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Abstract Background Agriculture is highly dependent on climate. Increases in temperature caused by global warming pose challenges for crop production. Heat stress induces oxidative damage to cell membranes and then causes cell death. Plants have developed various responses to elevated temperatures, including hormone signaling pathways and heat shock factors that elevate their thermotolerance. In response to heat stress, the gaseous hormone ethylene is produced through regulation of the expression of signaling-related genes to modulate resource allocation dynamics. For comprehensive understanding of the role of ethylene, this study used an ethylene precursor to analyze the ethylene signaling pathway involved in adjustment of the homeostasis of the antioxidant system and to evaluate heat shock factor expression in rice seedlings under heat stress. Results Levels of cell membrane oxidation and ion leakage were reduced in rice seedlings under heat treatment combined with ethylene precursor treatment, conferring enhanced thermotolerance. Reduction of the fresh weight and chlorophyll a/b ratio in rice seedlings was lower in rice seedlings under heat stress with ethylene precursor treatment than in those under heat stress only. Moreover, reduction of antioxidant response caused by heat stress was ameliorated by treatment with ethylene precursors such as catalase and total peroxidase. Quantitative reverse transcriptase-polymerase chain reaction showed higher expression levels of heat shock factors such as HSFA1a and HSFA2a, c, d, e, and f and ethylene-signaling-related genes such as ethylene insensitive 2, ethylene insensitive-like 1, and ethylene insensitive-like 2 in rice seedlings under heat stress with ethylene precursor treatment than in rice seedlings under heat stress only. Conclusion Ethylene-mediated signaling was involved in the reduction of oxidative damage, maintenance of chlorophyll content, and enhancement of thermotolerance in rice seedlings under heat stress. Furthermore, this study revealed heat shock factors and ethylene-signaling-related genes involved in complex network regulation that confers thermotolerance to rice seedlings.

Abstract Background Understanding the responses of rice to environmental stresses such as unscheduled submergence is of pressing important owing to increasing severity of weather thought to arise from global climate change. When rice is completely submerged, different types adopt either a quiescence survival strategy (i.e., minimal shoot elongation) or an escape strategy (i.e., enhanced shoot elongation). Each strategy can prolong survival depending on the circumstances. While submergence responses have been studied in rice typical of lowland and flood-prone areas, few studies have explored the physiological and molecular properties of upland rice under submergence. Here, we use seedlings of the upland rice ‘Tung Lu 3’ (‘TL3’) to analyze physiological and molecular responses to submergence. We compare them with those of ‘FR13A’, a lowland rice that tolerates submergence by adopting the quiescence strategy. Results Plant height and distance between leaf sheaths, increased rapidly in ‘TL3’ under submergence. Although this indicated a strong escape strategy the seedlings remained totally underwater for the duration of the experiments. In contrast, ‘FR13A’ elongated much less. Consequently, after 4 days complete submergence followed by drainage, ‘TL3’ lodged much more severely than ‘FR13A’. After 10 d complete submergence, 55% of ‘TL3’ seedlings survived compared to 100% in ‘FR13A’. Chlorophyll a, b and total chlorophyll concentrations of the 2nd oldest leaves of ‘TL3’ were also significantly above those of ‘FR13A’ (but were lower than ‘FR13A’ in the 3rd oldest leaves) and less hydrogen peroxide accumulated in ‘TL3’. Peroxidase activity in submerged ‘TL3’ was also greater than in ‘FR13A’ 1 day after submergence. Quantitative RT–PCR showed increased expression of sucrose synthase 1 and alcohol dehydrogenases 1 after 2 days complete submergence with significantly higher levels in ‘TL3’ compared to ‘FR13A’. Expression was also higher in ‘TL3’ under non-submerged conditions. Conclusions The upland rice line ‘TL3’ gave a stronger elongation response than ‘FR13A’ to complete submergence. This escape strategy is widely considered to prejudice survival when the plant remains totally submerged. However, contrary to expectations, ‘TL3’ survival rates were substantial although below those for ‘FR13A’ while physiological, biochemical and molecular parameters linked to adaptation differed in detail but appeared to be broadly comparable. These findings highlight that submergence tolerance is determine not only by the adoption of quiescence or escape strategies but maybe by metabolic and physiological properties unrelated to the underwater elongation rate.