Stress inducible glycosyltransferases in <em>Arabidopsis thaliana</em> and their impact on plant metabolism and defense mechanisms.

Abstract

Small molecule glycosylation in plants is crucial for the biosynthesis of secondary metabolites and the regulation of the activity of several signaling molecules and defense compounds. One hundred and twenty-two different UDP-dependent glycosyltransferases (UGTs) catalyzing these conjugations exist in the model plant Arabidopsis thaliana. Despite major advances in plant biology due to genome annotations and &lsquo;omics&rsquo; approaches, the vast majority are still uncharacterized enzymes without known specific substrates and physiological roles. In this project, the role of UGTs in plant stress response was investigated focusing on top stress responsive candidate genes. Transcriptional responsiveness of all UGT members of Arabidopsis was analyzed using publicly available expression data of plants exposed to several abiotic and biotic stress cues. A clear clustering of stress-dependent inductions was observed highlighting several highly responsive UGT genes with yet unknown function. The two top-ranking stress-induced and previously uncharacterized glucosyltransferases UGT76B1 and UGT87A2 were selected for further functional characterization. Both are broadly up-regulated by abiotic as well as biotic cues, suggesting an important stress related role. Using a reverse genetics approach (knockout and overexpression lines) metabolic and phenotypic changes correlating with the expression of the corresponding UGT gene were analyzed. In the case of UGT87A2, plants with altered UGT expression did not reveal any obvious phenotypes even when several stress cues were applied. Non-targeted FT-ICR-MS analyses in the negative mode of two knockout lines did not reveal significant metabolic changes, whereas independent overexpression lines showed several m/z peaks indicating up-regulated metabolites. Further characterization of these compounds led to the identification of a new metabolite in Arabidopsis, ascorbic acid 2-O-&szlig;-glucoside. Together with the upregulation of other putative compounds, the results suggest potential roles for UGT87A2 in ascorbic acid homeostasis or cell wall biosynthesis. UGT76B1 was identified as a novel player in plant defense affecting the antagonistic salicylic acid and jasmonate-dependent signaling pathways. Loss of the UGT76B1 function led to enhanced resistance to hemibiotrophic pathogens and accelerated senescence. This was accompanied by constitutively elevated SA levels and SA-related marker gene expression and repression of JA-dependent marker genes. The overexpression caused the opposite phenotypes. UGT76B1 therefore attenuates SA-dependent plant defense in the absence of infection, promotes JA response and suppresses the onset of senescence. Non-targeted metabolomic analyses of ugt76b1 knockout and UGT76B1-OE lines using ultra-high resolution Fourier-transform ion cyclotron mass spectrometry led to an unprecedented ab initio substrate identification. In vitro assays employing the recombinant enzyme confirmed isoleucic acid (2-hydroxy-3-methyl-pentanoic acid) as the UGT76B1 substrate. The findings indicate a novel link of amino acid-related molecules to plant pathogen defense pathways via small-molecule glucosylation. Together these findings emphasize the importance of plant secondary metabolite UGTs in plant defense mechanisms and provide a foundation for a detailed understanding of their role in plant stress response. Further, the results presented highlight the great potential of using high resolution metabolomic analysis for non-targeted screening plant mutants to identify new metabolites and reveal novel gene functions without any other prior knowledge