Silicon Alleviates Drought Stress of Rice Plants by Improving Plant Water Status,Photosynthesis and Mineral Nutrient Absorption

Drought is a major constraint for rice production in the rainfed lowlands in China. Silicon (Si) has been verified to play an important role in enhancing plant resistance to environmental stress. Two near-isogenic lines of rice (Oryza sativa L.), w-14 (drought susceptible) and w-20 (drought resistant), were selected to study the effects of exogenous Si application on the physiological traits and nutritional status of rice under drought stress. In wet conditions, Si supply had no effects on growth and physiological parameters of rice plants. Drought stress was found to reduce dry weight, root traits, water potential, photosynthetic parameters, basal quantum yield (F _v/F ₀), and maximum quantum efficiency of PSII photochemistry (F _v/F _m) in rice plants, while Si application significantly increased photosynthetic rate (Pr), transpiration rate (Tr), F _v/F ₀, and F _v/F _m of rice plants under drought stress. In addition, water stress increased K, Na, Ca, Mg, Fe content of rice plants, but Si treatment significantly reduced these nutrient level. These results suggested that silicon application was useful to increase drought resistance of rice through the enhancement of photochemical efficiency and adjustment of the mineral nutrient absorption in rice plants.     

籽粒镉低积累水稻地上部镉高积累遗传特性分析

农田土壤镉（Cd）污染日益严重,导致稻米Cd含量超标事件频繁出现,使粮食安全问题备受关注。因此,合理利用Cd污染农田、降低水稻籽粒Cd含量成为亟待解决的问题。籽粒Cd低积累水稻雅恢2816的地上部具有较强的Cd积累能力,研究旨在弄清其地上部Cd积累能力的遗传稳定性,进一步揭示控制该性状的遗传基础,为利用分子标记辅助选育地上部Cd富集能力强、籽粒Cd安全的水稻提供途径。以水稻雅恢2816和3个不同品种水稻分别组配获得的F₁为研究对象,分析地上部Cd积累相关性状的杂种优势。进一步以优势组合C268A/雅恢2816构建F₂作图群体,对地上部Cd积累相关性状进行QTL定位分析。结果表明：（1） F₁地上部Cd积累相关性状杂种优势明显,遗传稳定性强。地上部Cd积累相关性状属数量性状,F₂中/超亲分离现象明显。（2）在第4、6号染色体上共挖掘到1个控制水稻地上部生物量和3个控制地上部Cd积累量的QTL位点,分别为qSB-6、qSCdA-4、qSCdA-6-1和qSCdA-6-2,表型贡献率为10.6%—14.4%,且增效等位基因均来自雅恢2816。（3）地上部Cd积累量与地上部生物量、Cd含量,根、糙米的生物量、Cd积累量,根-地上部转移系数均呈极显著正相关,与地上部-籽粒转移系数呈极显著负相关,存在4个QTL集簇区Cl-4-1、Cl-6-1、Cl-6-2和Cl-6-3。（4）区间marker 04171-marker 04197控制着地上部生物量和Cd积累量,与控制糙米Cd含量的QTL不重叠。研究表明：籽粒Cd低积累水稻雅恢2816携带控制地上部Cd高积累的等位基因,可在世代间稳定遗传,QTL位点qSCdA-4、qSCdA-6-1、qSCdA-6-2是控制该性状的重要遗传基础,可为分子标记辅助选育地上部Cd高积累、籽粒Cd低积累水稻提供理论依据。     

Root aquaporins contribute to whole plant water fluxes under drought stress in rice (Oryza sativa L.)

Aquaporin activity and root anatomy may affect root hydraulic properties under drought stress. To better understand the function of aquaporins in rice root water fluxes under drought, we studied the root hydraulic conductivity (Lpr) and root sap exudation rate (Sr) in the presence or absence of an aquaporin inhibitor (azide) under well‐watered conditions and following drought stress in six diverse rice varieties. Varieties varied in Lpr and Sr under both conditions. The contribution of aquaporins to Lpr was generally high (up to 79% under well‐watered conditions and 85% under drought stress) and differentially regulated under drought. Aquaporin contribution to Sr increased in most varieties after drought, suggesting a crucial role for aquaporins in osmotic water fluxes during drought and recovery. Furthermore, root plasma membrane aquaporin (PIP) expression and root anatomical properties were correlated with hydraulic traits. Three chromosome regions highly correlated with hydraulic traits of the OryzaSNP panel were identified, but did not co‐locate with known aquaporins. These results therefore highlight the importance of aquaporins in the rice root radial water pathway, but emphasize the complex range of additional mechanisms related to root water fluxes and drought response.     

