Abstract
Lignocellulose, composed of cellulose, hemicellulose, and lignin, in the secondary cell wall constitutes wood and is the most abundant form of biomass on Earth. Enhancement of wood accumulation may be an effective strategy to increase biomass as well as wood strength, but currently only limited research has been undertaken. Here, we demonstrated that OsSWN1, the orthologue of the rice NAC Secondary-wall Thickening factor (NST) transcription factor, effectively enhanced secondary cell wall formation in the Arabidopsis inflorescence stem and poplar (Populus tremula×Populus tremuloides) stem when expressed by the Arabidopsis NST3 promoter. Interestingly, in transgenic Arabidopsis and poplar, ectopic secondary cell wall deposition in the pith area was observed in addition to densification of the secondary cell wall in fiber cells. The cell wall content or density of the stem increased on average by up to 38% and 39% in Arabidopsis and poplar, respectively, without causing growth inhibition. As a result, physical strength of the stem increased by up to 57% in poplar. Collectively, these data suggest that the reinforcement of wood by NST3pro:OsSWN1 is a promising strategy to enhance wood-biomass production in dicotyledonous plant species.
Highlights
NAC SECONDARY WALL THICKENING PROMOTING FACTOR (NST) 1‒2, and NST3/SECONDARY CELL WALL ASSOCIATED NAC DOMAIN PROTEIN1 (SND1), transcription factors containing the NAC domain, are master regulators of secondary cell wall formation except in vessel elements[1,2,3]
We further observed that overexpression of rice functional orthologue of NST1-3/SND1, Oryza sativa SECONDARY WALL NAC DOMAIN PROTEIN (OsSWN) 1, more strongly induced ectopic secondary cell wall deposition in epidermis cells of Arabidopsis than did Arabidopsis NSTs12, suggesting that OsSWN1 is a promising candidate gene to select for induction of secondary cell wall accumulation in dicot
In the previous study[12], we demonstrated that OsSWN1 could restore the phenotype of nst[1] nst[] double mutant which doesn’t possess any secondary cell wall in fiber cells when its expression was driven by NST3 promoter, which was previously shown that it is active in interfascicular fiber cells of the inflorescence stem of Arabidopsis[2]
Summary
NAC SECONDARY WALL THICKENING PROMOTING FACTOR (NST) 1‒2, and NST3/SECONDARY CELL WALL ASSOCIATED NAC DOMAIN PROTEIN1 (SND1), transcription factors containing the NAC domain, are master regulators of secondary cell wall formation except in vessel elements[1,2,3]. Wang et al.[7] reported expansion of the area of secondary cell wall formation was observed in the stem of wrky[12] knockout mutants of Arabidopsis and Medicago truncatula This phenomenon was due to upregulation of NST2 and other secondary-wall-associated transcription factors, which are negatively regulated by WRKY127. We demonstrated that expression of OsSWN1 under the control of the NST3 promoter in Arabidopsis induces ectopic secondary cell wall formation in the pith in addition to thickening of the secondary cell wall in fiber cells This enhanced secondary cell wall formation was observed in transgenic poplar (Populus tremula× Populus tremuloides) harboring the same construct without any growth retardation. These data indicated that enhancement of secondary cell wall formation by OsSWN1 driven by the AtNST3 promoter may be a valuable approach to boost biomass production for industrial uses of wood
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