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, SlHB8 showed stable high expression level during tomato stem development. Overexpression of SlHB8 gene decreased stem diameter with inhibited xylem width and xylem cell layers, while loss of function of SlHB8 gene increased the stem diameter and xylem width. The contents of lignin were decreased both in leaves and stems of SlHB8 overexpression plants. RNA-seq analysis on the stems of wild type and SlHB8 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 SlCCR (cinnamoyl-CoA reductase), SlCYP73A14/C4H (cinnamate 4-hydroxylase), SlC3H (coumarate 3-hydroxylase) and SlCAD (cinnamoyl alcohol dehydrogenase) were down-regulated in both stem and leaves of SlHB8 overexpression plants, indicating a negative regulatory role of SlHB8 in the lignin biosynthesis and stem development.
Highlights
Stems are the central part of the plant, connected with the leaves up and the roots down, and transport important substances for long-distance cell-to-cell communication
Previous study showed that SlHB8 gene belongs to the HD-Zip III transcription factor family, as it contains the four conserved domains of HD, bZip, START and MEKHLA in the HD-Zip III transcription factor [49]
The results showed that SlHB8 gene expressed in all the stages examined, including 20 D, 30 D, 45 D and 60 D stages stem tissues
Summary
Stems are the central part of the plant, connected with the leaves up and the roots down, and transport important substances for long-distance cell-to-cell communication. In Arabidopsis, five Class III HD-Zip transcription factors (REVOLUTA/IFL1 (REV), (PHABULOSA/AtHB14) PHB, PHAVOLUTA/AtHB9 (PHV), CORONA (CAN/ATHB-15), and ATHB-8) were isolated with four recognizable domains including a DNA binding homeodomain followed immediately by a leucine zipper motif (HD-Zip); a sterol/lipid binding (START) domain for binding small hydrophobic molecules such as steroid, phospholipids, or carotenoids; and a PAS (Per-ARNT-Sim) domain for protein-protein interaction [6,7] These five HD-Zip III transcription factors were reported to play roles in the regulation of primary and secondary vascular cell differentiation [8,9,10,11], meristem maintenance [7], leaf patterning [12] and so on. There were MiR165/166 target sites in the coding sequences of HD-Zip III genes and their expression levels were negatively regulated by MiR165/166 [15,17]
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