Identification of Down-regulated Genes Modulated by an Alternative Oxidase Pathway Under Cold Stress Conditions in Watermelon Plants

This study investigated the cold responses of watermelon plants modulated by the alternative oxidase (AOX) pathway. Salicylhydroxamic acid was used in watermelon (Citrullus lanatus) plants in order to chemically inhibit the AOX pathway. After exposure to cold temperature (10°C) for 24 h, the SHAM-treated plants showed more serious cold-induced damage compared to non-treated plants. To investigate how the AOX pathway modulated the watermelon plants’ responses to cold, a polymerase chain reaction (PCR)-based suppression subtractive hybridization technique was used to isolate genes differently expressed between cold-stressed leaves of SHAM-treated and non-treated watermelon germplasm “IVSM No. 9”. After sequencing 400 randomly chosen cDNA clones, 314 uniquely expressed sequence tags (ESTs) were obtained. A total of 114 ESTs encoded unknown proteins or had no similarity in the database. Identified genes with significant protein homology mainly participated in stress and defense responses, metabolism and energetic processes, regulation of gene expression, signal transduction, protein fate and synthesis, cellular transport and others. These results indicated that watermelon plants could not respond appropriately to cold stress and suffer greater damage with an inhibited AOX pathway, thus, the AOX pathway had an important role in defining the plants cold responses.