The SlHB8 Acts as a Negative Regulator in Stem Development and Lignin Biosynthesis

Abstract

The stem is an important organ in supporting plant body, transporting nutrients and communicating signals for plant growing. However, studies on the regulation of stem development in tomato are rather limited. In our study, we demonstrated that SlHB8 negatively regulated tomato stem development. SlHB8 belongs to homeo domain-leucine zipper Class III gene family transcription factors and expressed in all the organs examined including root, stem, leaves, flower, and fruit. Among these tissues, <i>SlHB8</i> showed stable high expression level during tomato stem development. Overexpression of <i>SlHB8</i> gene decreased stem diameter with inhibited xylem width and xylem cell layers, while loss of function of <i>SlHB8</i><i>gene</i> increased the stem diameter and xylem width. The contents of lignin were decreased both in leaves and stems of <i>SlHB8</i> overexpression plants. RNA-seq analysis on the stems of wild type and <i>SlHB8</i> transgenic plants showed that the 116 DEGs (differential expressed genes) with reversible expression profiles in SlHB8-ox and SlHB8-cr plants were significantly enriched in the phenylpropanoid biosynthesis pathway and plant-pathogen pathway which were related to lignin biosynthesis and disease resistance. Meanwhile, the key genes involved in the lignin biosynthesis pathway such as <i>SlCCR</i> (cinnamoyl-CoA reductase), <i>SlCYP73A14</i>/<i>C4H</i> (cinnamate 4-hydroxylase), <i>SlC3H</i> (coumarate 3-hydroxylase) and <i>SlCAD</i> (cinnamoyl alcohol dehydrogenase) were down-regulated in both stem and leaves of <i>SlHB8</i> overexpression plants, indicating a negative regulatory role of SlHB8 in the lignin biosynthesis and stem development.