Genome-Wide Identification and Expression Profiling of Brassinosteroid-Responsive WRKY Transcription Factors Potentially Involved in Natural Rubber Biosynthesis in Hevea brasiliensis

Natural rubber is synthesized in the laticifers of Hevea brasiliensis, but the transcriptional networks regulating latex-associated processes remain poorly understood. In particular, whether WRKY transcription factors participate in brassinosteroid (BR)-responsive regulation remains unclear. We identified the HbWRKY family using the haplotype A telomere-to-telomere reference genome of Hevea brasiliensis and integrated phylogeny, gene structure, duplication type, synteny, Ka/Ks, promoter cis-elements, tissue expression, and BR-treated transcriptomic data. The 109 HbWRKY proteins comprised 21 Group I, 72 Group II, and 14 Group III members, together with one atypical protein and one Group I-like protein. Whole-genome or segmental duplication accounted for 102 genes (93.6%), and Ka/Ks was below 1 for all 111 duplicated HbWRKY pairs, consistent with purifying selection. G-box elements occurred in 84 promoters (77.1%). Eighty-nine genes were expressed in bark and 37 in latex, with latex showing the highest value for nine genes. Seven HbWRKY genes were differentially expressed after BR treatment, and all were downregulated. HbWRKY7 (HbAchr10M0003572) and HbWRKY28 (HbAchr15M0012484) were prioritized because both combined differential expression following BR treatment, their highest recorded expression in bark among the seven tissues, and predicted promoter G-boxes. This updated family resource narrows the candidates for testing a BR-WRKY regulatory module in rubber tree.