LEVERAGING PSEUDOMONAS SYRINGAE TYPE III EFFECTORS TO UNDERSTAND THE GENETIC REQUIREMENTS FOR DISEASE RESISTANCE IN ARABIDOPSIS

Abstract

Plants employ a sophisticated immune system to recognize and respond to infecting pathogens. One of the most well-studied aspects of this immune system is the intracellular nucleotide-binding site leucine-rich repeat (NLR) receptor family. These proteins mediate pathogen perception by recognizing pathogen type III effector proteins (effectors) in the plant cell. Hundreds of NLR-cognate effector pairs have been characterized, yet much less work has been devoted to understanding 1) what effectors do in plant cells to subvert immune responses and 2) how individual NLRs work to signal immune responses in the context of the broader plant immune system. Here, I investigated the potential targeting of subfamily VII receptor-like cytoplasmic kinases (RLCK VIIs) in Arabidopsis and Nicotiana benthamiana by the Pseudomonas syringae effector, HopBA1. This work demonstrated that loss of function mutants in individual subgroups of RLCK VIIs in Arabidopsis polymutants were not sufficient to abolish HopBA1 recognition by the Arabidopsis immune system. I found that members of subgroup 6 of the NbRLCK VII family are apparently cleaved upon transient co-expression with HopBA1 in N. benthamiana. These data suggest that HopBA1 may act as a protease upon NbRLCK VII substrates, a previously undescribed enzymatic function for HopBA1. Through study of HopBA1 recognition by two NLRs in Ag-0 Arabidopsis, Response to HopBA1 (RBA1) and HopZ-Activated Resistance 1 (ZAR1), I characterize an unequal partnership in initiating cell death and growth restriction responses by distinct classes of plant immune receptor. This work also revealed a partial dependence of ZAR1 upon the canonical Toll-interleukin receptor (TIR)-NLR immune response regulator, ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1). I followed this observation with a genetic dissection of ZAR1-mediated responses to other pathogen effectors in Col-0. I found that ZAR1 generally requires EDS1 and the PHYTOALEXIN DEFICIENT 4 (PAD4) -ACTIVATED DISEASE RESISTANCE 1 (ADR1) signaling node to varying degrees to initiate immune responses, depending on the effector used to activate ZAR1. Taken together, the work presented here helps to fill some of the major gaps in understanding that exist within the field of molecular plant-pathology. I took steps to understand the particular enzymatic functions of HopBA1, which lays a foundation to better understand how effectors work to support pathogen colonization of their plant hosts. The finding that ZAR1 requires EDS1 to varying degrees to initiate complete immune responses in Arabidopsis represents a departure from the published work on ZAR1-mediated immune responses. These data support a more holistically integrated model for plant NLR-mediated immune responses and emphasize the importance of considering the whole immune system when describing signaling events initiated by a particular NLR.