Molecular and physiological screening of rice (Oryza sativa. L.) varieties for submergence tolerance

Rice (Oryza sativa L.) can successfully germinate and grow even when flooded. Rice varieties possessing the Sub1A gene display a distinct flooding-tolerant phenotype, associated with a lower carbohydrate consumption and restriction of the fast-elongation phenotype typical of flooding-intolerant rice varieties. CIPK15 was recently indicated as a key regulator of α-amylases under O2 deprivation, linked to both rice germination and flooding tolerance in adult plants. It is still unknown whether the Sub1A and CIPK15 mediated pathways act as complementary processes for rice survival under O2 deprivation. In adult plants Sub1A and CIPK15 may perhaps play an antagonistic role in terms of carbohydrate consumption, with Sub1A acting as a starch degradation repressor and CIPK15 as an activator. In this study, we analysed the sugar metabolism in the stem of rice plants under water submergence by selecting cultivars with different traits associated with flooding survival. The relation between the Sub1A and the CIPK15 pathways was investigated. The results showed that under O2 deprivation, the CIPK15 pathway is repressed in the tolerant, Sub1A-containing, FR13A variety. CIPK15 is likely to play a role in the up-regulation of Ramy3D in flooding intolerant rice varieties that display fast elongation under flooding and that do not possess Sub1A. In rice plant, lysigenous aerenchyma formation is a constitutive trait. Under water submersion, aerenchymatous areas are further enhanced likely to support the internal movement of gas. This phenomenon is promoted by ethylene that accumulates within the plant tissues under water submersion, but variation in the genotype response suggests further hypothesis. In Arabidopsis hypocotyls, a dual mechanism involving both ethylene and hydrogen peroxide signaling regulates the lysigenous aerenchyma formation under hypoxic condition. This hypothesis was investigated in the leaf sheets of the rice varieties FR13A and Arborio Precoce, showing different traits in flooding tolerance in terms of internode elongation and survival. Both the varieties displayed constitutive lysigenous arenchyma formation that was further enhanced under flooding. Arborio Precoce, characterised by a fast elongation when submerged, showed an active ethylene biosynthetic machinery associated with increased aerechymatous areas. FR13A, limiting the growth during oxygen deprivation, didn’t show any raise in the ethylene production. In this variety, hydrogen peroxide is likely to play a role in controlling lysigenous arenchyma formation under submergence.