a kinase known to be activated by ROS, is involved in the LYK5-CERK1 receptor complex, which recognizes fungal cell wall-derived chitin. The oxi1 mutant exhibits enhanced susceptibility to various pathogens and reduced chitin-induced MAPK activation and ROS burst. We showed that chitin induces the phosphorylation of OXI1 in an RbohD-dependent manner. H₂O₂ and chitin treatment causes the oxidation of OXI1 at Cys104 and Cys205, which is essential for the kinase activity of OXI1. These oxidation sites are required for chitin-induced MAPK activation and disease resistance. Activated OXI1 directly phosphorylates MAPKKK5 to regulate MAPK activation. Additionally, OXI1 phosphorylates RbohD, suggesting that it may activate RbohD to promote ROS burst to further enhance the long-term MAPK activation. Together, our findings reveal a pathway linking PRR-mediated ROS production to MAPK activation through OXI1.

As sessile organisms, plants have to cope with environmental change and numerous biotic and abiotic stress. Upon perceiving environmental cues and stress signals using different types of receptors, plant cells initiate immediate and complicated signaling to regulate cellular processes and respond to stress. Receptor-like cytoplasmic kinases (RLCKs) transduce signals from receptors to cellular components and play roles in diverse biological processes. Recent studies have revealed the hubbing roles of RLCKs in plant responses to biotic stress. Emerging evidence indicates the important regulatory roles of RLCKs in plant responses to abiotic stress, growth, and development. As a pivot of cellular signaling, the activity and stability of RLCKs are dynamically and tightly controlled. Here, we summarize the current understanding of how RLCKs regulate plant responses to biotic and abiotic stress.