Graduate Research Assistant University of Missouri Columbia, Missouri
Body of Abstract: In Arabidopsis thaliana, recognition of bacterial pathogens by plasma membrane-localized pattern recognition receptors (PRRs) initiates downstream signaling cascades resulting in pattern triggered immunity (PTI). Initiation of PTI involves various plant defense responses, including activation of mitogen activated protein kinase (MAPKs) cascades. The Peck lab identified mutants lacking MAPK phosphatase 1, MKP1, a negative regulator of the MAPK pathway, showing enhanced resistance against Pseudomonas syringae pv tomato DC3000. The resistance mechanism was found to be a decrease in extracellular plant-derived chemicals necessary for the bacterial pathogen to express its virulence program, the type III secretion system (T3SS; Anderson et al. 2014). Our lab identified a plasma membrane (PM) E3 ubiquitin ligase rapidly phosphorylated during and required for PTI. Mutants lacking this E3 ubiquitin ligase (e3l) show loss of PTI resistance comparable to PRR receptor mutants. However, receptor abundance and downstream events are not changed in e3l mutant. We found that absence of this E3 ubiquitin ligase leads to loss of enhanced resistance in mkp1; this suppression of resistance in mkp1 correlates with loss of restriction of T3SS. Although this result places E3L downstream of MKP1 regulation, the PTI-induced phosphorylation site on E3L is not a MAPK target, indicating that another kinase must be involved in regulation of this E3 ligase.
Calcineurin B-Like proteins (CBLs) are a class of calcium-dependent signaling molecules, that regulate CBL-Interacting Protein Kinases (CIPKs). A quintuple knockout (cbl-5ko) of CBL proteins eliminating all PM-localized CIPK activity leads to loss of PTI to similar levels as seen in e3l. Intriguingly, phosphorylation sites on E3 ligase are consistent with the upstream kinase being a CIPK. These results indicate CBL proteins and their associated CIPKs may be the missing piece(s) downstream of the MAPK cascade, resulting in phosphorylation and activation of the E3 ligase. Further characterization of mutants will be discussed.