Seed germination initiates the plant life cycle, but it exhibits high sensitivity to salt stress, which is a significant environmental factor limiting rice production. Brassinosteroid (BR) is a growth-promoting phytohormone that mitigates various stresses in rice including salt, drought, and extreme temperatures. However, the mechanisms by which BR alleviates salt stress during seed germination remain inadequately characterized. This study demonstrates that seed-specific overexpression of OsDWF4, a rate-limiting gene in BR biosynthesis, enhances rice germination. The DWF4-OX lines, which have greater endogenous BR content in the seeds, showed better germination under salt stress, corroborating the results obtained through exogenous BR application. Antioxidant enzyme analyses demonstrated that BR enhances the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). Metabolomic analysis revealed that BR mitigates salt stress primarily through the biosynthesis of phenylpropanoids and secondary metabolites. Transcriptomic analysis indicated that both endogenous and exogenous BR share five co-regulated target genes and utilize a common biosynthetic pathway for stilbenoids, diarylheptanoids, and gingerols. These findings confirm the capacity of BR to enhance seed germination under salt stress and identified several BR-mediated targets for developing salt-tolerant rice varieties that are suitable for direct seeding cultivation.

Amylose content, the key determinant of rice eating and cooking quality, is regulated primarily by the Waxy (Wx) gene. We adjusted the amylose content and transparency of semi-glutinous japonica rice carrying the Wx^mp allele by genome editing of upstream open reading frame 6 (uORF6) of Wx.

Amylose content (AC) is a crucial determinant of the eating and cooking quality (ECQ) of rice, with low AC varieties exhibiting a softer texture and greater stickiness −attributes that enhance palatability and are desirable in specific culinary contexts. To harness these traits, significant efforts have been made to manipulate AC to improve rice ECQ. Our research utilized the MutMap⁺ approach to identify LAC6/TL1, a gene that is an allele of Du13, responsible for low AC. LAC6 encodes a C2H2 zinc finger protein, which specifically increases the splicing efficiency of the Wx^b allele without affecting the Wx^a allele. Functional studies of LAC6 revealed that its proper integration could rectify the undesirable AC phenotype, whereas mutations within this gene led to reduced AC and were associated with shorter grain length and decreased thousand-grain weight. Despite these drawbacks, such mutations positively impact rice palatability, presenting a trade-off between grain size and eating quality. To address the challenges posed by the reduced grain weight associated with LAC6 mutations, we developed a specific molecular marker for LAC6, which has been effectively used in breeding programs to select lac6/tl1/du13 homozygous individuals with larger grain size. Our findings demonstrate that the “small grain” trait associated with lac6/tl1/du13 can be effectively mitigated through combined phenotype-based and marker-assisted selection. This study highlights the potential of lac6/tl1/du13 as a valuable gene for breeding novel, high-quality soft rice varieties through targeted breeding strategies.

In animals, serotonin is a neurotransmitter and mood regulator. In plants, serotonin functions in energy acquisition, tissue maintenance, delay of senescence, and response to biotic and abiotic stresses. In this study, we examined the effect of serotonin enrichment of rice endosperm on plant growth, endosperm development, and grain quality. To do so, TDCs and T5H were selected as targets for serotonin fortification. Overexpression of TDC1 or TDC3 increased serotonin accumulation relative to overexpression of T5H in rice grain. Transgenic lines of target genes driven by the Gt1 promoter showed better field performance than those driven by the Ubi promoter. Overexpression of T5H showed little effect on plant growth or grain physicochemical quality. In neuronal cell culture assays, serotonin induced neuroprotective action against apoptosis. Breeding of rice cultivars with high serotonin content may be beneficial for health and nutrition.

Seed germination is the beginning of a new lifecycle, and involves many complex physiological and biochemical reactions including seed reserve mobilization in the endosperm and nutrient transport and reuse in the embryo. Although glutelin is a dominant storage protein in rice, its contribution to seed germination and its regulatory mechanisms are mostly unknown. Gibberellin (GA) and brassinosteroid (BR), two major growth-promoting phytohormones, also play positive roles in controlling seed germination. However, how GA and BR interact and coordinate seed germination and facilitate glutelin mobilization remains unclear. In the present study, biochemical and physiological analyses of seed germination indicated that both GA and BR promote seed germination and post-germination growth. Exogenous application of GA restored germination defects caused by BR deficiency or insensitivity. Proteomic and qRT-PCR results showed that the expression of several glutelin proteins and their encoding genes was induced by BR and GA in the embryo. Expression assays suggested that the increased accumulation of glutelin protein in the embryo was due to the accelerated degradation of glutelin by a cysteine proteinase (REP-1) in the endosperm. The breakdown of glutelin in the endosperm showed a strict positive correspondence with the length of the shoot. The GluA2 mutation led to reduced degradation rate of glutelin and defects in seed germination, and the promotion effect of GA on seed germination was weakened in the glua2 mutant. In vitro culture assay of rice embryos showed that glutelin mobilization functioned downstream of the GA and BR pathways to promote shoot elongation. These findings suggest a mechanism that mediates crosstalk between BR and GA in co-regulating rice seed germination and embryo growth.

The amylose content (AC) of rice endosperm starch varies from 0 to 35%, and is associated with rice cooking and eating quality. Soft rice has low AC, generally between 6% and 15%, and its eating quality is high whether it is consumed hot or cold. However, the appearance quality of current soft rice cultivars needs to be improved, especially opaque endosperm. Conventional genetic engineering has improved some agronomic traits of soft rice varieties, but not endosperm appearance. In the present study, a RNAi construct of the soluble starch synthase II-2 (SSSII-2) and the hygromycin phosphotransferase (HPT) gene were introduced into an elite japonica rice variety, Kangtiaowuyunjing (KWY8) by co-transformation. Several selectable marker-free (SMF) transgenic lines were obtained, and SSSII-2 expression was significantly downregulated in selected transgenic lines, resulting in lower AC of the endosperm. The physicochemical properties of the transgenic rice kernels, including gel consistency (GC) and rapid visco analyzer (RVA) profile, differed significantly from those of wild-type rice and were similar to those of a soft rice variety, Nanjing 46 (NJ46). These findings indicate that the cooking, eating, and processing qualities of transgenic rice are comparable to those of NJ46. However, the transgenic rice endosperm retained a transparent appearance under low-moisture conditions. Thus, SMF SSSII-2 RNAi rice provides a resource for breeding soft rice with transparent endosperm.