Functional Characterization of a Leucine-Rich Repeat and Cysteine-Rich Receptor-Like Kinases Involved in Arabidopsis Defense Responses

Abstract I
Plants can acquire enhanced resistance to pathogens after treatment with beta- aminobutyric acid (BABA). BABA primes plant’s defense responses. Previously, we showed that a leucine-rich repeat protein kinase (LRRPK) contributes to disease resistance to Pseudomonas syringae pv. tomato (Pst) DC3000. Indeed, lrrpk mutants were more susceptible to Pst DC3000 hrcC-, suggesting this LRRPK is required for pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). We have demonstrated that lrrpk mutants were defective in BABA-mediated blockage of stomatal reopening after Pst DC3000 infection. lrrpk mutants also demonstrated reduced PTI marker gene induction and impaired callose deposition after flg22 treatment. However, lrrpk mutants demonstrated a normal reactive oxygen species burst. The aim of this study was to identify whether LRRPK can enhance disease resistance to another bacterial pathogen and to analyze the phenotype of LRRPK over-expression (OE) lines. lrrpk mutants were found to be more susceptible to virulent Pseudomonas syringae pv. maculicola (Psm) ES4326 and defective in BABA-mediated plant defense. Moreover, OE lines were more resistant to Pst DC3000 and accumulated more callose after treatment with the PAMP flg22. In addition, stomata of OE lines did not reopen 3 hours post inoculation with bacteria. Knock-out mutants and OE lines phenotypes suggest that LRRPK plays a role in PTI and BABA-induced resistance.

Abstract II
Genome-wide microarray analysis of an Arabidopsis Lectin Receptor Kinase-VI.2 (LecRK-VI.2) over-expression line shows that 12 cysteine-rich repeat like kinases (Cysteine-rich receptor like kinases, CRK-A, -B, -C, -D. –E, -F, -G, -H, -I , -J, -K, and -L) were up-regulated at least 4-fold when compared to WT. In Arabidopsis, CRKs are a sub-family of receptor-like protein kinases that contain two copies of the C-X8-C-X2-C motif in their extracellular domains. It has been shown that CRK genes can be induced by the defense hormone salicylic acid and bacteria Pst DC3000 (Pseudomonas syringae pv. tomato DC3000) bacteria. Using a T-DNA knock-out approach, we did not observe any altered phenotype after Pst DC3000 inoculation, suggesting that these genes are functionally redundant. Therefore, we decided to use a gain-of-function approach by over-expressing these CRKs in wild-type Col-0 background. Our preliminary results indicate that CRK-H and CRK-K over-expression lines are more resistant to Pst DC3000. In addition, these two CRKs over-expression lines showed constitutive callose deposition and expression of PTI marker gene PR1 (PATHOGENESIS-RELATIVE 1) and FRK1 (FLG22-INDUCED RECEPTOR-LIKE 1), and an altered stomatal immunity. We suggest that CRK-H and CRK-K are involved in plant defense.