Apigenin pretreatment enhances growth and salinity tolerance of rice seedlings

Soil salinity is a limiting factor in rice production. Since flavonoids present in most plant tissues play multiple roles in plant-environment interactions, in this study, we focused on the contribution of flavone aglycone (Apigenin) to the adaptation of salinity-sensitive rice cultivar 'Koshihikari,' to salinity stress, for the first time. Rice seeds were soaked in Apigenin solution (10 ppm) for 24 h, then air-dried and grown hydroponically under 50 mM NaCl for 14 days. Apigenin pretreatment improved the growth of rice seedlings by enhancing shoot elongation and dry mass accumulation under both unstressed and NaCl-stress conditions, compared with that in the non-pretreated seedlings. Apigenin pretreatment significantly reduced Na+ accumulation in the salinity stressed seedlings, and helped to maintain a lower Na+/K+ ratio in all plant organs, compared with that in the non-pretreated seedlings, possibly by regulating the expression of some important Na+ transporter-encoding genes (OsHKT2;1, OsCHGC1, OsSOS1). Higher levels of lipid peroxidation and hydrogen peroxide (H2O2) concentrations were observed in the shoots of the salinity-stressed seedlings; however, lower levels of lipid per oxidation and H2O2 concentration were detected in the Apigenin-treated seedlings. Apigenin pretreatment was associated with the induction of the rice antioxidant defense system represented by the induced activities of the antioxidant enzymes Catalase (CAT) and Ascorbate peroxidase (APX) in the roots, as well as by increased accumulation of the non-enzymatic antioxidants carotenoids and flavonoids in the shoots, relative to that in the untreated seedlings, under salinity stress conditions. Together, these results suggest that Apigenin pretreatment can alleviate the damaging effects of salinity on rice seedlings, presumably by regulating selective ion uptake by the roots and translocation to the shoots, thereby maintaining higher K+/Na+ ratios critical for normal plant growth under salinity stress, and by triggering the induction of the antioxidant defense system.