Similar to animals, plants rely on cell-surface and intracellular immune receptors to defend against pathogens. The front line of innate immunity in plants involves the recognition of highly conserved microbe/pathogen associated molecular patterns (MAMPs/PAMPs) such as bacterial flagellin or fungal chitin by cell-surface localized pattern recognition receptors (PRRs) leading to PAMP-triggered immunity (PTI) to limit pathogen growth. Highly evolved pathogens deliver effector proteins to interfere with the PTI response. Plants have evolved intracellular nucleotide-binding domain leucine-rich repeat (NLR) class of receptors which recognize pathogen effectors and activate effector-triggered immunity (ETI). The PTI and ETI pathways share many components; ETI has increased in the amplitude of the signals during ETI often culminates in hypersensitive response programmed cell death (HR-PCD) that physically isolates the infection. Our long-term goal is to understand the molecular mechanisms by which immune receptors recognize pathogens and initiate immune signaling.
Pattern-triggered immunity: One of the main areas of research in our group is to understand how cell-surface PRRs recognize PAMPs to induce PTI. BIK1, a receptor-like cytoplasmic kinase (RLCK), functions as a central integrator of signaling from multiple PRRs. To understand how BIK1 decodes pathogen inputs into immune signaling outputs, we determined the BIK1 crystal structure. To integrate the BIK1 structure data, we mapped trans-phosphorylation sites of BIK1 by the PRR EFR, a receptor-like kinase that recognizes bacterial elongation factor peptide elf18 to activate immune signaling. Based on the trans-phosphorylation and structure data, we identified several important amino acid residues that could control immune signaling. Analyses of phosophonull and phosphomimetic transgenic plants identified S89 and T90 residues in BIK1 play important roles in response to Pseudomonas bacterial infection. Interestingly, the S89 and T90 reside in a uniquely extended loop in the BIK1 structure compared to homologous kinases. Phosphonull mutants were more susceptible and phosphomimetic mutants were more resistant to bacterial infection. Interestingly, S89D/T90d phosphomimetic mutants have increased levels of the phytohormone jasmonic acid (JA), which plays a role in immune signaling. BIK1 also localizes to the nucleus where it phosphorylates WRKY transcription factors to regulate JA and SA hormone responsive genes. These findings provided the mechanistic basis of signal transduction from PRR to phytohormones during immune signaling (PMC6266874).
Extracellular reactive oxygen species (ROS) produced through NADPH oxidase RbohD plays an important role in PRR-mediated immune signaling to limit pathogen infection. We recently contributed towards understanding the role of MAPKKKK (MAP4K) called SIK1 that interacts and stabilizes BIK1 during PRR signaling and promotes ROS burst through RbohD (PMC6279242). In another collaborative work with Professor Gitta Coaker laboratory, we demonstrated that RbohD activity is switched off after initiation of PRR signaling through phosphorylation of RbohD by PBL13 kinase and RbohD ubiquitination through a previously uncharacterized E3 ubiquitin ligase PIRE1 (PMC7160206).
Effector-triggered immunity: Another main area of research in our group is to understand how intracellular NLRs recognize pathogen effectors and activate defense signaling. To this end, we study N immune receptor-mediated defense against Tobacco Mosaic Virus (TMV) as a model. N is the first member of plant Toll-Interleukin-1 receptor homology Region (TIR) domain containing NLR class of immune receptor (TIR-NLR). N is a nucleocytoplasmic receptor and recognizes the helicase domain (referred to as p50) within the TMV replicase. N recognizes p50 indirectly through a chloroplast localized N Receptor Interacting Protein 1 (NRIP1). In the presence of TMV, a subset of available NRIP1 relocalizes from the chloroplast to the cytoplasm and nucleus. Cytoplasmic NRIP1 associates with the TMV replicase leading to the recruitment of N (PMC1820829; PMC2267721). Interestingly, N associates directly with a transcription factor, Squamosa Promoter-binding-protein-Like 6 (SPL6), and this association occurs only upon recognition of the p50 effector. Silencing of SPL6 compromises N-mediated resistance to TMV. SPL6 is also required for Arabidopsis TIR-NLR RPS4-mediated resistance to Pseudomonas syringae bacteria expressing the AvrRps4 effector. These findings point to SPL6 as one of the conserved nuclear components of TIR-NLR signaling (PMC3597514).
Recently, we optimized and used a proximity-labeling technique in plants and identified number of novel proteins which play a role in N-mediated resistance to TMV. Our findings indicated that a novel UBR-box containing E3 ubiquitin ligase, UBR7, plays an important role in modulating N protein levels and immune response to TMV (PMC6642208).
Extracellular reactive oxygen species (ROS) produced through NADPH oxidase RbohD plays an important role in PRR-mediated immune signaling to limit pathogen infection. We recently contributed towards understanding the role of MAPKKKK (MAP4K) called SIK1 that interacts and stabilizes BIK1 during PRR signaling and promotes ROS burst through RbohD (PMC6279242). In another collaborative work with Professor Gitta Coaker laboratory, we demonstrated that RbohD activity is switched off after initiation of PRR signaling through phosphorylation of RbohD by PBL13 kinase and RbohD ubiquitination through a previously uncharacterized E3 ubiquitin ligase PIRE1 (PMC7160206).
Effector-triggered immunity: Another main area of research in our group is to understand how intracellular NLRs recognize pathogen effectors and activate defense signaling. To this end, we study N immune receptor-mediated defense against Tobacco Mosaic Virus (TMV) as a model. N is the first member of plant Toll-Interleukin-1 receptor homology Region (TIR) domain containing NLR class of immune receptor (TIR-NLR). N is a nucleocytoplasmic receptor and recognizes the helicase domain (referred to as p50) within the TMV replicase. N recognizes p50 indirectly through a chloroplast localized N Receptor Interacting Protein 1 (NRIP1). In the presence of TMV, a subset of available NRIP1 relocalizes from the chloroplast to the cytoplasm and nucleus. Cytoplasmic NRIP1 associates with the TMV replicase leading to the recruitment of N (PMC1820829; PMC2267721). Interestingly, N associates directly with a transcription factor, Squamosa Promoter-binding-protein-Like 6 (SPL6), and this association occurs only upon recognition of the p50 effector. Silencing of SPL6 compromises N-mediated resistance to TMV. SPL6 is also required for Arabidopsis TIR-NLR RPS4-mediated resistance to Pseudomonas syringae bacteria expressing the AvrRps4 effector. These findings point to SPL6 as one of the conserved nuclear components of TIR-NLR signaling (PMC3597514).
Recently, we optimized and used a proximity-labeling technique in plants and identified number of novel proteins which play a role in N-mediated resistance to TMV. Our findings indicated that a novel UBR-box containing E3 ubiquitin ligase, UBR7, plays an important role in modulating N protein levels and immune response to TMV (PMC6642208).