Three-week old plants of rice (Oryza sativa L. cv CT9993 and cv IR62266) developed gradual water stress over 23 days of transpiration without watering, during which period the mid-day leaf water potential declined to approximately -2.4 MPa, compared with approximately -1.0 MPa in well-watered controls. More than 1000 protein spots that were detected in leaf extracts by proteomic analysis showed reproducible abundance within replications. Of these proteins, 42 spots showed a significant change in abundance under stress, with 27 of them exhibiting a different response pattern in the two cultivars. However, only one protein (chloroplast Cu-Zn superoxide dismutase) changed significantly in opposite directions in the two cultivars in response to drought. The most common difference was for proteins to be up-regulated by drought in CT9993 and unaffected in IR62266; or down-regulated by drought in IR62266 and unaffected in CT9993. By 10 days after rewatering, all proteins had returned completely or largely to the abundance of the well-watered control. Mass spectrometry helped to identify 16 of the drought-responsive proteins, including an actin depolymerizing factor, which was one of three proteins detectable under stress in both cultivars but undetectable in well-watered plants or in plants 10 days after rewatering. The most abundant protein up-regulated by drought in CT9993 and IR62266 was identified only after cloning of the corresponding cDNA. It was found to be an S-like RNase homologue but it lacked the two active site histidines required for RNase activity. Four novel drought-responsive mechanisms were revealed by this work: up-regulation of S-like RNase homologue, actin depolymerizing factor and rubisco activase, and down-regulation of isoflavone reductase-like protein.

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their expression, and may demonstrate variation that is subsequently modified by adaptive traits. Adaptive traits In the rainfed lowlands, rice (Oryza sativa L.) develops roots under will be defined as those, such as root penetration index anaerobic soil conditions with ponded water, prior to exposure to aerobic soil conditions and water stress. Constitutive root system or osmotic adjustment (Zhang et al., 2001), which are development in anaerobic soil conditions has been reported to have expressed in response to water deficit or soil physical/ a positive effect on subsequent expression of adaptive root traits chemical barriers. Less research attention has been and water extraction during water stress. We examined effects of given to constitutive traits than to adaptive traits. phenotyping environment on identification of quantitative trait lociA deep and thick root system has been thought advan-(QTLs) for constitutive root morphology traits using 220 doubledtageous for improved drought tolerance in the rainfed haploid lines (DHLs) from the cross of 'CT9993-5-10-1-M' (CT9993; lowland ecosystem, based on extrapolation from experijaponica, upland adapted) ϫ 'IR62266-42-6-2' (IR62266; indica, lowence with upland rice (O'Toole, 1982; and Fukai and land adapted) in four greenhouse experiments. Broad sense heritabil-Cooper, 1995). Under anaerobic well-watered condiity (h 2 ) was 75, 60, and 64% on average for shoot biomass, deep root tions, root system development had a positive effect on morphology, and root thickness traits, respectively. Quantitative trait loci analysis identified 18 genomic regions associated with deep root subsequent plant growth during progressive water stress morphology traits, but only three were identified consistently across (Azhiri-Sigari et al., 2000; Kamoshita et al., 2000; and experiments. Three out of a total of eight QTLs for root thickness Hoque and Kobata, 1998). Azhiri-Sigari et al. (2000) traits were found in more than one experiment. The maximum genetic and Kamoshita et al. (2000) demonstrated genotypic effects caused by a single QTL were increments of 0.05 g of deep variation in constitutive root traits, and subsequent reroot mass below a 30-cm soil depth, 0.9% of deep root ratio, 1.6 cm sponses of adaptive root traits, especially in deeper soil of rooting depth, and 0.09 cm of root thickness, with phenotypiclayers. Greater root elongation to depth resulted in imvariation explained by a single QTL ranging from 6.8 to 51.8%. The proved water extraction. Improved seedling vigor was results demonstrate the importance of phenotyping environment and also valuable to growth afterward (Mitchell et al., 1998).suggest prospects for selection of QTLs for deep root morphology,

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