Abscisic acid and hydrogen peroxide regulate proline homeostasis in melon seedlings under cold stress by forming a bidirectional closed loop

Abstract

Low temperatures severely limit melon cultivation and production in winter and spring. Cold stress can induce proline accumulation in melon seedlings. However, the accumulation mechanism is unclear and the way by which abscisic acid (ABA) and hydrogen peroxide (H 2 O 2 ) act as signaling molecules in the process remains unknown. We first identified key enzyme genes in the proline metabolism pathway, and identified six genes, namely, two CmP5CSs and one each of CmP5CR , CmOAT , ProDH , and CmP5CDH . Low temperature induced proline accumulation by up-regulating three synthase genes, CmP5CS1 , CmP5CR , and CmOAT , while suppressing the expressions of two catabolic genes, namely, CmProDH and CmP5CDH . Cold stress triggered ABA and H 2 O 2 synthase genes to activate their accumulation. Both H 2 O 2 and ABA pretreatments enhanced melon seedling’s cold tolerance, but the two behaved divergently in proline metabolism. Exogenous H 2 O 2 enhanced or maintained the aforementioned induction efficiency of low temperature. In contrast, ABA only induced CmP5CR and CmOAT expression, with no effect on CmP5CS1 , and synchronously induced CmProDH and CmP5CDH expressions, leading to a slight reduction in proline content. Alternatively, ABA controlled proline homeostasis under cold stress, at least in part, by inhibiting H 2 O 2 generation. Overall, the results suggest that ABA and H 2 O 2 formed a fine bidirectional closed loop to regulate proline homeostasis in melon seedlings. • Proline metabolism genes in melon were identified for the first time. • Cold induced proline accumulation by promoting CmP5CS1 , CmP5CR , CmOAT expression, and inhibiting CmProDH , CmP5CDH expression. • Exogenous H 2 O 2 induced proline and ABA accumulation for cold tolerance improvement. • ABA inhibited H 2 O 2 generation to keep proline homeostasis under cold stress.