Genome-wide identification and expression analysis of WRKY family genes under soft rot in Chinese cabbage.

Complex transcriptional networks regulate plant defense against pathogen attack, and plant transcription factors act as key regulators of the plant immune responses. The differences between transcription factor expression and regulation in Chinese cabbage soft rot (Pectobacterium carotovorum; Pc) have not been revealed. In this study, a total of 148 putative Chinese cabbage WRKY genes (BrWRKYs) were identified from the Chinese cabbage genome (v3.0). These genes were divided into seven subgroups (groups I, IIa-e, and III) based on phylogenomic analysis, with distinct motif compositions in each subgroup. Time-series RNA-seq was carried out to elucidate the dynamic expression patterns of the BrWRKYs on the resistant mutant (sr) and the susceptible wild-type (inbred WT) challenged by Pc. Transcriptional analysis showed that 48 WRKY transcription genes at 0-24 hpi were significantly upregulated in sr under soft rot stress. At the 12-h post-inoculation critical time point, we identified three specifically upregulated genes and two downregulated genes in the resistant mutant, which may provide potential applications for genetic improvement against soft rot. The findings improved our understanding of the WRKY-mediated soft rot stress response regulation in Chinese cabbage. The study thus lays a foundation for the genetic improvement of soft rot resistance.

Mutation in a chlorophyll-binding motif of Brassica ferrochelatase enhances both heme and chlorophyll biosynthesis.

The heme branch of tetrapyrrole biosynthesis contributes to the regulation of chlorophyll levels. However, the mechanism underlying the balance between chlorophyll and heme synthesis remains elusive. Here, we identify a dark green leaf mutant, dg, from an ethyl methanesulfonate (EMS)-induced mutant library of Chinese cabbage. The dg phenotype is caused by an amino acid substitution in the conserved chlorophyll a/b-binding motif (CAB) of ferrochelatase 2 (BrFC2). This mutation increases the formation of BrFC2 homodimer to promote heme production. Moreover, wild-type BrFC2 and dBrFC2 interact with protochlorophyllide (Pchlide) oxidoreductase B1 and B2 (BrPORB1 and BrPORB2), and dBrFC2 exhibits higher binding ability to substrate Pchlide, thereby promoting BrPORBs-catalyzed production of chlorophyllide (Chlide), which can be directly converted into chlorophyll. Our results show that dBrFC2 is a gain-of-function mutation contributing to balancing heme and chlorophyll synthesis via a regulatory mechanism in which dBrFC2 promotes BrPORB enzymatic reaction to enhance chlorophyll synthesis.