To resist pathogen invasion, plants have evolved several mec...

created [InstanceEdit:9611567] Naithani, Sushma, 2018-06-18
dbId 9611517
displayName To resist pathogen invasion, plants have evolved several mec...
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text To resist pathogen invasion, plants have evolved several mechanisms to recognize non-self or modified-self molecules using plasma membrane-associated receptor proteins (pattern recognition receptors; PRRs) that trigger cell signaling leading to reactive oxygen species (ROS) production, programmed cell death (PCD), and defense-related (PR) gene expression, and immunity. Upon plant microbe interactions, often the components of the microbial cell walls (e.g. fungal chitin, and/or bacterial peptidoglycan) serve as elicitors in plant innate immunity (PTI). In rice, CEBiP, a chitin elicitor binding protein (Akamatsu et al. 2013), and a lysin motif containing plasma membrane proteins LYP4 and PYP6 (Liu et al. 2012; Kouzai et al. 2014) act as the chitin receptor. LYP4 and LYP6 can also bind to peptidoglycan, an essential cell wall component of both Gram-positive and Gram-negative bacteria. Upon chitin binding, the CEBiP forms a homodimer, then forms signaling-active receptor complex with CERK1 receptor kinase in the plasma membrane (Akamatsu et al. 2013; Kouzai et al. 2014). The LYP4 and LYP6 receptor proteins also associate with CERK1 after binding to chitin or peptidoglycan (Liu et al. 2012). The activated receptor kinase protein CERK1 (in association with RacGEF1, a guanine nucleotide exchange factor) phosphorylates Rac1, a central component of the signaling pathway that integrates multiple biotic stress signals(Kouzai et al. 2014). The active, GTP-bound Rac1 (in the plasma membrane) then associates with the NADPH oxidases RBOHB to trigger ROS generation (Liu et al. 2015), which triggers expression of defense related genes and PCD leading to immunity (resistance) to pathogens. RBOHB is specifically activated in response to pathogen attack, while downregulated in response to submergence. In rice, Rac1 also directly interacts with rice blast NB-LRR R protein Pit and contributes to Pit-mediated ROS production and HR. Rac1-mediated immune signaling is negatively regulated by a RhoGAP protein SPIN6, the E3 ligase SPL11 by dephosphorylating Rac1:GTP into Rac1:GDP (Liu et al. 2015). Mutation in the Spin6 gene causes accumulation of ROS and PR proteins, i.e., PR1a, PR5 and PBZ1, that results in plant cell death and immunity (Liu et al. 2015). Recently, another receptor like kinase, SDS2, has been identified to positively regulate PCD and immunity in rice. SDS2 kinase activates two cytoplasmic kinases RLCK117 and RLCK118, which in turn activate RBOHB (Fan et al. 2018). In Arabidopsis ortholog(s) of CEBiP have not been identified and CERK1 is known to directly binds chitin. However, the homologs of LYP4 and LYP6 exist in Arabidopsis. Thus, Arabidopsis and rice (possibly other plants) differ in sensing microbial elicitor or same type of pattern recognition receptors can act differently in different species of plants.