拟南芥半纤维素多糖修饰对抗铝性的影响及其分子机制

Al toxicity is one of the major factors limiting crop production in acidic soils as ionic Al will be released from the minerals when the soil pH is decreased to lower than 5.5, and Al ions at micromolar level will inhibit root elongation and damage the structure and function of root tip, thus affects the uptake of nutrients and water, and finally reduce the crop yield. Cell wall is the first plant organ to contact with Al from the growth medium, and most Al entering plant roots is also found to be fixed in the cell wall, therefore, cell wall plays a very important role in conferring Al resistance in plant. However, how Al is bound to cell wall components and its underlying physiological and molecualr mechanisms are largely unknown. With the support of the previous NSFC project, we demonstrated that one of the XTH genes involved in hemicellulose biosynthesis and modification, XTH31, could control the xyloglucan content in the root cell wall, thus affect the amount of Al accumulated in the root. The mutation of XTH31 resulted in the decrease of xyloglucan content and Al accumulation in the root, thus the mutant xth31 became Al resistance. The current project is based on the problems we found in the last project, and will foucus on uncovering the XTHx that can form dimers with XTH31 to encode XET activity to conrol xyloglucan content, and key genes encoding acetylase of polysaccharides of hemicellulose that are expressed in roots and reponse to Al stress by testing various XTH and acetylase related Arabidopsis mutants, then, we will investigate the physiological and molecular mechanisms how these genes affect the components and the degree of acetylation of polysaccharide components, which in turn affect the Al binding capacity of the cell wall and confering Al resistance or sensitivity. With these studies, we hope to deep our understanding on the contribution of hemicellulose polysaccharides and its acetylation to Al resistance and provide the theoretical bases to genetically improve crop resistance to Al toxicity by modifying cell wall components and their properies.

铝毒是限制酸性土壤上作物生产力的主要因素。细胞壁是第一个接触生长介质中的铝的植物器官，同时，进入根系的铝极大部分被细胞壁所固定，因此，细胞壁在植物抗铝作用中起着十分重要的作用，但对其结合铝的机制及其调控分子基础还不清楚。前期研究我们发现与拟南芥细胞壁半纤维素合成与修饰相关基因XTH31通过调控木葡聚糖含量，影响细胞壁结合铝的能力，并进而改变植物抗铝性,说明半纤维素多糖在调控植物抗铝性上起着关键作用。本项目将以拟南芥相关突变体为材料，探明与XTH31互作的XTHx蛋白基因和在根尖表达、且响应铝胁迫的多糖乙酰化的关键基因，在此基础上，解析上述基因如何调控细胞壁半纤维多糖组分和乙酰化程度改变细胞壁对铝的结合能力及其对植物抗铝性的影响。通过上述研究可望深入了解细胞壁XTH基因和多糖乙酰化基因在调控植物抗铝性中的分子生理基础，丰富植物抗铝机制的理论知识。

铝毒是限制酸性土壤上作物生产力的主要因素。细胞壁是第一个接触生长介质中的铝的植物器官，同时，进入根系的铝极大部分被细胞壁所固定，因此，细胞壁在植物抗铝作用中起着十分重要的作用，但对其结合铝的机制及其调控分子基础还不清楚。前期研究我们发现细胞壁的半纤维素才是结合铝的主要组分。而作为调控组成半纤维素的主要多糖木葡聚糖含量的XTH31，通过影响半纤维素中的能结合铝的木葡聚糖含量，而改变植物的抗铝性。但同时我们也发现，XTH31本身主要表现为水解酶的活性(XTH),无法解释xth31突变体XET活性大幅度下降的现象；此外，木葡聚糖还存在结构上的差异与乙酰化程度的差异，那这两者的差异是否会影响半纤维素结合铝的能力也有待解明。本项目正是针对上述两个根本问题提出的，经过4年来的研究，取得了以下富有创新意义的成果：我们通过找到了能与XTH31互作并富于XET活性的XTH17蛋白，而且xth17和xth31的表型基本相近，均表现为抗铝性提高， xth17细胞壁的铝含量显著降低，并伴随着细胞壁半纤维素总糖含量的降低。我们的结果表明XTH17与XTH31以二聚体的形式调控XET酶活，进而调节拟南芥根系细胞壁对铝的积累量，最终影响拟南芥的耐铝性；其次我们发现2个木葡聚糖O-乙酰化程度降低的突变体axy4-1，axy4-3耐铝性显著下降与突变体根系和细胞壁积累更多的铝相关，但突变体细胞壁果胶含量，果胶甲酯酶（PME）活性，果胶中的铝含量，半纤维素含量均与野生型没有显著差异，由此推测，木葡聚糖O-乙酰化程度的降低使铝在植物细胞壁积累增加，致使突变体对铝更敏感；最后，我们发现木葡聚糖岩藻化糖程度增加和减少的各2个突变体，其细胞壁结合铝的量与野生型相比分别提高和减少，从而使进入细胞内的铝分别减少和增加，最终导致铝抗性分别减弱和增强。通过上述研究使我们深入了解细胞壁XTH基因、多糖乙酰化基因和岩藻糖化基因在调控植物抗铝性中的分子生理基础，丰富植物抗铝机制的理论知识。上述成果分别发表在2篇Pant Physiology和1篇Scientific reports上。