No abstract

Reductions in crop yields as a consequence of global climate change threaten worldwide food security. It is therefore imperative to develop high-yielding crop plants that show sustainable production under stress conditions. In order to achieve this aim through breeding or genetic engineering, it is crucial to have a complete and comprehensive understanding of the molecular basis of plant architecture and the regulation of its sub-components that contribute to yield under stress. Rice is one of the most widely consumed crops and is adversely affected by abiotic stresses such as drought and salinity. Using it as a model system, in this review we present a summary of our current knowledge of the physiological and molecular mechanisms that determine yield traits in rice under optimal growth conditions and under conditions of environmental stress. Based on physiological functioning, we also consider the best possible combination of genes that may improve grain yield under optimal as well as environmentally stressed conditions. The principles that we present here for rice will also be useful for similar studies in other grain crops.

Rice yield and heading date are the two discrete traits controlled by quantitative trait loci (QTLs). Both traits are influenced by the genetic make‐up of the plant as well as the environmental factors where it thrives. Drought and salinity adversely affect crop productivity in many parts of the world. Tolerance to these stresses is multigenic and complex in nature. In this study, we have characterized a QTL, DTH8 (days to heading) from Oryza sativa L. cv IR64 that encodes a putative HAP3/NF‐YB/CBF subunit of CCAAT‐box binding protein (HAP complex). We demonstrate DTH8 to be positively influencing the yield, heading date, and stress tolerance in IR64. DTH8 up‐regulates the transcription of RFT1, Hd3a, GHD7, MOC1, and RCN1 in IR64 at the pre‐flowering stage and plays a role in early flowering, increased number of tillers, enhanced panicle branching, and improved tolerance towards drought and salinity stress at the reproductive stage. The presence of DTH8 binding elements (CCAAT) in the promoter regions of all of these genes, predicted by in silico analysis of the promoter region, indicates the regulation of their expression by DTH8. In addition, DTH8 overexpressing transgenic lines showed favorable physiological parameters causing less yield penalty under stress than the WT plants. Taken together, DTH8 is a positive regulator of the network of genes related to early flowering/heading, higher yield, as well as salinity and drought stress tolerance, thus, enabling the crops to adapt to a wide range of climatic conditions.

Lysine cannot be synthesized by the human body, and it is, therefore, an indispensable amino acid in the human that must be consumed in adequate amounts to prevent disease. Unfortunately, lysine is not abundant in many important food sources such as rice grains. Efforts to fortify rice and other monocotyledonous crops with high lysine have proved to be challenging because of their effects on plant growth. The present study was designed to avoid the problems that are often encountered in engineering lysine metabolism. We generated different over‐expression constructs for the key anabolic enzyme, dihydrodipicolinate synthase (DHDPS and cdapA) and knockdown (KD) constructs for the catabolic enzyme—lysine ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH), with expression under the control of either the CaMV35S promoter or the endosperm‐specific glutelinD1 promoter. The use of the endosperm‐specific promoter together with the lysine feedback‐insensitive cdapA for over‐expression provides distinct advantages, as does silencing of LKR/SDH under the control of this promoter. Rice plants produced by over‐expressing cdapA together with silencing of LKR/SDH showed a 58% increase in grain lysine content, as well as higher amounts of total seed protein levels. Moreover, the transgenic seedlings were more tolerant to salinity. These results demonstrate that it is possible to generate high lysine stress‐tolerant rice varieties, which have the potential to reduce the need for dietary supplementation.

Mutation breeding has a long track record in the development of crop cultivars with improved tolerance to abiotic stresses such as heat, salinity and drought. Oryza sativa L. cv IR64 is a very popular high yielding rice, but susceptible to major abiotic stresses, such as low and high temperatures, salinity and drought. We subjected IR64 to gamma irradiation and generated a mutant population (M3) with ~2,000 families. These were screened at the seedling stage for tolerance to high-temperature stress using hydroponics and controlled-environment chambers, resulting in the identification of three mutant lines showing a robust seedling phenotype. Under heat stress, higher CO2 assimilation (10-30%), higher spikelet fertility (40-45%) and higher antioxidant activity (15-20% catalase activity) confirmed superiority of the selected mutant lines over wild type plants at seedling and flowering stages. Upon exposure to salinity and drought stress, the three selected lines also exhibited better tolerance than wild type in terms of higher CO2 assimilation, stomatal conductance, transpiration and chlorophyll fluorescence. Transcript and protein abundance analyses confirmed higher constitutive levels of heat shock proteins and antioxidant enzymes in the mutant lines relative to wild type. Tolerance to multiple abiotic stresses was reflected in higher (25-30%) grain yield than wild type. It is anticipated that the mutant lines identified will be useful for developing new improved cultivars for dry and saline areas and may be exploited to dissect the molecular basis of multiple stress tolerance in crop plants