Physiological and biochemical characterization of NERICA‐L‐44: a novel source of heat tolerance at the vegetative and reproductive stages in rice

The predicted increase in the frequency and magnitude of extreme heat spikes under future climate can reduce rice yields significantly. Rice sensitivity to high temperatures during the reproductive stage is well documented while the same during the vegetative stage is more speculative. Hence, to identify and characterize novel heat‐tolerant donors for both the vegetative and reproductive stages, 71 rice accessions, including approximately 75% New Rice for Africa (NERICAs), were phenotyped across field experiments during summer seasons in Delhi, India, and in a controlled environment study at International Rice Research Institute , Philippines. NERICA‐L‐44 (NL‐44) recorded high seedling survival (52%) and superior growth and greater reproductive success exposed to 42.2°C (sd ± 2.3) under field conditions. NL‐44 and the heat‐tolerant check N22 consistently displayed lower membrane damage and higher antioxidant enzymes activity across leaves and spikelets. NL‐44 recorded 50–60% spikelet fertility, while N22 recorded 67–79% under controlled environment temperature of 38°C (sd ±1.17), although both had about 87% fertility under extremely hot field conditions. N22 and NL‐44, exposed to heat stress (38°C), had similar pollen germination percent and number of pollen tubes reaching the ovary. NL‐44 maintained low hydrogen peroxide production and non‐photochemical quenching (NPQ) with high photosynthesis while N22 avoided photosystem II damage through high NPQ under high‐temperature stress. NL‐44 with its reproductive stage resilience to extreme heat stress, better antioxidant scavenging ability in both vegetative tissue and spikelets and superior yield and grain quality is identified as a novel donor for increasing heat tolerance at both the vegetative and reproductive stages in rice.     

Molecular approaches for designing heat tolerant wheat

Global warming is causing changes in temperature rapidly for over two decades. The increased temperature during reproductive phase of plant growth has emerged as a serious problem all over the world. Constant or transitory high temperatures may affect the plant growth and development which may lead to diverse morphological, physiological and biochemical changes in plants ultimately decrease in yield. Genetic approaches leading to improved thermo-tolerance can mitigate the reduction in yield. In this backdrop, several indirect traits or parameters have been developed for identification of heat tolerant plants/lines. The traits like stay green/delayed senescence are reported to contribute toward capability of plants to tolerate heat stress. In addition, understanding of biochemical and molecular basis of thermo-tolerance in combination with genetic approaches like identification and mapping of heat tolerant QTLs will not only assist conventional breeders to develop heat tolerant cultivars but also help molecular biologists to clone and characterize genes associated with heat tolerance, which could be used in genetically modified heat tolerant plants. Therefore, overviews of different strategies for developing heat tolerant wheat are discussed in this review.     

High day–night transition temperature alters nocturnal starch metabolism in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Transitory starch plays a vital role in maintenance respiration as its degradation products provide substrate for the night respiration. A study was conducted with two contrasting rice cultivars: Vandana (high night temperature susceptible) and Nagina 22 (high night temperature tolerant) by subjecting them to increase in transition temperature from anthesis to physiological maturity. Night respiration on plant area basis increased by 35% in Vandana at 5 days after anthesis but was unaffected in tolerant cultivar. A simultaneous 18% decrease in starch content was observed in the susceptible cultivar. An analysis of the starch-metabolizing enzymes showed that activity of β-amylase increased markedly in Vandana whereas both β and α-amylase decreased in Nagina 22 following high day to night transition temperature exposure. The level of starch breakdown product, maltose, increased in the susceptible cultivar but glucose levels declined in both the cultivars. Concurrently, expression of chloroplastic isoforms α-amylase OsAMY1, OsAMY2 and β-amylase OsBAM2 increased in Vandana. A lower accumulation of dry matter was recorded in the susceptible than the tolerant cultivar. Our study elucidated the regulatory role of transitory starch in supporting the high day to night transition temperature-induced night-time respiration which is mediated by the increased activity of β-amylase through enhanced expression of OsBAM2 in flag leaves of susceptible cultivar.     