Dynamic QTL analysis of the Na+ content,K+ content,and Na+/K+ ratio in rice roots during the field growth under salt stress

Rice (Oryza sativa L.) is seriously impacted by global soil salinization. To determine the quantitative trait loci (QTLs) related to salt tolerance in rice roots, F_2:3 and BC₁F_2:3 populations derived from a cross between the cv. Dongnong 425 of high quality and yield and the salt-tolerant cv. Changbai 10, were studied at different development stages. Two genetic linkage maps of F_2:3 and BC1F_2:3 populations were constructed. A 66 mM NaCl solution was used to irrigate the field and to analyze the dynamic QTL of some rice root traits. Using unconditional and conditional QTL mapping methods, 30 unconditional QTLs and 16 conditional QTLs related to the 6 root traits were detected on the 9 rice chromosomes during different developmental stages. Fourteen pairs of unconditional and conditional QTLs were detected at the identical developmental stage in the identical population. A number of QTLs were detected at different developmental stages, however, many did not appear at the last stage. Remarkably, qRKC1 appeared continuously at multiple stages in both the populations suggesting its key role in regulating the salt tolerance of rice roots.     

Responses of plant biomass,photosynthesis and lipid peroxidation to warming and precipitation change in two dominant species (Stipa grandis and Leymus chinensis) from North China Grasslands

Influential factors of global change affect plant carbon uptake and biomass simultaneously. Although the effects from warming and precipitation change have been extensive studied separately, the responses of plant biomass, photosynthesis, and lipid peroxidation to the interaction of these factors are still not fully understood. In this study, we examined the physiological responses of two dominant plant species from grasslands of northern China with different functional traits to combinations of five simulated warming patterns and five simulated precipitation patterns in environment‐controlled chambers. Our results showed that the biomass, net CO₂ assimilation rate (P_n), maximal efficiency of photosystem II photochemistry (F_v/F_m), and chlorophyll content (Chl) of Stipa grandis and Leymus chinensis were enhanced by moderate warming and plus precipitation, but they declined drastically with high temperature and drought. High temperature and drought also led to significant malondialdehyde (MDA) accumulation, which had a negative correlation with leaf biomass. The lower level of lipid peroxidation in leaves of S. grandis suggests that this species is better protected from oxidative damage under heat stress, drought stress and their interactive conditions than L. chinensis. Using the subordinate function values method, we found S. grandis to be more sensitive to climate change than L. chinensis and the gross biomass and root biomass of S. grandis and the leaf biomass of L. chinensis were most sensitive to climate change. Furthermore, the P_n of both S. grandis and L. chinensis had a significant linear relationship with F_v/F_m and Chl, indicating that carbon assimilation may be caused by nonstomatal limitations.     

Impact of phosphorus application on drought resistant responses of Eucalyptus grandis seedlings

Eucalyptus grandis is the most widely planted tree species worldwide and can face severe drought during the initial months after planting because the root system is developing. A complete randomized design was used to study the effects of two water regimes (well‐watered and water‐stressed) and phosphorus (P) applications (with and without P) on the morphological and physio‐biochemical responses of E. grandis. Drought had negative effects on the growth and metabolism of E. grandis, as indicated by changes in morphological traits, decreased net photosynthetic rates (P_n), pigment concentrations, leaf relative water contents (LRWCs), nitrogenous compounds, over‐production of reactive oxygen species (ROS) and higher lipid peroxidation. However, E. grandis showed effective drought tolerance strategies, such as reduced leaf area and transpiration rate (E), higher accumulation of soluble sugars and proline and a strong antioxidative enzyme system. P fertilization had positive effects on well‐watered seedlings due to improved growth and photosynthesis, which indicated the high P requirements during the initial E. grandis growth stage. In drought‐stressed seedlings, P application had no effects on the morphological traits, but it significantly improved the LRWC, P_n, quantum efficiency of photosystem II (F_v/F_m), chlorophyll pigments, nitrogenous compounds and reduced lipid peroxidation. P fertilization improved E. grandis seedling growth under well‐watered conditions but also ameliorated some leaf physiological traits under drought conditions. The effects of P fertilization are mainly due to the enhancement of plant N nutrition. Therefore, P can be used as a fertilizer to improve growth and production in the face of future climate change.     

