| dc.description.abstract | In most organisms, threonine deaminase (TD) functions as a housekeeping enzyme to convert threonine to α-ketobutyrate and ammonia as the committed step in the biosynthesis of isoleucine (Ile). However, tomato plants have two paralogous copies of the TD gene: TD1 and TD2. Besides its housekeeping function in Ile biosynthesis, TD2 also plays a defensive role against insect herbivores and necrotrophic pathogens. For insect herbivores, TD2 acts as a feeding deterrent in insect gut. A proteolytically cleaved, but active TD2 protein is delivered to the insect gut and reduces Thr availability by conversation to α-ketobutyrate (α-KB). In response to both insect feeding and necrotrophs, TD2 expression level is enhanced by necrotroph or insect attacks and TD2-gerenarted Ile is utilized for the production of the jasmonic acid-isoleucine (JA-Ile) conjugate, a highly active phytohormone required for host defense against insects and necrotrophs. However, a role for TD2 in defense against bacterial pathogens has not been reported to date.
Recently, it was shown that TD2 was able to be unexpectedly detected by an anti-phospho-Adi3 (α-pAdi3) antibody, which was initially developed to detect the phosphorylated version of the Adi3 protein kinase. Furthermore, when tomato leaves were treated with flg22 (22-amino acid peptide of bacterial flagellin), detection of TD2 by the α-pAdi3 antibody was quickly decreased over 50 minutes, but detection was restored after 1 hour and returned to the initial detection level.
It was determined that TD2 is possibly modified by proteolytic cleavage at the C-terminus identified by MS analysis and this possible C-terminal cleavage induces the loss of TD2 detection by the α-pAdi3 antibody. Interestingly, TD2 as modified during the response to flg22 showed reduced enzymatic activity and enhanced sensitivity to feedback inhibition by an allosteric effector, Ile. In defense phenotype analyses, TD2 RNAi knockdown (KD) plants showed lower reactive oxygen production compared to wild type plants. Also, TD2 KD plants were more resistant to the bacterial pathogen, P. syringae pathovar tomato (Pst), but more susceptible to necrotrophic pathogens, Botrytis cinerea. Because phytohormome salicylic acid (SA) and jasmonic acid (JA) control defense in response to Pst and B. cinerea with distinctly different life styles, respectively, SA and JA pathways act antagonistically to each other during plant defenses. Therefore, it was suggested that TD2 regulates defense-related hormone crosstalk between SA and JA. In qRT-PCR analysis, TD2 KD plants showed high expression levels of SA-responsive genes, which positively control bacterial infection, but low expression levels of JA-responsive genes, which would lead to less resistance to necrotrophic pathogens.
This study provides an insight into a novel TD2 function in the elaborate crosstalk between SA and JA signaling induced by bacterial infection. | en |
