Distinct mechanisms for aerenchyma formation in leaf sheaths of rice genotypes displaying a quiescence or escape strategy for flooding tolerance.

BACKGROUND AND AIMS Rice is one of the few crops able to withstand periods of partial or even complete submergence. One of the adaptive traits of rice is the constitutive presence and further development of aerenchyma which enables oxygen to be transported to submerged organs. The development of lysigenous aerenchyma is promoted by ethylene accumulating within the submerged plant tissues, although other signalling mechanisms may also co-exist. In this study, aerenchyma development was analysed in two rice (Oryza sativa) varieties, 'FR13A' and 'Arborio Precoce', which show opposite traits in flooding response in terms of internode elongation and survival. METHODS The growth and survival of rice varieties under submergence was investigated in the leaf sheath of 'FR13A' and 'Arborio Precoce'. The possible involvement of ethylene and reactive oxygen species (ROS) was evaluated in relation to aerenchyma formation. Cell viability and DNA fragmentation were determined by FDA/FM4-64 staining and TUNEL assay, respectively. Ethylene production was monitored by gas chromatography and by analysing ACO gene expression. ROS production was measured by using Amplex Red assay kit and the fluorescent dye DCFH(2)-DA. The expression of APX1 was also evaluated. AVG and DPI solutions were used to test the effect of inhibiting ethylene biosynthesis and ROS production, respectively. KEY RESULTS Both the varieties displayed constitutive lysigenous aerenchyma formation, which was further enhanced when submerged. 'Arborio Precoce', which is characterized by fast elongation when submerged, showed active ethylene biosynthetic machinery associated with increased aerenchymatous areas. 'FR13A', which harbours the Sub1A gene that limits growth during oxygen deprivation, did not show any increase in ethylene production after submersion but still displayed increased aerenchyma. Hydrogen peroxide levels increased in 'FR13A' but not in 'Arborio Precoce'. CONCLUSIONS While ethylene controls aerenchyma formation in the fast-elongating 'Arborio Precoce' variety, in 'FR13A' ROS accumulation plays an important role.