Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9.
Plant Physiol. 2011 Jul;156(3):1351-63. doi: 10.1104/pp.111.172676. Epub 2011 May 19.
Bast (phloem) fibers, tension wood fibers, and other cells with gelatinous-type secondary walls are rich in crystalline cellulose. In developing bast fibers of flax (Linum usitatissimum), a galactan-enriched matrix (Gn-layer) is gradually modified into a mature cellulosic gelatinous-layer (G-layer), which ultimately comprises most of the secondary cell wall. Previous studies have correlated this maturation process with expression of a putative β-galactosidase. Here, we demonstrate that β-galactosidase activity is in fact necessary for the dynamic remodeling of polysaccharides that occurs during normal secondary wall development in flax fibers. We found that developing stems of transgenic (LuBGAL-RNAi) flax with reduced β-galactosidase activity had lower concentrations of free Gal and had significant reductions in the thickness of mature cellulosic G-layers compared with controls. Conversely, Gn-layers, labeled intensively by the galactan-specific LM5 antibody, were greatly expanded in LuBGAL-RNAi transgenic plants. Gross morphology and stem anatomy, including the thickness of bast fiber walls, were otherwise unaffected by silencing of β-galactosidase transcripts. These results demonstrate a specific requirement for β-galactosidase in hydrolysis of galactans during formation of cellulosic G-layers. Transgenic lines with reduced β-galactosidase activity also had biochemical and spectroscopic properties consistent with a reduction in cellulose crystallinity. We further demonstrated that the tensile strength of normal flax stems is dependent on β-galactosidase-mediated development of the phloem fiber G-layer. Thus, the mechanical strength that typifies flax stems is dependent on a thick, cellulosic G-layer, which itself depends on β-galactosidase activity within the precursor Gn-layer. These observations demonstrate a novel role for matrix polysaccharides in cellulose deposition; the relevance of these observations to the development of cell walls in other species is also discussed.
韧皮纤维、张力木纤维和其他具有凝胶型次生壁的细胞富含结晶纤维素。在亚麻（Linum usitatissimum）发育中的韧皮纤维中，富含半乳糖的基质（Gn 层）逐渐被改造成成熟的纤维素凝胶层（G 层），最终构成次生细胞壁的大部分。以前的研究将这个成熟过程与一个假定的β-半乳糖苷酶的表达相关联。在这里，我们证明β-半乳糖苷酶活性实际上是亚麻纤维正常次生壁发育过程中多糖动态重塑所必需的。我们发现，与对照相比，β-半乳糖苷酶活性降低的转基因（LuBGAL-RNAi）亚麻发育中的茎具有较低浓度的游离半乳糖，并且成熟的纤维素 G 层的厚度显著减小。相反，用半乳糖特异性 LM5 抗体强烈标记的 Gn 层在 LuBGAL-RNAi 转基因植物中大大扩展。总体形态和茎解剖结构，包括韧皮纤维壁的厚度，否则不受β-半乳糖苷酶转录物沉默的影响。这些结果表明，β-半乳糖苷酶在形成纤维素 G 层时对半乳糖苷的水解具有特定的要求。β-半乳糖苷酶活性降低的转基因系也具有与纤维素结晶度降低相一致的生化和光谱特性。我们进一步证明，正常亚麻茎的拉伸强度依赖于β-半乳糖苷酶介导的韧皮纤维 G 层的发育。因此，典型的亚麻茎的机械强度依赖于厚的、纤维素 G 层，而 G 层本身依赖于 Gn 层中的β-半乳糖苷酶活性。这些观察结果表明基质多糖在纤维素沉积中的一个新作用；还讨论了这些观察结果对其他物种细胞壁发育的相关性。
