The rice DSM2 gene significantly contributes to control of the xanthophyll cycle and ABA synthesis, both of which play critical roles in the establishment of drought resistance in rice . The functional analysis of the DSM2 suggested that DSM2 is essential for drought resistance in rice.
GO assignment(s): GO:0003824,GO:0008152
- T-DNA insertion mutants in the background of japonica Zhonghua11 (ZH11) rice, selected from the Rice Mutant Database, for drought resistance under field conditions was screened by the researchers to identify critical genes required for drought resistance in rice. One of the drought-hypersensitive mutants showing drought sensitivity at both the seedling and panicle development stages, designated as dsm2-1, was further characterized in this study.
T-DNA Insertion Mutation
- The T-DNA insertion sites of dsm2-1 and dsm2-2 are located in the third intron and the first exon, respectively.Under normal conditions, the homozygous mutants showed no obvious phenotypic change compared with the wild-type genotype segregated from the heterozygous mutant.
- To verify the drought-sensitive phenotype, mutant and wild-type plants at the four-leaf stage grown in sandy soil were subjected to drought stress. Under the moderate stress condition, the dsm2 mutant lines wilted faster than the wild type. After severe drought stress treatment followed by rewatering, almost all the mutant plants died, whereas wild-type plants had a significantly higher survival rate (Fig. 1C). Cosegregation analysis also suggested that the drought sensitivity was due to the T-DNA insertion in the OsBCH1 gene (data not shown). The dsm2-1 mutant was also more sensitive than the wild type to salt stress, but no significant difference was observed under cold or heat shock stress.
- Drought sensitivity of the dsm2-1 mutant was also tested at the reproductive stage by growing the mutant and the wild type in a paddy field facilitated with a removable rain-off shelter and in polyvinyl chloride (PVC) tubes filled with sandy soil. During the course of drought stress development, dsm2-1 showed wilting earlier than the wild type. After moderate drought stress in the PVC tubes, the total grain yield of dsm2-1 was reduced by about 52% compared with the wild type, and the pollen fertility of dsm2-1 (23%) was also significantly lower than that of the wild type (58%; Fig. 1D). The root depth and volume of dsm2-1 were significantly reduced compared with the wild type (Fig. 1D), and contents of chlorophyll and Pro in dsm2-1 were also reduced prominently.
Overexpression of DSM2
- To test whether DSM2 overexpression has a significant effect on improving drought resistance, the full-length cDNA of DSM2 under the control of the cauliflower mosaic virus 35S promoter (Fig. 2A) was transformed into japonica rice ZH11. After severe drought stress (no watering for 1 week), the overexpression lines had a significantly higher survival rate (approximately 74%) than the negative control (completely died; Fig. 2, C and D). After drought treatments at the reproductive stage, the overexpression lines had more green leaves and higher spikelet fertility than negative transgenic lines. The overexpression lines had less oxidative damage on the leaves and higher seed-setting rates than the negative control (Fig. 2, E and F). These results suggest that overexpression of DSM2 has a significant effect on the improvement of drought resistance in rice.
- Overexpression of DSM2 in rice resulted in significantly increased resistance to drought and oxidative stresses and increases of the xanthophylls and nonphotochemical quenching. Some stress-related ABA responsive genes were up-regulated in the overexpression line.
- Transcript analysis suggested that the expression of DSM2 was abolished in the two allelic dsm2 mutants. The DSM2 transcript level was induced (7- to 9-fold) by drought and salt treatments and slightly induced by ABA. However, DSM2 was not induced by the other treatments (and was slightly suppressed by cold stress).
- A strong GFP signal was observed in the stamen, plumule, hull, pistil,mature leaf, and root, and a weak GFP signal was detected in calli, young shoot and root, and endosperm, suggesting an organ/ tissue-dependent differential expression pattern of DSM2 in rice.
In the rice genome, there are two homologs of DSM2/ OsBCH1, designated OsBCH2 (LOC_Os04g48880) and OsBCH3 (LOC_Os10g38940), showing 82% and 75% identity to OsBCH1, respectively. DSM2 belongs to the BCH family.
- Abiotic stresses such as drought, salinity, and adverse temperatures are major limiting factors for plant growth and reproduction. To respond to environmental cues, plants have evolved a variety of biochemical and physiological mechanisms to adapt to adverse conditions during their growth and development. Abscisic acid (ABA) has been recognized as a stress hormone that coordinates the complex networks of stress responses.
- Under drought or salt stress conditions, plant endogenous ABA level can rise to about 40-fold, triggering the closure of stomata and accumulating reactive oxygen species (ROS), dehydrins, and late embryogenesis abundant proteins for osmotic adjustment (Verslues et al., 2006). The endogenous ABA level is determined by ABA biosynthesis, catabolism, and release of ABA from ABA-Glc conjugates. Therefore, identification of all the components affecting active ABA content is essential for a complete understanding of the action of the hormone.
- The CYP97 and BCH gene pairs are primarily responsible for hydroxylation of α- and β-carotenes, respectively, but exhibit some overlapping activities, most notably in hydroxylation of the β-ring of α-carotene. The BCH duplicates encode isozymes that show significant expression divergence in reproductive organs.
- The two BCH isozymes could hydroxylate the β-ring of α-carotene, though again to a lower extent than the full four enzyme complement in the wild type.
Labs working on this gene
- National Key Laboratory of Crop Genetic Improvement, National Center of Plant Gene Research (Wuhan), Huazhong Agricultural University, Wuhan 430070, China
- Du H, Wang N, Cui F, et al. Characterization of the β-carotene hydroxylase gene DSM2 conferring drought and oxidative stress resistance by increasing xanthophylls and abscisic acid synthesis in rice[J]. Plant Physiology, 2010, 154(3): 1304-1318.
- Vallabhaneni R, Gallagher C E, Licciardello N, et al. Metabolite sorting of a germplasm collection reveals the hydroxylase3 locus as a new target for maize provitamin A biofortification[J]. Plant physiology, 2009, 151(3): 1635-1645.
- Kim J, Smith J J, Tian L, et al. The evolution and function of carotenoid hydroxylases in Arabidopsis[J]. Plant and cell physiology, 2009, 50(3): 463-479.