Transcriptomic and metabolic profiling of watermelon uncovers the role of salicylic acid and flavonoids in the resistance to cucumber green mottle mosaic virus.

Understanding the mechanisms underlying plant resistance to virus infections is crucial for viral disease management in agriculture. However, the defense mechanism of watermelon (Citrullus lanatus) against cucumber green mottle mosaic virus (CGMMV) infection remains largely unknown. In this study, we performed transcriptomic, metabolomic, and phytohormone analyses of a CGMMV susceptible watermelon cultivar Zhengkang No.2 (ZK) and a CGMMV resistant wild watermelon accession PI 220778 (PI) to identify the key regulatory genes, metabolites, and phytohormones of watermelon responsible for CGMMV resistance. We then tested several phytohormones and metabolites for their roles in watermelon CGMMV resistance via foliar application, followed by CGMMV inoculation. Several phenylpropanoid metabolism-associated genes and metabolites, especially those involved in the flavonoid biosynthesis pathway, were found to be significantly enriched in the CGMMV-infected 'PI' plants compared with the CGMMV-infected 'ZK' plants. We also identified a gene encoding UDP-glycosyltransferase (UGT), which is involved in kaempferol-3-O-sophoroside biosynthesis, leads to dwarf stature, and promotes disease resistance. Additionally, salicylic acid (SA) biogenesis increased in the CGMMV-infected 'ZK' plants, resulting in the activation of a downstream signaling cascade. The level of SA in the assayed watermelon plants correlated with that of total flavonoids, and SA pre-treatment up-regulated the expression of flavonoid biosynthesis genes, thus increasing the total flavonoid content. Furthermore, application of exogenous SA or flavonoids extracted from watermelon leaves suppressed CGMMV infection. In summary, our study demonstrates the role of SA-induced flavonoid biosynthesis in plant development and CGMMV resistance, which could be used to breed for CGMMV resistance in watermelon.