Brassinosteroids-Induced Systemic Stress Tolerance was Associated with Increased Transcripts of Several Defence-Related Genes in the Phloem in Cucumis sativus

Brassinosteroids (BRs), a group of naturally occurring plant steroidal compounds, are essential for plant growth, development and stress tolerance. Recent studies showed that BRs could induce systemic tolerance to biotic and abiotic stresses; however, the molecular mechanisms by which BRs signals lead to responses in the whole plant are largely unknown. In this study, 24-epibrassinosteroid (EBR)-induced systemic tolerance in Cucumis sativus L. cv. Jinyan No. 4 was analyzed through the assessment of symptoms of photooxidative stress by chlorophyll fluorescence imaging pulse amplitude modulation. Expression of defense/stress related genes were induced in both treated local leaves and untreated systemic leaves by local EBR application. With the suppressive subtractive hybridization (SSH) library using cDNA from the phloem sap of EBR-treated plants as the tester and distilled water (DW)-treated plants as the driver, 14 transcripts out of 260 clones were identified. Quantitative Real Time-Polymerase Chain Reaction (RT-qPCR) validated the specific up-regulation of these transcripts. Of the differentially expressed transcripts with known functions, transcripts for the selected four cDNAs, which encode an auxin-responsive protein (IAA14), a putative ankyrin-repeat protein, an F-box protein (PP2), and a major latex, pathogenesis-related (MLP)-like protein, were induced in local leaves, systemic leaves and roots after foliar application of EBR onto mature leaves. Our results demonstrated that EBR-induced systemic tolerance is accompanied with increased transcript of genes in the defense response in other organs. The potential role of phloem mRNAs as signaling components in mediating BR-regulated systemic resistance is discussed.