Determining the role of tomato atypical receptor kinase 1 in pattern triggered immunity
Keywords: Microbiology and Immunology.

Plants rely on innate immunity to recognize and defend against pathogenic microbes and pests. In the initial stages of plant immunity, cell surface localized protein complexes that contain pattern recognition receptors (PRRs) perceive microbe-associated molecular patterns (MAMPs) through a process called pattern triggered immunity (PTI). The initiation of PTI is characterized by the generation of reactive oxygen species, and the transcription of defense genes which are all geared towards suppressing pathogen growth. Despite the extensive study of PTI and associated PRR containing protein complexes in the model system Arabidopsis thaliana, recent work indicates that there is a range of PTI responses in diverse plant species, questioning the notion that PTI responses are broadly conserved in plants. This indicates there is a growing need to elucidate the molecular basis of PTI responses and isolate their immune related protein complexes in economically important crop plants, such as the Solanaceae family. In the Solanaceae, very few PRRs have been characterized to date. However, there is growing evidence for the role of receptor like kinases in immune signaling is emerging. For example, the Tomato Atypical Receptor Kinase 1 (TARK1) is required for PTI and is the target for Xanthomonas euvesicatoria (Xe) effector protein, XopN. Previous research in the Mudgett laboratory demonstrated that TARK1 is required for full resistance to Xe indicating that TARK1 is a positive regulator of PTI. In addition to positively regulating defense signaling, TARK1 protein was shown to accumulate in response to infection and its accumulation correlates with disease resistance. Despite initial evidence to indicate TARK's involvement in promoting PTI, there was a limited understanding of TARK1's specific role within tomato immune signaling and this became the central focus of my thesis. In this work, I demonstrate that TARK1 associates with a number of proteins linked to plant defense. In particular, TARK1 interacted with two a plasma membrane (PM) H+ ATPase (LHA4) and a leucine-rich repeat receptor-like kinase (RLK15), which I hypothesize are associated with a form of pre-invasion defense called stomatal immunity. Unexpectedly, my research indicates that TARK1 negatively regulates pre-invasion defense. Manipulation of TARK1 protein levels alters stomatal movement in response to bacteria and bacteria-associated biotic elicitors. My biochemical analyses reveal that TARK1 positively regulates PM H+ ATPase activity. Collectively, my results support a model that TARK1 promotes stomatal opening through the activation of PM H+ ATPases. In addition to the study of TARK1, I have shown that RLK15 is an active kinase and plays diverse roles in pre- and post-invasion resistance to bacterial and fungal infection. It remains to be determined how and/or if TARK1 and RLK15 regulate each other's activity in different immune related contexts. Overall, my thesis work uncovers new roles for TARK1 and RLK15 in a multitude of tomato defense responses and serves as fountain to further our understanding of immune-related protein complexes in tomato and their functional roles within immune signal transduction