Rice (Oryza SativaL.) is an essential constituent of the global food chain. Drought stress significantly diminished its productivity and threatened global food security. This review concisely discussed how drought stress negatively influenced the rice’s optimal growth cycle and altered its morpho-physiological, biochemical, and molecular responses. To withstand adverse drought conditions, plants activate their inherent drought resistance mechanism (escape, avoidance, tolerance, and recovery). Drought acclimation response is characterized by many notable responses, including redox homeostasis, osmotic modifications, balanced water relations, and restored metabolic activity. Drought tolerance is a complicated phenomenon, and conventional breeding strategies have only shown limited success. The application of molecular markers is a pragmatic technique to accelerate the ongoing breeding process, known as marker-assisted breeding. This review study compiled information about quantitative trait loci (QTLs) and genes associated with agronomic yield-related traits (grain size, grain yield, harvest index, etc.) under drought stress. It emphasized the significance of modern breeding techniques and marker-assisted selection (MAS) tools for introgressing the known QTLs/genes into elite rice lines to develop drought-tolerant rice varieties. Hence, this study will provide a solid foundation for understanding the complex phenomenon of drought stress and its utilization in future crop development programs. Though modern genetic markers are expensive, future crop development programs combined with conventional and MAS tools will help the breeders produce high-yielding and drought-tolerant rice varieties.

Rice drought resistance is a complicated quantitative feature involving a range of biological and agronomic variables, but little is known about the underlying genetics and regulatory mechanisms that regulate drought tolerance. This study used 120 recombinant inbred lines (RILs), derived from a cross between drought tolerant Lvhan 1 and susceptible Aixian 1. The RILs were subjected to drought stress at the first ear stage, and phenotypic data of 16 agronomic and physiological traits under varying conditions were investigated. Genome-wide association study (GWAS) on the drought resistance index of traits was carried out. A total of 9 quantitative trait loci (QTLs) associated with drought-related traits were identified on chromosomes 2, 6, 7, 8, 9, and 10, which includes QTLs for plant height (PH) qPH10.1, effective panicles number (EPN) qEPN6.1, panicle length (PL) qPL9.1, thousand-grain weight (TGW) qTGW2.1, qTGW6.1, qTGW8.1, leaf length (LL) qLL7.1, leaf width (LW) qLW7.1, and leaf area (LA) qLA7.1. The fraction of phenotypic variation explained by individual QTL varied from 10.6% to 13.9%. Except for days to flowering (DTF), the mean values of all traits under normal water management conditions were considerably higher than those under drought conditions. Except for the DTF, the drought resistance index of all rice traits was less than 1, indicating that drought treatment reduced the EPN, FGPP, SSR, PH, and LA, which affected the growth and development of rice. The drought resistance index of DTF was 1.02, indicating that drought prolonged the heading time of rice and diminish the yield parameters. Along with identifying QTLs, the results also predicted ten candidate genes, which are directly or indirectly involved in various metabolic functioning related to drought stress. The identification of these genomic sites or QTLs that effectively respond to water scarcity will aid in the quest of understanding the drought tolerance mechanisms. This study will facilitate the marker-assisted rice breeding and handy in the breeding of drought-tolerant rice varieties.

Japonica/geng and indica/xian are two major rice (Oryza sativa) subspecies with multiple divergent traits, but how these traits are related and interact within each subspecies remains elusive. Brassinosteroids (BRs) are a class of steroid phytohormones that modulate many important agronomic traits in rice. Here, using different physiological assays, we revealed that japonica rice exhibits an overall lower BR sensitivity than indica. Extensive screening of BR signaling genes led to the identification of a set of genes distributed throughout the primary BR signaling pathway with divergent polymorphisms. Among these, we demonstrate that the C38/T variant in BR Signaling Kinase2 (OsBSK2), causing the amino acid change P13L, plays a central role in mediating differential BR signaling in japonica and indica rice. OsBSK2^L13 in indica plays a greater role in BR signaling than OsBSK2^P13 in japonica by affecting the auto‐binding and protein accumulation of OsBSK2. Finally, we determined that OsBSK2 is involved in a number of divergent traits in japonica relative to indica rice, including grain shape, tiller number, cold adaptation, and nitrogen‐use efficiency. Our study suggests that the natural variation in OsBSK2 plays a key role in the divergence of BR signaling, which underlies multiple divergent traits between japonica and indica.