Ectopic expression of a rice plasma membrane intrinsic protein (OsPIP1;3) promotes plant growth and water uptake

Plasma membrane intrinsic proteins (PIPs) are known to be major facilitators of the movement of a number of substrates across cell membranes. From a drought‐resistant cultivar of Oryza sativa (rice), we isolated an OsPIP1;3 gene single‐nucleotide polymorphism (SNP) that is mostly expressed in rice roots and is strongly responsive to drought stress. Immunocytochemistry showed that OsPIP1;3 majorly accumulated on the proximal end of the endodermis and the cell surface around the xylem. Expression of GFP‐OsPIP1;3 alone in Xenopus oocytes or rice protoplasts showed OsPIP1;3 mislocalization in the endoplasmic reticulum (ER)‐like neighborhood, whereas co‐expression of OsPIP2;2 recruited OsPIP1;3 to the plasma membrane and led to a significant enhancement of water permeability in oocytes. Moreover, reconstitution of 10×His‐OsPIP1;3 in liposomes demonstrated water channel activity, as revealed by stopped‐flow light scattering. Intriguingly, by patch‐clamp technique, we detected significant NO₃^? conductance of OsPIP1;3 in mammalian cells. To investigate the physiological functions of OsPIP1;3, we ectopically expressed the OsPIP1;3 gene in Nicotiana benthamiana (tobacco). The transgenic tobacco plants exhibited higher photosynthesis rates, root hydraulic conductivity (Lp_r) and water‐use efficiency, resulting in a greater biomass and a higher resistance to water deficit than the wild‐type did. Further experiments suggested that heterologous expression of OsPIP1;3 in cyanobacterium altered bacterial growth under different conditions of CO₂ gas supply. Overall, besides shedding light on the multiple functions played by OsPIP1;3, this work provides insights into the translational value of plant AQPs.     

The hybrid protein interactome contributes to rice heterosis as epistatic effects

Heterosis is the phenomenon in which hybrid progeny exhibits superior traits in comparison with those of their parents. Genomic variations between the two parental genomes may generate epistasis interactions, which is one of the genetic hypotheses explaining heterosis. We postulate that protein?protein interactions specific to F₁ hybrids (F₁‐specific PPIs) may occur when two parental genomes combine, as the proteome of each parent may supply novel interacting partners. To test our assumption, an inter‐subspecies hybrid interactome was simulated by in silico PPI prediction between rice japonica (cultivar Nipponbare) and indica (cultivar 9311). Four‐thousand, six‐hundred and twelve F₁‐specific PPIs accounting for 20.5% of total PPIs in the hybrid interactome were found. Genes participating in F₁‐specific PPIs tend to encode metabolic enzymes and are generally localized in genomic regions harboring metabolic gene clusters. To test the genetic effect of F₁‐specific PPIs in heterosis, genomic selection analysis was performed for trait prediction with additive, dominant and epistatic effects separately considered in the model. We found that the removal of single nucleotide polymorphisms associated with F₁‐specific PPIs reduced prediction accuracy when epistatic effects were considered in the model, but no significant changes were observed when additive or dominant effects were considered. In summary, genomic divergence widely dispersed between japonica and indica rice may generate F₁‐specific PPIs, part of which may accumulatively contribute to heterosis according to our computational analysis. These candidate F₁‐specific PPIs, especially for those involved in metabolic biosynthesis pathways, are worthy of experimental validation when large‐scale protein interactome datasets are generated in hybrid rice in the future.     

