Abstract
In contrast to wild species, drought-tolerance in crops requires a fully functional metabolism during drought (particularly photosynthetic processes). However, the link between drought-tolerance, photosynthetic regulation during drought, and the associated transcript and metabolic foundation, remains largely unknown. For this study, we used two rice cultivars with contrasting drought-tolerance (the drought-intolerant cultivar IRAT109 and the drought-tolerant cultivar IAC1246) to explore transcript and metabolic responses to long-term drought. The drought-tolerant cultivar represented higher osmotic adjustment and antioxidant capacity, as well as higher relative photosynthesis rate under a progressive drought stress occurred in a modified field with shallow soil-layers. A total of 4059 and 2677 differentially expressed genes (DEGs) were identified in IRAT109 and IAC1246 between the drought and well-watered conditions, respectively. A total of 69 and 47 differential metabolites (DMs) were identified between the two treatments in IRAT109 and IAC1246, respectively. Compared to IRAT109, the DEGs of IAC1246 displayed enhanced regulatory amplitude during drought. We found significant correlations between DEGs and the osmolality and total antioxidant capacity (AOC) of both cultivars. During the early stages of drought, we detected up-regulation of DEGs in IAC1246 related to photosynthesis, in accordance with its higher relative photosynthesis rate. The contents of six differential metabolites were correlated with the osmotic potential and AOC. Moreover, they were differently regulated between the two cultivars. Particularly, up-regulations of 4-hydroxycinnamic acid and ferulic acid were consistent with the performance of photosynthesis-related DEGs at the early stages of drought in IAC1246. Therefore, 4-hydroxycinnamic acid and ferulic acid were considered as key metabolites for rice drought-tolerance. DEGs involved in pathways of these metabolites are expected to be good candidate genes to improve drought-tolerance. In conclusion, well-maintained photosynthesis under drought should contribute to improved drought-tolerance in rice. Metabolites play vital roles in protecting photosynthesis under dehydration via osmotic adjustments and/or antioxidant mechanisms. A metabolite-based method was thus an effective way to explore drought candidate genes. Metabolic accompanied by transcript responses to drought stress should be further studied to find more useful metabolites, pathways, and genes.
Highlights
With increased global water shortage, enhancing droughtresistance in cereal crops breeding is one of the most important goals (Sivamani et al, 2000; Luo, 2010; Hu and Xiong, 2014)
We address the following questions: (1) How will osmotic adjustment, antioxidant mechanisms, and photosynthesis respond to the progressive drought in rice cultivars of contrasting droughttolerances? (2) How will transcripts and metabolic pathways respond to the progressive drought in rice cultivars of contrasting drought-tolerances? (3) Are there any featured metabolites in rice tolerance to drought? (4) How are these candidate metabolites and their related genes enhancing drought-tolerance?
Relative water content (RWC) was maintained at normal levels throughout the drought treatment, the osmolality was significantly increased after time point C and subsequently decreased after re-watering (Figure 2B)
Summary
With increased global water shortage, enhancing droughtresistance in cereal crops breeding is one of the most important goals (Sivamani et al, 2000; Luo, 2010; Hu and Xiong, 2014). A plant with improved drought-avoidance might utilize more water via a well-developed root system, while a plant with improved drought-tolerance maintains its basic life activities even when dehydrated (Blum, 2005; Luo, 2010). Drought-tolerance plays an important role during the later stage of drought and has great potential to enhance drought-resistance. Due to their complexity, the molecular mechanisms of drought-tolerance remain largely unknown (Hu and Xiong, 2014)
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