Acetic acid (HAC) is a crucial signal molecule in plant stress responses; however, its role in conferring heat tolerance to rice remains unclear. This study aims to investigate the effect of HAC in protecting pistil function under heat stress and its potential role in facilitating pollen germination and tube growth via HAC-induced synthesis of methyl jasmonate (MeJA) and quercetin (QR). Physiological analysis, including pollen germination, pollen tube growth into the ovule, reactive oxygen species (ROS), as well as the levels of HAC, acetyl coenzyme A (acetyl-CoA), MeJA, and QR in the pistils of heat stress-treated early indica rice cultivars Zhongzao39 (ZZ39) and Zhongjiazao17 (ZJZ17), were conducted. RNA sequencing (RNA-seq) was performed to identify differentially expressed genes involved in this process. Effect of exogenous acetate (NaAC), MeJA, and QR on spikelet fertility were also investigated. Compared with ZJZ17, severe inhibition of spikelet fertility, pollen germination, and pollen tube growth was observed in ZZ39, due to the ROS burst and an irregular distribution across the stigma, style, and ovule. RNA-seq and physiological data indicate that HAC may activate acetyl-CoA to enhance heat tolerance by inducing the synthesis of MeJA and QR. Exogenous NaAC enhanced spikelet fertility under heat stress, accompanied by elevated antioxidant enzyme activities, improved energy status, and increased levels of acetyl-CoA, MeJA, and QR in the pistils. Additionally, NaAC, MeJA, and QR, either alone or in combination, effectively augmented spikelet fertility under heat stress, while the combination of MeJA and QR inhibitors significantly reduced fertility. Acetate activates acetyl-CoA to induce the synthesis of both MeJA and QR, thereby alleviating heat-induced pistil dysfunction by maintaining ROS homeostasis and enhancing the pollen germination, pollen tube growth and spikelet fertility. Our results offer a promising strategy to enhance the heat tolerance of crops.

Drought is a primary abiotic stress that inhibits rice (Oryza sativa L.) growth and development, and during the reproductive stage it has a negative impact on the rice seed-setting rate. This research study examined two rice lines, La-96 (drought sensitive) and La-163 (drought resistant), for drought stress treatment (with soil moisture at 20% for 7 days) and control (normal irrigation and kept soil moisture ≥40%). To elucidate the photosynthesis and molecular mechanisms underlying drought tolerance in rice, leaf photosynthetic traits and transcriptome sequencing were used to compare differences between two contrasting recombinant inbred lines (RIL) during drought and subsequent recovery at the booting stage. The rice line La-96 showed a significant decrease in seed-setting rate after being treated for seven days’ drought stress (from 86.64% to 22.75%), while La-163 was slightly affected (from 89.04% to 79.33%). The photosynthetic activities of both lines significantly decreased under the drought treatment, and these traits of La-163 recovered to a comparable level with the control after three days of rewatering. The transcriptome of both lines in three treatments (the control, drought stress, and subsequent recovery) were tested, and a total of 16,051 genes were identified, among which 10,566 genes were differentially expressed in various treatments and rice lines. Comprehensive gene expression profiles revealed that the specifically identified DEGs were involved in the ribosome synthesis and the metabolic pathway of photosynthesis, starch, and sucrose metabolism. The DEGs that are activated and respond quickly, as seen during recovery in the tolerant rice line, may play essential roles in regulating subsequent growth and development. This study uncovered the molecular genetic pathways of drought tolerance and extended our understanding of the drought tolerance mechanisms and subsequent recovery regulation in rice.

Impact of dataset on the study of crop disease image recognition