# Comprehensive analysis of bHLH transcription factors reveals candidate regulators of flower development and heat stress response in Rhododendron simsii

## TL;DR

This study identifies bHLH genes in Rhododendron simsii that may regulate flower development and help the plant tolerate heat stress.

## Contribution

The first comprehensive analysis of bHLH genes in R. simsii, identifying candidate regulators for flower development and heat stress response.

## Key findings

- 116 bHLH genes were identified and grouped into 17 phylogenetic clusters.
- 12 flower-expressed genes showed altered expression under high-temperature conditions.
- RsbHLH53 and RsbHLH59 are orthologs of key heat stress regulators in Arabidopsis.

## Abstract

The basic helix–loop–helix (bHLH) transcription factors constitute one of the largest families in plants and play vital roles in growth, development, and environmental stress responses. However, their involvement in flower development and heat stress response in Rhododendron simsii remains unclear.

A total of 116 bHLH genes were identified in the R. simsii genome and classified into 17 groups based on phylogenetic relationships with Arabidopsis thaliana bHLHs. Gene duplication analysis revealed that 57 genes originated from whole-genome duplication events, and 49 from ancient segmental duplications. Promoter analysis identified abundant cis-regulatory elements associated with light, gibberellin, abscisic acid, and stress responses, suggesting regulatory roles in flower development and stress adaptation. Expression profiling showed that 23 and 13 genes were specifically expressed in vegetative organs and flowers, respectively. Analysis of five flower developmental stages grouped 49 genes into four distinct expression patterns. Under high-temperature treatment, 12 flower-expressed genes exhibited altered expression, with nine being significantly upregulated. Functional annotation indicated that two genes, RsbHLH53 and RsbHLH59, are orthologous to Arabidopsis MYC2 and JAM2, which are key regulators of heat stress response.

This study provides the first comprehensive characterization of the bHLH gene family in R. simsii and identifies candidate genes potentially involved in flower development and thermotolerance. These findings lay a theoretical foundation for understanding the molecular mechanisms underlying heat tolerance and for future molecular breeding of heat-resistant R. simsii varieties.

The online version contains supplementary material available at 10.1186/s12870-025-07868-x.