Objective Castanea mollissima is one of the major species in China’s distinctive nut industry. Its fruits possess both nutritional and medicinal properties, making them of high economic value. Improving fruit yield and quality is thus crucial for promoting industrial development. This paper aims to elucidate the molecular regulatory mechanisms, by which bHLH transcription factors mediate MeJA signaling to influence fruit development, quality, and nut set in C. mollissima. It also seeks to clarify the key role of MeJA during fruit development, thereby providing valuable genetic resources and technical strategies for breeding of high-yield and high-quality C. mollissima cultivars.

Method In this study, C. mollissima ‘Zunhuaduanci’ was used as experimental material. The physicochemical properties, sequence characteristics, and evolutionary relations of C. mollissima bHLH transcription factor family were systematically analyzed. Gene expression patterns were detected using transcriptomic data and quantitative real-time PCR technology. Different concentrations of MeJA and its inhibitor (DIECA) were sprayed during the key periods of C. mollissima pollination and fertilization as growth regulators, and various fruit quality indicators (including single fruit mass, soluble sugar content, starch content) and changes in endogenous hormone levels were measured in different treatment groups.

Result (1) A total of 93 CmbHLH genes were identified in C. mollissima genome. Among them, 63% encoded acidic proteins. Phylogenetic analysis divided it into 20 subgroups (A–T). Jasmonate-responsive cis-elements were found in 70 of CmbHLH genes, and numerous light-responsive, endosperm-specific, and meristem-specific regulatory elements were also detected, indicating a close association with MeJA signaling and plant developmental processes. (2) A majority of CmbHLH genes exhibited significant differential expression (P < 0.05) between normally developing ovules and abortive ovules at the 18th day after pollination, suggesting their involvement in early ovule development and providing molecular evidence for identifying key developmental stages of fruit set and abortion. (3) Exogenous MeJA treatment modulated the expression of CmbHLH genes. Furthermore, MeJA application improved bract morphology, significantly increasing transverse, longitudinal, and lateral diameters, as well as bract mass. Notably, 400 mg/L MeJA treatment produced the most pronounced effects, significantly increasing the number of nuts per bract and the fruit set rate. (4) MeJA application elevated the levels of MeJA, gibberellins (GAs), and abscisic acid (ABA) in C. mollissima fruits, with higher concentrations producing more significant effects. In contrast, DIECA treatment reduced these hormone levels. The three hormones exhibited coordinated changes, with the most notable increases observed under the 400 mg/L MeJA treatment. (5) In terms of nutritional composition, MeJA treatments enhanced fruit protein and starch contents, with the 400 mg/L mass concentration showing the strongest effects. These results suggest that CmbHLH genes may regulate nutritional quality through MeJA-mediated signaling pathways.

Conclusion CmbHLHs genes could respond to MeJA signaling and modulate both morphological traits and nutritional quality in C. mollissima. These findings provide new insights into the molecular mechanisms underlying C. mollissima fruit development and establish a theoretical and technical foundation for the application of MeJA in high-quality cultivation of C. mollissima.