Early recognition and immune signaling in crop plants

Atypical receptor-like kinases & immune signaling in tomato

Progress in elucidating the defense machinery that inhibits pathogen growth has come from genetic and biochemical studies using model systems. We have learned that plants have a complex, multilayer signaling network that controls the recognition of pathogen molecules and mediates appropriate anti-microbial defenses. In addition, pathogens evolved virulence factors to suppress nodes of this defense network. The nature and function of immune complexes in agriculturally important plants; however, are in its infancy. We study the Xanthomonas euvesicatoria (Xcv)-tomato system to elucidate pathways suppressed during pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). Two central questions that we are addressing in this project are: (1) What are the plasma membrane (PM)-associated immune complexes in tomato required for anti-Xcv immunity? (2) How does Xcv modulate signaling transmitted from these complexes in tomato? The questions are motivated by our data showing that tomato atypical receptor kinase TARK1 and 14-3-3 phospho-binding protein TFT1 are required for Xcv immunity, which is suppressed by the Xcv type III effector XopN.

The over-arching goal of this area of research is to study the TARK1-dependent signaling pathway in tomato required for Xcv immunity. We hypothesize that: (1) Xcv infection stimulates TARK1 accumulation at the PM and TARK1-associated complexes initiate defense signaling that leads to basal immunity. (2) Tomato MAPKs may be involved in TARK1-dependent immune responses. (3) TARK1 is differentially phosphorylated during Xcv infection and may be regulated by the PP2C phosphatase. (4) TARK1 may operate with 14-3-3s to regulate immunity. (5) XopN suppresses TARK1 and 14-3-3 function to inhibit defense.

Significance: Tomato is a representative crop in the Solanaceae, the third most valuable crop family in the world and the most valuable among vegetable crops. Little is known about early immune signal transduction events in tomato, which is central to plant health in the field. Moreover, Xcv pathology is highly specific for the Solanaceae arguing that elucidating the molecular basis of anti-Xcv immunity in the natural host tomato will uncover novel defense machinery and signaling mechanisms defining the evolution of this host-pathogen interaction.