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The main goals of rice breeding nowadays include increasing yield, improving grain quality, and promoting complete mechanized production to save labor costs. Rice grain shape, specified by three dimensions, including grain length, width and thickness, has a more precise meaning than grain size, contributing to grain appearance quality as well as grain weight and thus yield. Furthermore, the divergence of grain shape characters could be utilized in mechanical seed sorting in hybrid rice breeding systems, which has been succeeded in utilizing heterosis to achieve substantial increase in rice yield in the past decades. Several signaling pathways that regulate rice grain shape have been elucidated, including G protein signaling, ubiquitination-related pathway, mitogen-activated protein kinase signaling, phytohormone biosynthesis and signaling, microRNA process, and some other transcriptional regulatory pathways and regulators. This review summarized the recent progress on molecular mechanisms underlying rice grain shape determination and the potential of major genes in future breeding applications.

Chilling stress is one of the main abiotic factors affecting rice growth and yield. In rice, chlorophyllide a oxygenase encoded by OsCAO1 is responsible for converting chlorophyllide a to chlorophyllide b, playing a crucial role in photosynthesis and thus rice growth. However, little is known about the function of OsCAO1 in chilling stress responses. The presence of the cis-acting element involved in low-temperature responsiveness (LTR) in the OsCAO1 promoter implied that OsCAO1 probably is a cold-responsive gene. The gene expression level of OsCAO1 was usually inhibited by low temperatures during the day and promoted by low temperatures at night. The OsCAO1 knockout mutants generated by the CRISPR-Cas9 technology in rice (Oryza sativa L.) exhibited significantly weakened chilling tolerance at the seedling stage. OsCAO1 dysfunction led to the accumulation of reactive oxygen species and malondialdehyde, an increase in relative electrolyte leakage, and a reduction in antioxidant gene expression under chilling stress. In addition, the functional deficiency of OsCAO1 resulted in more severe damage to chloroplast morphology, such as abnormal grana thylakoid stacking, caused by low temperatures. Moreover, the rice yield was reduced in OsCAO1 knockout mutants. Therefore, the elevated expression of OsCAO1 probably has the potential to increase both rice yield and chilling tolerance simultaneously, providing a strategy to cultivate chilling-tolerant rice varieties with high yields. [ABSTRACT FROM AUTHOR]

Hybrid rice needs to prevent lodging due to excessive grain weight during maturity, which can lead to yield loss, grain quality deterioration and difficulty in harvesting. Breeding excellent parents (restorer and male-sterile lines) is the foundation for cultivating lodging-resistant hybrid rice. However, there is still limited information about restorers with improved lodging resistance. To identify a new restorer with significantly enhanced resistance to lodging, the restorer line Huazhan, which has been used for many years in hybrid rice breeding in China, was used as the control, to evaluate the lodging resistance of the new restorer R382. A two-year field experiment was executed to investigate the difference on lodging-related traits of the basal two internodes above-ground, such as out diameters of stems with or without leaf sheath (DWS, DWOS), culm thickness (CT), the section modulus (SM), bending stress (BS), and bending moment at break (M). The results indicated that DWS, CT, BS, M of R382 was at least 19.9%, 13.3%, 36.5% and 52.6% higher than that of Huazhan. However, the negative indicator lodging index of R382 was over 17.5% lower. The number and area of vascular bundles, and the contents of the chemical components of the cell wall including cellulose, lignin, and hemicellulose, together with the accumulation of starch granules, were significantly greater in R382. Furthermore, in the basal stems during the jointing stage, a series of genes that promote cell wall formation exhibited significantly higher expression levels in R382. Meanwhile, there was no significant difference in the yield of R382 and Huazhan. Thus, this new restorer could be a valuable donor in the future hybrid rice breeding programs for improving lodging resistance. [ABSTRACT FROM AUTHOR]

Heat stress (HS) has become one of the major abiotic stresses that severely constrain rice growth. Abscisic acid (ABA) plays an important role in plant development and stress response. However, the effect of different concentrations of exogenous ABA on HS tolerance in rice still needs to be further elucidated. Here, we found that high concentrations of exogenous ABA increased HS damage in seedlings, whereas 10−12 M ABA treatment increased fresh and dry weight under HS relative to mock seedlings. Our further data showed that, in response to HS, 10−5 M, ABA-treated seedlings exhibited a lower chlorophyll content, as well as transcript levels of chlorophyll biosynthesis and antioxidant genes, and increased the accumulation of reactive oxygen species (ROS). In addition, the transcript abundance of some heat-, defense-, and ABA-related genes was downregulated on 10−5 M ABA-treated seedlings under HS. In conclusion, high concentrations of exogenous ABA reduced the HS tolerance of rice seedlings, and this negative effect could be achieved by regulating the accumulation of ROS, chlorophyll biosynthesis, and the transcription levels of key genes in seedlings under HS. [ABSTRACT FROM AUTHOR]