Drought stress severely impairs plant growth and production. Lipoxygenase (LOX), a master regulator for lipid peroxidation, is critical for direct or indirect response to abiotic stresses. Here, we found that drought stress induced the transcription of CmLOX10 in leaves of oriental melon seedlings. Reverse genetic approaches and physiological analyses revealed that silencing CmLOX10 increased drought susceptibility and stomatal aperture in oriental melon seedlings, and that ectopic overexpression of CmLOX10 in Arabidopsis enhanced drought tolerance and decreased the stomatal aperture. Moreover, the transcription of jasmonic acid (JA)-related genes and JA accumulation were significantly induced in CmLOX10-overexpressed Arabidopsis, which were reversely suppressed in CmLOX10-silenced seedlings during the stage of drought stress. Foliar application of JA further verified that JA enhanced drought tolerance and induced stomatal closure in leaves of melon seedlings. In addition, the feedback regulation of CmLOX10 was induced by JA signaling, and the expression level of CmMYC2 was increased by JA and drought treatment. Yeast one-hybrid analysis showed that CmMYC2 directly bound to the promoter of CmLOX10. In summary, we identified the important roles of CmLOX10 in the regulation of drought tolerance in oriental melon seedlings through JA- mediated stomatal closure and JA signaling-mediated feedback through CmMYC2.

No abstract

No abstract

To better understand the function role of the melon CmLOX18 gene in the biosynthesis of C6 volatiles during fruit ripening, we biochemically characterized CmLOX18 and identified its subcellular localization in transgenic tomato plants. Heterologous expression in yeast cells showed that the molecular weight of the CmLOX18 protein was identical to that predicted, and that this enzyme possesseed lipoxygenase activity. Linoleic acid was demonstrated to be the preferred substrate for the purified recombinant CmLOX18 protein, which exhibited optimal catalytic activity at pH 4.5 and 30 °C. Chromatogram analysis of the reaction product indicated that the CmLOX18 protein exhibited positional specificity, as evidenced by its release of only a C-13 oxidized product. Subcellular localization analysis by transient expression in Arabidopsis protoplasts showed that CmLOX18 was localized to non-chloroplast organelles. When the CmLOX18 gene was transgenically expressed in tomato via Agrobacterium tumefaciens-mediated transformation, it was shown to enhance expression levels of the tomato hydroperoxide lyase gene LeHPL, whereas the expression levels of six TomLox genes were little changed. Furthermore, transgenic tomato fruits exhibited increases in the content of the C6 volatiles, namely hexanal, (Z)-3-hexanal, and (Z)-3-hexen-1-ol, indicating that CmLOX18 probably plays an important role in the synthesis of C6 compounds in fruits.

Light signals and plant hormones are involved in regulating the growth, development and stress resistance of plants; however, it remains unclear whether light affects hormones and thus pathogen resistance in oriental melon. Here, we found that red light promoted salicylic acid (SA) accumulation and powdery mildew resistance by activating the transcription of CmICS, the key gene for SA biosynthesis, and silencing CmICS seriously weakened the induction effect of red light on powdery mildew resistance in oriental melon leaves. Further studies showed that red light induced the expression of CmWRKY42 under powdery mildew stress, and CmWRKY42 directly bound to the CmICS promoter to activate its expression and promote the accumulation of SA under red light. Furthermore, we found that PHYTOCHROME INTERACTING FACTOR 8 (PIF8), as a negative regulator of SA biosynthesis, inhibits CmWRKY42 transcriptional activation by binding to the CmWRKY42 promoter, and thus inhibits transcriptional activation of CmICS by CmWRKY42. Also, CmPIF8 binds to the CmICS promoter and directly inhibits its transcription. In conclusion, our study revealed a new molecular mechanism of the relationship between red light-SApowdery mildew resistance and provided a theoretical basis for resistance breeding of oriental melon.