Expression profiling of rice cultivars differing in their tolerance to long-term drought stress

Understanding the molecular basis of plant performance under water-limiting conditions will help to breed crop plants with a lower water demand. We investigated the physiological and gene expression response of drought-tolerant (IR57311 and LC-93-4) and drought-sensitive (Nipponbare and Taipei 309) rice (Oryza sativa L.) cultivars to 18 days of drought stress in climate chamber experiments. Drought stressed plants grew significantly slower than the controls. Gene expression profiles were measured in leaf samples with the 20 K NSF oligonucleotide microarray. A linear model was fitted to the data to identify genes that were significantly regulated under drought stress. In all drought stressed cultivars, 245 genes were significantly repressed and 413 genes induced. Genes differing in their expression pattern under drought stress between tolerant and sensitive cultivars were identified by the genotype x environment (G x E) interaction term. More genes were significantly drought regulated in the sensitive than in the tolerant cultivars. Localizing all expressed genes on the rice genome map, we checked which genes with a significant G x E interaction co-localized with published quantitative trait loci regions for drought tolerance. These genes are more likely to be important for drought tolerance in an agricultural environment. To identify the metabolic processes with a significant G x E effect, we adapted the analysis software MapMan for rice. We found a drought stress induced shift toward senescence related degradation processes that was more pronounced in the sensitive than in the tolerant cultivars. In spite of higher growth rates and water use, more photosynthesis related genes were down-regulated in the tolerant than in the sensitive cultivars.     

Water deficit and aluminum tolerance are associated with a high antioxidative enzyme capacity in Indica rice seedlings

Plant growth and productivity are greatly affected due to changes in the environmental conditions. In the present investigation, the interactive effects of two important abiotic stresses, i.e., water deficit and Al toxicity, were examined in the seedlings of two rice (Oryza sativa L.) cvs. Malviya-36 (water deficit/Al sensitive) and Vandana (water deficit/Al tolerant). When 15 days grown seedlings were exposed to water deficit (created with 15 % polyethylene glycol 6000) or Al (1 mM AlCl₃) treatment or both the treatments together for 48 h, the lengths of root/shoot, relative water content, and chlorophyll greatly declined in the seedlings of the sensitive cultivar, whereas in the tolerant seedlings, either little or insignificant decline in these parameters was observed due to the treatments. Seedlings subjected to water deficit or Al treatment alone or in combination showed increased intensity of the isoenzyme activity bands of superoxide dismutase (SOD), guaiacol peroxidase (GPX), and ascorbate peroxidase (APX) in in-gel activity staining studies. Water deficit caused decrease in intensity of catalase (CAT) activity bands; however, when seedlings were exposed to AlCl₃ alone or in combination with water deficit, the intensity of the CAT isoforms increased in both the rice cultivars. The level of expression of the activity bands of SOD, CAT, GPX, and APX was always higher in the seedlings of tolerant cv. Vandana compared to the sensitive cv. Malviya-36 under both controls as well as stress treatments. Higher intensity of isozymes representing higher activity levels of antioxidative enzymes in the rice seedlings and their further increase under water deficit, Al exposure, or in combination of both the stresses appears to serve as useful marker for specifying a combination of water deficit and Al tolerance in rice.     

Heat-tolerant basmati rice engineered by over-expression of hsp101

Rice is sensitive to high-temperature stress at almost all the stages of its growth and development. Considering the crucial role of heat shock protein 101 (Hsp101) in imparting thermotolerance to cells, we introduced Arabidopsis thaliana hsp101 (Athsp101) cDNA into the Pusa basmati 1 cultivar of rice (Oryza sativa L.) by Agrobacterium-mediated transformation. Stable integration and expression of the transgene into the rice genome was demonstrated by Southern, northern and western blot analyses. There appeared no adverse effect of over-expression of the transgene on overall growth and development of transformants. The genetic analysis of tested T₁ lines showed that the transgene segregated in a Mendelian fashion. We compared the survival of T₂ transgenic lines after exposure to different levels of high-temperature stress with the untransformed control plants. The transgenic rice lines showed significantly better growth performance in the recovery phase following the stress. This thermotolerance advantage appeared to be solely due to over-expression of Hsp101 as neither the expression of low-molecular-weight heat shock proteins (HSPs) nor of other members of Clp family proteins was altered in the transgenic rice. The production of high temperature tolerant transgenic rice cultivars would provide a stability advantage under supra-optimal temperature regime thereby improving its overall performance.     

