The basic/helix–loop–helix (bHLH) family is a major family of transcription factors in plants. Although it has been reported that bHLH plays a defensive role against pathogen infection in plants, there is no comprehensive study on the bHLH-related defence response in rose (Rosa sp.). In this study, a genome-wide analysis of bHLH family genes (RcbHLHs) in rose was carried out, including their phylogenetic relationships, gene structure, chromosome localization and collinearity analysis. Via phylogenetic analysis, a total of 121 RcbHLH genes in the rose genome were divided into 21 sub-groups. These RcbHLHs are unevenly distributed in all 7 chromosomes of rose. The occurrence of gene duplication events indicates that whole-genome duplication and segmental duplication may play a key role in gene duplication. Ratios of non-synonymous to synonymous mutation frequency (Ka/Ks) analysis showed that the replicated RcbHLH genes mainly underwent purification selection, and their functional differentiation was limited. Gene expression analysis showed that 46 RcbHLHs were differentially expressed in rose petals upon B. cinerea infection. It is speculated that these RcbHLHs are candidate genes that regulate the response of rose plants to B. cinerea infection. Virus-induced gene silencing (VIGS) confirmed that RcbHLH112 in rose is a susceptibility factor for infection with B. cinerea. This study provides useful information for further study of the functions of the rose bHLH gene family.

Fruit rot caused by Fusarium species represents a significant threat to watermelon (Citrullus lanatus (Thunb.) Matsum. et Nakai) quality and economic value. In this study, a fungal pathogen designated strain X2 was isolated from rotten watermelon fruit and identified as Fusarium proliferatum (Matsush.) Nirenberg ex Gerlach & Nir through integrated morphological, microscopic, and multi-locus molecular phylogenetic analyses. Koch’s postulates were fully satisfied, confirming that F. proliferatum strain X2 induces water-soaking and softening symptoms on watermelon fruit at relatively high temperatures. However, low-temperature storage at 4 °C strongly suppressed fungal growth and symptom development. Strain X2 produced fumonisins (FB1, FB2, and FB3) in vitro, but all of these mycotoxins were below the limit of detection in infected fruit tissues. Dual-culture assay revealed that three antagonistic Bacillus strains (G3-2, KP3P9 and K13C) can significantly inhibit the growth of F. proliferatum strain X2. Consistently, fruit co-inoculation assays verified that these strains differentially alleviated external and internal rot symptoms caused by the fungus. Biochemically, F. proliferatum strain X2 infection markedly increased cell wall-degrading enzyme (CWDE) activities, whereas co-inoculation with biocontrol Bacillus bacteria suppressed the activities of polygalacturonase (PG), pectin lyase (PL) and cellulase (CL) in a strain-specific manner. This study identified F. proliferatum strain X2 as a causal agent of watermelon fruit rot and highlights the roles of low-temperature storage and Bacillus-mediated biocontrol in disease suppression.