Jasmonic acid causes short- and long-term alterations to the transcriptome and the expression of defense genes in sugarbeet roots

Jasmonic acid (JA) induces native defense responses in plants and increases the resistance of postharvest sugarbeet roots to three common storage-rot causing organisms. To gain insight into the defense responses induced by JA in harvested sugarbeet roots, RNA was isolated from roots treated with water or 10 μM JA and incubated for 2 or 60 d post-treatment. RNA was sequenced, and sequence data was analyzed for short-term (2 d) and long-term (60 d) effects of JA on the sugarbeet root transcriptome. A total of 283 and 326 differentially expressed unigenes were identified in JA-treated roots at 2 and 60 d after treatment, respectively. Of these, > 96% were uniquely altered in expression at 2 or 60 d, indicating that long-term JA-induced changes to the sugarbeet root transcriptome were substantially different from short-term JA-induced changes. Categorization of differentially expressed unigenes by gene ontology (GO) or Kyoto Encyclopedia of Genes and Genomes (KEGG) identifiers indicated that JA altered expression of a large number of unigenes involved in metabolism, including many unigenes encoding enzymes and unigenes that participate in secondary metabolism. In addition, 88 unigenes, or 15% of all unigenes whose expression were altered by JA, were putatively involved in plant defense. Included were unigenes for pathogenesis-related proteins, regulatory proteins, plant disease resistance proteins, secondary metabolite biosynthetic enzymes, oxidases, and plant cell wall-modifying proteins. Detailed, time-course expression analysis of 19 JA-altered defense unigenes revealed similarities in expression for three plant disease resistant (R) genes and three laccase genes, indicating possible co-regulation of these genes. No relationships, however, were observed between enzyme activities and transcript levels for any of four major families of upregulated defense genes for which enzyme activities were determined, consistent with JA functioning as a primer, rather than an inducer, of plant defenses. Overall, this research supports JA's role as a signaling molecule for plant defense and provides evidence that a variety of defense mechanisms, including the production of antifungal and antimicrobial compounds, stimulation of antioxidant defenses, and stiffening and strengthening of cell walls, may contribute to JA-induced storage rot resistance in sugarbeet roots.