Bisulphite sequencing reveals dynamic DNA methylation under desiccation and salinity stresses in rice cultivars

DNA methylation governs gene regulation in plants in response to environmental conditions. Here, we analyzed role of DNA methylation under desiccation and salinity stresses in three (IR64, stress-sensitive; Nagina 22, drought-tolerant and Pokkali, salinity-tolerant) rice cultivars via bisulphite sequencing. Methylation in CG context within gene body and methylation in CHH context in distal promoter regions were positively correlated with gene expression. Hypomethylation in Nagina 22 and hypermethylation in Pokkali in response to desiccation and salinity stresses, respectively, were correlated with higher expression of few abiotic stress response related genes. Most of the differentially methylated and differentially expressed genes (DMR-DEGs) were cultivar-specific, suggesting an important role of DNA methylation in abiotic stress responses in rice in cultivar-specific manner. DMR-DEGs harboring differentially methylated cytosines due to DNA polymorphisms between the sensitive and tolerant cultivars in their promoter regions and/or coding regions were identified, suggesting the role of epialleles in abiotic stress responses.     

Effect of short-term heat exposure on physiological traits of indica rice at grain-filling stage

Heat stress impacts the quantity and quality of rice grains, particularly during grain-filling stage needed for grain development. In this study, the effect of short heat stress (42 °C, 30 min) on indica rice plants at the grain-filling stage (dough grain stage) was found by determining their physiological and growth traits F_v/F_m, ?F/F_m′, chlorophyll content, leaf water potential (LWP), membrane stability, relative leaf area (RLA), relative plant height (RPH), total grain weight per panicle (TGW) and 1000 GW. Thai economic rice cvs. KDML105 and Pathumthani 1 (PTT1) were compared to the heat-tolerant rice cultivars N22 and Dular and to the heat-sensitive rice cultivar IR64. The results showed that short heat stress exhibited effects on physiology and growth greater than the control (35 °C, 30 min) by reducing of F_v/F_m, ?F/F_m′, chlorophyll content, LWP, membrane stability and RLA. This result impacted the TGW and 1000 GW. A higher reduction of physiological traits was shown in IR64, followed by KDML105. In contrast, N22 and Dular were minimally affected by heat stress and were able to adapt and recover based on their grain weight that exhibited less of an effect. PTT1 was also impacted by heat stress similarly to Dular. Thus, short heat stress affected the physiological parameters of five rice cultivars at the dough grain stage. In addition, the five indica rice cultivars were classified into three groups: (1) the heat-tolerant group (N22, Dular and PTT1), (2) the moderately heat-tolerant group (KDML105), and (3) the heat-sensitive group (IR64) by PC-ORD program at 50% of similarity of the 13 physiological traits.     

miRNA对水稻生长发育的调控

MicroRNA(miRNA)是一类小的非编码RNA,它们主要在转录后水平对靶mRNA进行切割或抑制mRNA的翻译来调控基因的表达. miRNA通过对靶基因的调控参与植物的生长发育、胁迫应答和代谢过程.在水稻中,已经发现并初步验证了许多与生长发育相关的miRNA,它们对水稻不同器官和形态发育发挥着重要作用.本文综述了水稻miRNA的发生和调控机制、靶基因的预测,重点介绍了miRNA对水稻生长发育和形态建成的研究进展,并对研究过程中存在的问题进行了讨论.为更好地了解miRNA及其靶基因在提高水稻产量和品质方面的作用,进一步解析miRNA在水稻中的调控机制提供参考.