The level of endogenous brassinosteroid regulated by CYP90C1 is associated with tetraploid birch (Betula pendula) leaf morphology variations

Polyploid plants typically exhibit phenotypes distinct from diploids. Understanding the mechanism underlying plant polyploid phenotype variation is a critical prerequisite for effectively utilizing polyploid resources. In this study, we induced and obtained autotetraploid birch along with its diploid parent. Comparative analysis revealed significant differences in the morphology of leaves and stems between them. Transcriptome analysis indicated that 3.86% of genes showed significant differential expression between diploids and tetraploids. The genes significantly downregulated in tetraploids were primarily associated with functional terms related to hormone regulation, plant development, and morphogenesis. Notably, a group of genes involved in brassinosteroid (BR) biosynthesis was downregulated in tetraploids, and the level of active BRs in tetraploids was significantly lower than that in diploids. A Cas9/gRNA gene editing method was used to perform functional deletion mutations on BpCYP90C1. The knockout of BpCYP90C1 resulted in limited biosynthesis of 6-deoxoCS and CS in plants. Compared with wild-type (WT) plants, the bpcyp90c1 mutants exhibited a significant increase in leaf epidermal cell and a decrease in the number of leaf epidermal cells. The bpcyp90c1 mutants had curled leaves with obvious serrated edges and cordate base. Furthermore, the height and internode spacing of the bpcyp90c1 mutants were shorter. These phenotypic variations were similar to those of tetraploid birch. This indicates that the decrease in active BR levels was an important factor affecting the variation of tetraploid leaves and stems. Our research provides important insights into the molecular mechanisms of phenotypic variation in autopolyploid plants.