Molecular Dissection of a Genomic Region Governing Root Traits Associated with Drought Tolerance Employing a Combinatorial Approach of QTL Mapping and RNA-seq in Rice

Drought is considered as one of the major obstacles for progressive yield enhancement and stability in rice, especially in rain-fed conditions. Being a complex trait, drought is regulated by numerous quantitative trait loci (QTL), of which, however, very few underlying genes have been cloned. In the present investigation, we made an attempt to uncover the candidate gene(s) behind a major QTL, rdw8.1 governing drought tolerance traits viz., root dry weight and root length. The targeted QTL has been delimited to 366.75 kb from 10.17 Mb by QTL mapping in BC₁F₂ population. Further, the targeted region was delineated employing next-generation sequencing based RNA-seq. Based on the QTL mapping and RNA-seq approaches, the plausible candidate gene underlying the QTL region was identified as a wound inducible protein (LOC_Os08g08090). This gene can be of potential value to enhance the drought tolerance of the elite rice varieties through molecular breeding.     

Identification of QTLs for hybrid fertility in inter-subspecific crosses of rice (Oryza sativa L.)

Two subspecies in rice, japonica and indica, have their own ecotypic traits. However, reproductive barriers such as spikelet sterility in hybrid progenies between subspecies have been an obstacle in breeding programs for combining desirable traits from both subspecies through inter-subspecific hybridization. The 166 F₉ RILs and two BC₁F₁s’ were analyzed for spikelet and pollen fertility with the parents and F₁ between Dasanbyeo (DS, indica) / TR22183 (TR, japonica). A frame map was constructed using a total of 218 polymorphic STS and SSR markers. In both BC₁F₁s’ of DS//DS/TR and TR//DS/TR, clusters of significant QTLs for spikelet and pollen fertility were identified on the short arm of chromosome 5 and chromosome 8. Nine and ten digenic epistatic interactions for DS//DS/TR and TR//DS//TR were identified, respectively. HF-QTLs were detected at the similar position with subspecies-specific markers and segregation distortion loci, implying that HF-QTLs might be associated with the differentiation of indica and japonica. Hybrid fertility/sterility and its relationship with other traits are discussed in relation to the reproductive barriers between subspecies of rice.     

Ospapst1, a useful mutant for identifying seed purity and authenticity in hybrid rice

The stability and completeness of male sterility is still a challenge in some male sterile rice lines, especially those of photoperiod/thermo-sensitive genic male sterility (P/TGMS). Leaf color marker is a widely practiced approach to reduce the impact of self-pollinated seeds of male sterile lines. The papst1 is a leaf color mutant. The newly emerged leaves of papst1 are chlorosis and have an impaired photosynthesis. But the other agronomic traits, such as germination rate, duration of maturation and seed weight, are not changed. The papst1/PAPST1 F₁ showed the wild-type leaf phenotype. The papst1/PAPST1 F₂ progenies displayed an approximately 3:1 segregation ratio of WT phenotype:mutant phenotype (72: 28, χ² = 0.48, p > 0.05), suggesting that papst1 mutant phenotype is caused by a single repressive gene. Map-based cloning and sequencing analysis revealed that a point mutation was occurred in Os01 g16040 (OsPAPST1). Given these results, the Ospapst1 mutant is a useful mutant for identifying seed purity and authenticity in hybrid rice.     

Lettuce, a shallow-rooted crop, and Lactuca serriola, its wild progenitor, differ at QTL determining root architecture and deep soil water exploitation

Wild plant species are often adapted to more stressful environments than their cultivated relatives. Roots are critical in exploiting soil resources that enable plants to withstand environmental stresses, but they are difficult to study. Cultivated lettuce (Lactuca sativa L.) and wild L. serriola L. differ greatly in both shoot and root characteristics. Approximately 100 F_2:3 families derived from an interspecific cross were evaluated in greenhouse and field experiments. In the greenhouse, root traits (taproot length, number of laterals emerging from the taproot, and biomass) and shoot biomass were measured 4 weeks after planting. In the field, plants were grown for 9 weeks (close to harvest maturity of the cultivated parent); mild drought stress was induced by withholding water for 1 week, and gravimetric moisture of soil was then determined for five depth increments between 0–100 cm. The families were genotyped using codominantly scored AFLP markers distributed throughout the genome. Composite interval mapping was used to analyze marker-trait associations. Quantitative trait loci were identified for differences between wild and cultivated lettuce for root architectural traits and water acquisition. Thirteen QTL were detected that each accounted for 28–83% of the phenotypic variation. The loci for taproot length (i.e., cm taproot length g^–1 plant biomass) and the ability to extract water from deep in the soil profile co-localized in the genome. These coincident loci were identified in separate experiments. The wild L. serriola is therefore a potential source of agriculturally important alleles to optimize resource acquisition by cultivated lettuce, thereby minimizing water and fertilizer inputs and ultimately enhancing water quality. Received: 25 February 2000 / Accepted: 31 March 2000     

