Drought-induced ABA-responsive glutamate dehydrogenase activation is a dominant pathway for proline synthesis in Brassica napus leaves

초록

Abscisic acid (ABA), commonly referred to as the "stress hormone," is a key regulator of plant stress responses and tolerance. Accumulation of proline is one of the distinctive responses to drought stress. This study aimed to characterize the role of ABA in the glutamate (Glu) pathway linked to proline metabolism under drought conditions. In a preliminary experiment, drought stress simultaneously increased the levels of H2O2, ABA, NH4+, Glu, and proline as leaf water potential decreased. In the second experiment, the role of drought-induced ABA in regulating the Glu pathway to support proline metabolism was characterized by comparing responses to ABA biosynthesis inhibitor (fluridone) under drought (Drought + Flu), and to exogenous ABA under well-watered conditions (Exo-ABA). The drought-induced increases in drought-responsive parameters were significantly suppressed in the Drought + Flu treatment, whereas they remained largely unchanged in the Exo-ABA treatment. Drought-induced proteolysis contributed to NH4+ accumulation, which activated glutamate dehydrogenase (GDH) while concurrently suppressed the expression of genes involved in the GS/GOGAT pathway (GS2 and Fd-GOGAT1). Proline accumulation under drought was mainly driven by an increased Glu pool through NAD(H)-GDH activation, mediated by dehydration-responsive element binding protein 2 (DREB2) and NAC protein (NAC55) in a drought-induced ABA-and H2O2-dependent manner, in which proline biosynthesis-related genes (P5CS1 and P5CR) were upregulated and proline degradation-related gene (PDH) was downregulated. However, exogenous ABA-responsive transcription factors (AREB2 and SnRK2) had minimal effect on these responses. These results indicate that drought-induced, ABA-mediated activation of GDH pathway is a dominant route for Glu supply in proline synthesis.

키워드