Abscisic acid (ABA) and proline play important roles in rice acclimation to different stress conditions. To study whether cross-talk exists between ABA and proline, their roles in rice acclimation to hypoxia, rice growth, root oxidative damage and endogenous ABA and proline accumulation were investigated in two different rice genotypes ('Nipponbare' (Nip) and 'Upland 502' (U502)).Compared with U502 seedlings, Nip seedlings were highly tolerant to hypoxic stress, with increased plant biomass and leaf photosynthesis and decreased root oxidative damage. Hypoxia significantly stimulated the accumulation of proline and ABA in the roots of both cultivars, with a higher ABA level observed in Nip than in U502, whereas the proline levels showed no significant difference in the two cultivars. The time course variation showed that the root ABA and proline contents under hypoxia increased 1.5- and 1.2-fold in Nip, and 2.2- and 0.7-fold in U502, respectively, within the 1 d of hypoxic stress, but peak ABA production (1 d) occurred before proline accumulation (5 d) in both cultivars. Treatment with an ABA synthesis inhibitor (norflurazon, Norf) inhibited proline synthesis and simultaneously aggravated hypoxia-induced oxidative damage in the roots of both cultivars, but these effects were reversed by exogenous ABA application. Hypoxia plus Norf treatment also induced an increase in glutamate (the main precursor of proline). This indicates that proline accumulation is regulated by ABA-dependent signals under hypoxic stress. Moreover, genes involved in proline metabolism were differentially expressed between the two genotypes, with expression mediated by ABA under hypoxic stress. In Nip, hypoxia-induced proline accumulation in roots was attributed to the upregulation of OsP5CS2 and downregulation of OsProDH, whereas upregulation of OsP5CS1 combined with downregulation of OsProDH enhanced the proline level in U502.These results suggest that the high tolerance of the Nip cultivar is related to the high ABA level and ABA-mediated antioxidant capacity in roots. ABA acts upstream of proline accumulation by regulating the expression of genes encoding the key enzymes in proline biosynthesis, which also partly improves rice acclimation to hypoxic stress. However, other signaling pathways enhancing tolerance to hypoxia in the Nip cultivar still need to be elucidated.
