Elevated salicylic acid levels conferred by increased expression of <scp>ISOCHORISMATE SYNTHASE</scp> 1 contribute to hyperaccumulation of <scp>SUMO</scp>1 conjugates in the Arabidopsis mutant <i>early in short days 4</i>

Abstract

<jats:title>Summary</jats:title><jats:p>Post‐translational modification of proteins by attachment of small ubiquitin‐like modifier (<jats:styled-content style="fixed-case">SUMO</jats:styled-content>) is essential for plant growth and development. Mutations in the <jats:styled-content style="fixed-case">SUMO</jats:styled-content> protease early in short days 4 (<jats:styled-content style="fixed-case">ESD</jats:styled-content>4) cause hyperaccumulation of conjugates formed between <jats:styled-content style="fixed-case">SUMO</jats:styled-content> and its substrates, and phenotypically are associated with extreme early flowering and impaired growth. We performed a suppressor mutagenesis screen of <jats:italic>esd4</jats:italic> and identified a series of mutants called <jats:italic>suppressor of esd4</jats:italic> (<jats:italic>sed</jats:italic>), which delay flowering, enhance growth and reduce hyperaccumulation of <jats:styled-content style="fixed-case">SUMO</jats:styled-content> conjugates. Genetic mapping and genome sequencing indicated that one of these mutations (<jats:italic>sed111</jats:italic>) is in the gene salicylic acid induction‐deficient 2 (<jats:italic>SID2</jats:italic>), which encodes <jats:styled-content style="fixed-case">ISOCHORISMATE SYNTHASE</jats:styled-content> I, an enzyme required for biosynthesis of salicylic acid (<jats:styled-content style="fixed-case">SA</jats:styled-content>). Analyses showed that compared with wild‐type plants, <jats:italic>esd4</jats:italic> contains higher levels of <jats:italic>SID2 </jats:italic><jats:styled-content style="fixed-case">mRNA</jats:styled-content> and about threefold more <jats:styled-content style="fixed-case">SA</jats:styled-content>, whereas <jats:italic>sed111</jats:italic> contains lower <jats:styled-content style="fixed-case">SA</jats:styled-content> levels. Other <jats:italic>sed</jats:italic> mutants also contain lower <jats:styled-content style="fixed-case">SA</jats:styled-content> levels but are not mutant for <jats:italic>SID2</jats:italic>, although most reduce <jats:italic>SID2 </jats:italic><jats:styled-content style="fixed-case">mRNA</jats:styled-content> levels. Therefore, higher <jats:styled-content style="fixed-case">SA</jats:styled-content> levels contribute to the small size, early flowering and elevated <jats:styled-content style="fixed-case">SUMO</jats:styled-content> conjugate levels of <jats:italic>esd4</jats:italic>. Our results support previous data indicating that <jats:styled-content style="fixed-case">SUMO</jats:styled-content> homeostasis influences <jats:styled-content style="fixed-case">SA</jats:styled-content> biosynthesis in wild‐type plants, and also demonstrate that elevated levels of <jats:styled-content style="fixed-case">SA</jats:styled-content> strongly increase the abundance of <jats:styled-content style="fixed-case">SUMO</jats:styled-content> conjugates.</jats:p>