Previous studies suggest that abscisic acid (ABA) stimulates the activities of antioxidant enzymes under normal and chilling temperature and enhanced chilling resistance in Stylosanthes guianensis. The objective of this study was to test whether nitric oxide (NO) is involved in the ABA-induced activities of the antioxidant enzymes in Stylosanthes guianensis due to its nature as a second messenger in stress responses. Plants were treated with NO donors, ABA, ABA in combination with NO scavengers or the nitric oxide synthase (NOS) inhibitor and their effects on the activity of antioxidant enzymes and NO production were compared. The results showed that ABA increased the activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). The effect of ABA on antioxidant enzyme activities was suppressed by the NOS inhibitor, N^ω-nitro-L-arginine (L-NNA), and the NO scavenger, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide (PTIO). NO content increased after 5 h of ABA treatment. The NO-scavenger, PTIO, and the NOS-inhibitor, L-NNA, inhibited the accumulation of NO in ABA-treated Stylosanthes guianensis. NO donor treatment enhanced the activities of SOD, CAT, and APX. The results suggested that NO was involved in the ABA-induced activities of SOD, CAT, and APX in Stylosanthes guianensis. ABA triggered NO production that may lead to the stimulation of antioxidant enzyme activities.

Summary: Nuclear factor Y (NF‐Y) is a ubiquitous transcription factor formed by three distinct subunits, namely NF‐YA, NF‐YB and NF‐YC. A stress‐responsive cDNA of NF‐YC (Cdt‐NF‐YC1) was isolated from triploid bermudagrass (Cynodon dactylon × Cynodon transvaalensis), and its role in abiotic stress tolerance was investigated in this study. Cdt‐NF‐YC1 transcript was detected in all vegetative tissues with higher levels being observed in roots. Transcription of Cdt‐NF‐YC1 in leaves was induced by dehydration, salinity, and treatments with abscisic acid (ABA), hydrogen peroxide (H₂O₂) or nitric oxide (NO), but not altered by cold. The dehydration‐ or salt‐induced transcription of Cdt‐NF‐YC1 was blocked by inhibitor of ABA synthesis and scavenger of H₂O₂ or NO, indicating that ABA, H₂O₂ and NO were involved in the dehydration‐ and salt‐induced transcription of Cdt‐NF‐YC1. Overexpression of Cdt‐NF‐YC1 resulted in elevated tolerance to drought and salt stress and increased sensitivity to ABA in transgenic rice. Transcript levels of stress/ABA responsive genes (OsLEA3, OsRAB16A, OsLIP9 and OsP5CS1), ABA synthesis and signalling genes (OsNCED3 and OsABI2), and ABA‐independent genes (OsDREB1A, OsDREB1B and OsDREB2A) were substantially higher in transgenic rice than in wild‐type plants. The results suggested that that Cdt‐NF‐YC1 is a good candidate gene to increase drought and salinity tolerance in transgenic rice through modulating gene regulation in both ABA‐dependent and ABA‐independent pathways.