Water stress and recovery in the performance of two Eucalyptus globulus clones: physiological and biochemical profiles

Eucalyptus plantations are among the most productive forest stands in Portugal and Spain, being mostly used for pulp production and, more recently, as an energy crop. However, the region's Mediterranean climate, with characteristic severe summer drought, negatively affects eucalypt growth and increases mortality. Although the physiological response to water shortage is well characterized for this species, evidence about the plants' recovery ability remains scarce. In order to assess the physiological and biochemical response of Eucalyptus globulus during the recovery phase, two genotypes (AL‐18 and AL‐10) were submitted to a 3‐week water stress period at two different intensities (18 and 25% of field capacity), followed by 1 week of rewatering. Recovery was assessed 1 day and 1 week after rehydration. Drought reduced height, biomass, water potential, NPQ and gas exchange in both genotypes. Contrarily, the levels of pigments, chlorophyll fluorescence parameters (F_v/F_m and φ_PSII), MDA and ABA increased. During recovery, the physiological and biochemical profile of stressed plants showed a similar trend: they experienced reversion of altered traits (MDA, ABA, E, g_s, pigments), while other parameters did not recover (φ_PSII, NPQ). Furthermore, an overcompensation of CO₂ assimilation was achieved 1 week after rehydration, which was accompanied by greater growth and re‐establishment of oxidative balance. Both genotypes were tolerant to the tested conditions, although clonal differences were found. AL‐10 was more productive and showed a more rapid and dynamic response to rehydration (namely in carotenoid content, φ_PSII and NPQ) compared to clone AL‐18.     

Identification of QTLs for Drought-Related Traits in Alien Introgression Lines Derived from Crosses of Rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> cv. IR64) × <Emphasis Type="Italic">O. glaberrima</Emphasis> under Lowland Moisture Stress

Drought is a major abiotic stress that limits rice productivity in rain-fed and upland ecosystems. African rice, Oryza glaberrima, has low yields but is tolerant to drought and other stresses. We evaluated 513 BC₂F₃ progenies from alien introgression lines (AILs) that were derived from crosses of Oryza sativa (IR64) × O. glaberrima. They were assessed for yield and other traits when grown under drought at two locations. Such conditions reduced grain production by 59% compared with the recurrent parent (IR64). However, 33 AILs had higher yields, thus demonstrating their potential as genetic material for transferring drought-related traits from O. glaberrima to O. sativa. A set of 200 AILs was selectively genotyped with 173 simple sequence repeat and sequenced tagged site markers. Molecular analysis showed that a mean of 4.5% of the O. glaberrima genome was introgressed in BC₂F₃ AILs. Our analysis revealed 33 quantitative trait loci (QTLs; including 10 novel) for different traits. O. glaberrima contributed 50% of the alleles to those newly identified QTLs, with one for grain yield per plant (ypp9.1) being new. A QTL at RM208 on chromosome 2 positively affected yield under stress, accounting for 22% of the genetic variation. Our identification of drought-related QTLs for yield and yield components will be useful to future research efforts in marker-assisted selection.     

QTL mapping of grain quality traits from the interspecific cross <Emphasis Type="Italic">Oryza sativa</Emphasis> × <Emphasis Type="Italic">O. glaberrima</Emphasis>

