Photosynthetic light curve, chlorophyll (Chl) content, Chl fluorescence parameters, malondialdehyde (MDA) content, phosphoenolpyruvate carboxylase (PEPC) activity and reactive oxygen metabolism were studied under drought stress in two autotetraploid rice lines and corresponding diploid rice lines. Net photosynthetic rate decreased dramatically, especially under severe drought stress and under high photosynthetic active radiation in diploid rice, while it declined less under the same conditions in autotetraploid lines. Compared with the corresponding diploid lines, the Chl content, maximum photochemical efficiency of photosystem (PS) II, and actual photochemical efficiency of PSII were reduced less in autotetraploid lines. PEPC activities were higher in autotetraploid rice lines. PEPC could alleviate inhibition of photosynthesis caused by drought stress. The chromosome-doubling enhanced rice photoinhibition tolerance under drought stress. The lower MDA content and superoxide anion production rate was found in the autotetraploid rice indicating low peroxidation level of cell membranes. At the same time, the superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities were higher in autotetraploid rice lines. SOD, POD, and CAT could effectively diminish the reactive oxygen species and reduced the membrane lipid peroxidation.

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Although recent studies have established a significant regulatory role for abscisic acid (ABA) and ethylene response factor (ERF) proteins in plant pathogen resistance, it is not clear whether and how ABA performs this role. Previously, it was reported that an ERF protein, TSRF1, activates the expression of GCC box-containing genes and significantly enhances the resistance to Ralstonia solanacearum in both tobacco and tomato plants. Here, it is reported that TSRF1-regulated pathogen resistance is modified by ABA application. TSRF1 activates the expression of ABA biosynthesis-related genes, resulting in the increase of ABA biosynthesis, which further stimulates ethylene production. More interestingly, ABA application decreases, while the inhibitor of ABA biosynthesis fluridone increases, the TSRF1-enhanced resistance to R. solanacearum. This observation is further supported by the finding that ABA and fluridone reversibly modify the ability of TSRF1 to bind the ethylene-responsive GCC box, consequently altering the expression of element-controlled genes. These results therefore establish that TSRF1-regulated resistance to R. solanacearum can be modified in tobacco by ABA.

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