Functional characterization of the orthologs of AtNST1/2 in Glycine soja (Fabaceae) and the evolutionary implications

The secondary cell wall in plant cells is a highly organized structure that provides mechanical support to the tissue and the entire plant body. Glycine soja, the presumed ancestor of domesticated soybean, differs from the domesticated soybean in an array of morphological characters. The remarkable difference is the stem phenotype, which is often attributed to the thickness of secondary cell walls. Hence, it is of great importance to understand the genetic basis underlying the regulation of secondary wall thickening in these two species. Here, we report that GlycineNST1A/B are functional orthologs of AtNST1/2, which have been shown to act as the master switches of secondary cell wall thickening. GlycineNST1A/B are preferentially expressed in organs undergoing secondary wall deposition, and GmNST1A/B are expressed at twofold the level of GsNST1A/B in mature stem. The overexpression of GsNST1A/B disrupts overall plant development by inducing ectopic deposition of massive secondary walls in normally parenchyma cells in Arabidopsis. Furthermore, expression analyses suggest that the ectopic secondary wall is achieved by upregulating the expression of a series of cell wall-associated transcription factors and secondary wall biosynthetic genes. Together, these results indicate that GsNST1A/B, as the key regulators, are capable of activating the entire secondary wall biosynthetic pathway in Arabidopsis, and also suggest that the genetic circuit of secondary wall development is conserved between Fabaceae and Brassiaceae.