International rice export markets are increasing demands for rapid improvements in grain quality characteristics. The African rice Oryza glaberrima is a new potential source of genes that will enhance the eating, cooking, and milling properties of the rice grain. The objective of this research was to identify and characterize quantitative trait loci (QTLs) among 312 doubled haploid lines derived from the BC₃F₁ of an interspecific cross of O. sativa × O. glaberrima. Genetic material was planted in replicated plots and evaluated for ten grain quality traits in 2001 in Colombia. A linkage map was constructed with 100 polymorphic microsatellite markers using the mapdisto software program to adjust for segregation distortion. Transgressive segregation was observed for all traits. Interval and composite interval analyses identified 27 QTLs for nine characters located on 11/12 chromosomes. The chromosomal positions of QTLs for percentage amylose, alkali-spreading score, and percentage protein were in agreement with data reported by others, whereas QTL markers for percentage head rice, percentage milled rice, percentage protein, and percentage brown rice were different in our mapping population. Five major QTLs were found to be associated with improved percentage rice bran, percentage amylose, and alkali-spreading score. Seven QTLs for improved percentage rice bran, percentage milled rice, alkali-spreading score, percentage protein, and grain length/width ratio were derived from the O. glaberrima accession. Three new QTLs for percentage rice bran are reported here for the first time. Results from this study suggest that the African rice might be a valuable new source for introgression and improvement of several traits that affect quality traits demanded by the different rice export markets.Approved for publication by the Director of the Louisiana Agricultural Experiment Station as paper no. 04-14-0054.     

Eddy covariance captures four‐phase crassulacean acid metabolism (CAM) gas exchange signature in Agave

Mass and energy fluxes were measured over a field of Agave tequilana in Mexico using eddy covariance (EC) methodology. Data were gathered over 252 d, including the transition from wet to dry periods. Net ecosystem exchanges (F_N,EC) displayed a crassulacean acid metabolism (CAM) rhythm that alternated from CO₂ sink at night to CO₂ source during the day, and partitioned canopy fluxes (F_A,EC) showed a characteristic four‐phase CO₂ exchange pattern. Results were cross‐validated against diel changes in titratable acidity, leaf‐unfurling rates, energy exchange fluxes and reported biomass yields. Projected carbon balance (g C m^?2 year^?1, mean ± 95% confidence interval) indicated the site was a net sink of ?333 ± 24, of which contributions from soil respiration were +692 ± 7, and F_A,EC was ?1025 ± 25. EC estimated biomass yield was 20.1 Mg (dry) ha^?1 year^?1. Average integrated daily F_A,EC was ?234 ± 5 mmol CO₂ m^?2 d^?1 and persisted almost unchanged after 70 d of drought conditions. Regression analyses were performed on the EC data to identify the best environmental predictors of F_A. Results suggest that the carbon acquisition strategy of Agave offers productivity and drought resilience advantages over conventional semi‐arid C3 and C4 bioenergy candidates.     

Evaluation of near-isogenic lines of rice introgressed with QTLs for root depth through marker-aided selection

Drought is one of the main abiotic constraints in rice. A deep root system contributes efficiently to maintaining the water status of the crop through a stress period. After identifying QTLs affecting root parameters in a doubled-haploid (DH) population of rice derived from the cross IR64/Azucena, we started a marker-assisted backcross program to transfer the Azucena allele at four QTLs for deeper roots (on chromosomes 1, 2, 7 and 9) from selected DH lines into IR64. We selected the backcross progenies strictly on the basis of their genotypes at the marker loci in the target regions up to the BC₃F₂. We assessed the proportion of alleles remaining from Azucena in the non-target areas of the BC₃F₂ plants, which was in the range expected for the backcross stage reached. Twenty nine selected BC₃F₃ near-isogenic lines (NILs) were developed and compared to IR64 for the target root traits and three non-target traits in replicated experiments. Of the three tested NILs carrying target 1, one had significantly improved root traits over IR64. Three of the seven NILs carrying target 7 alone, as well as three of the eigth NILs carrying both targets 1 and 7, showed significantly improved root mass at depth. Four of the six NILs carrying target 9 had significantly improved maximum root length. Five NILs carrying target 2 were phenotyped, but none had a root phenotype significantly different from that of IR64. A re-analysis of the initial data with the composite interval mapping technique revealed two linked QTLs with opposite effects in this area. Some NILs were taller than IR64 and all had a decreased tiller number because of a likely co-introgression of linked QTLs. The usefulness of NILs, the efficiency of marker-aided selection for QTLs and the relationship between root traits are discussed. The NILs with an improved root system will permit testing the importance of root depth for water-limited environments. Received: 17 July 2000 / Accepted: 20 October 2000