Integrative analysis from the transcriptome to proteome uncovers brassinosteroid-mediated regulation of cambium initiation and patterning in woody stems

Wood formation involves sequential developmental events, requiring the coordination of multiple hormones. Brassinosteroids (BRs) play a key role in wood development, but little is known about the cellular and molecular processes that underlie wood formation in tree species. Here, we generated transgenic poplar lines editing one or several members of PdBRI1s, the orthologs of Arabidopsis vascular-enriched BR receptors, and showed how inhibition of BR signaling influences wood development at the mRNA level. Six Populus PdBRI1 genes form three gene pairs and each exhibits high expression in the basal stems. Simultaneous mutation of PdBRI1-1, 2, 3 and 6, the orthologs of Arabidopsis vascular-enriched BR receptors BRI1, BRL1 and BRL3, resulted in a severe defect in growth. Particularly, the stems of these mutant lines displayed a discontinuous ring of cambium and patterning defects in derived secondary vascular tissues. Abnormal formation of cambia within cortical parenchyma was also observed in the stem of pdbri1-1;2;3;6. Transgenic poplars editing PdBRI1-1 or PdBRI1-1;2;6 had phenotypic alterations in stem development at 4.5 months of growth, indicating functional redundancy among these PdBRI1 genes. An integrated analysis of the transcriptome from pdbri1-1;2;3;6 stems revealed differential expression of a number of genes associated with wood development and hormones. Overall design: mRNA of stem profiles of 45-day old wild type (WT) and pdbri1-1;2;3;6 in Populus