Chitosan Mediates Germling Adhesion in Magnaporthe oryzae and Is Required for Surface Sensing and Germling Morphogenesis.
Chitosan Mediates Germling Adhesion in Magnaporthe oryzae and Is Required for Surface Sensing and Germling Morphogenesis.
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DOI：
10.1371/journal.ppat.1005703
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发表时间：
2016-06
期刊：
影响因子：
6.7
通讯作者：
Gurr SJ
中科院分区：
文献类型：
--
作者：
Geoghegan IA;Gurr SJ
真菌细胞壁不仅在维持细胞完整性方面起着至关重要的作用，而且还构成了真菌与环境之间的界面。因此，细胞壁的组成可以影响真菌与其物理和生物环境的相互作用。甲壳素是壳壁的主要多糖成分之一，可以通过脱乙酰基进行化学修饰。这一反应由一系列称为甲壳素脱乙酰酶(CDA)的酶催化，并导致壳聚糖的形成，这是一种β1，4-氨基葡萄糖的聚合物。此前已有研究表明，在植物病原真菌的侵染结构中，壳聚糖会积聚在细胞壁中。在这里，它一直被认为是一种“隐形”分子，是完整发病机制所必需的。在这项研究中，我们使用作物病原菌和模式生物稻瘟菌来验证这一假说。我们首先证实了壳聚糖定位于胚管和附着胞，然后根据CDA基因在附着胞分化过程中转录水平的升高而将其删除。缺失菌株的种苗表现出甲壳素脱乙酰基的缺失，并在人工疏水表面附着和形成附着体的能力受到损害。令人惊讶的是，外源壳聚糖的加入完全恢复了种子的附着和附着胞的发育。尽管人工表面缺乏附着胞的发育，但突变菌株的致病性没有受到影响。进一步的分析表明，在植物表面附着胞发育过程中，角质蜡足以超过对壳聚糖的需求。因此，壳聚糖并不起到“隐形”分子的作用，而是起到了促进附着胞发育所必需的物理刺激的作用。这项研究揭示了壳聚糖在植物病原真菌中的新作用，并进一步深入了解了米根霉附着胞发育的机制。稻瘟病菌(Magnaporthe Oryzae)是一种丝状真菌，对水稻造成毁灭性的损失。对寄主的成功入侵取决于真菌保持不被植物固有免疫系统检测的能力，该系统识别真菌细胞壁的保守成分，如甲壳素。先前的研究表明，与感染相关的细胞壁成分的变化是必要的，以使真菌在感染期间保持不被检测到。长期以来，人们一直认为这种变化具有“隐形机制”的作用，那就是被称为甲壳素脱乙酰酶的酶对甲壳素多糖的脱乙酰基作用。甲壳素的脱乙酰基产生一种被称为壳聚糖的多糖，以前已经证明这种多糖专门积累在植物病原真菌的感染结构上。然而，在这项研究中，我们表明胚芽定位的壳聚糖不是致病所必需的，在这个阶段反对作为一种‘隐形机制’的作用。相反，附着胞的发育需要壳聚糖，附着胞是植物细胞穿透所需的一种关键的真菌感染结构。这一要求可以归因于壳聚糖介导的细菌对表面的黏附，这是感知物理刺激所必需的。
The fungal cell wall not only plays a critical role in maintaining cellular integrity, but also forms the interface between fungi and their environment. The composition of the cell wall can therefore influence the interactions of fungi with their physical and biological environments. Chitin, one of the main polysaccharide components of the wall, can be chemically modified by deacetylation. This reaction is catalyzed by a family of enzymes known as chitin deacetylases (CDAs), and results in the formation of chitosan, a polymer of β1,4-glucosamine. Chitosan has previously been shown to accumulate in the cell wall of infection structures in phytopathogenic fungi. Here, it has long been hypothesized to act as a 'stealth' molecule, necessary for full pathogenesis. In this study, we used the crop pathogen and model organism Magnaporthe oryzae to test this hypothesis. We first confirmed that chitosan localizes to the germ tube and appressorium, then deleted CDA genes on the basis of their elevated transcript levels during appressorium differentiation. Germlings of the deletion strains showed loss of chitin deacetylation, and were compromised in their ability to adhere and form appressoria on artificial hydrophobic surfaces. Surprisingly, the addition of exogenous chitosan fully restored germling adhesion and appressorium development. Despite the lack of appressorium development on artificial surfaces, pathogenicity was unaffected in the mutant strains. Further analyses demonstrated that cuticular waxes are sufficient to over-ride the requirement for chitosan during appressorium development on the plant surface. Thus, chitosan does not have a role as a 'stealth' molecule, but instead mediates the adhesion of germlings to surfaces, thereby allowing the perception of the physical stimuli necessary to promote appressorium development. This study thus reveals a novel role for chitosan in phytopathogenic fungi, and gives further insight into the mechanisms governing appressorium development in M.oryzae. Magnaporthe oryzae is a filamentous fungal pathogen which causes devastating crop losses in rice. Successful invasion of the host is dependent upon the ability of the fungus to remain undetected by the innate immune system of the plant, which recognizes conserved components of the fungal cell wall, such as chitin. Previous studies have demonstrated that infection-related changes in cell wall composition are necessary to allow the fungus to remain undetected during infection. One such change that has long been hypothesized to have a role as a 'stealth mechanism' is the deacetylation of the polysaccharide chitin by enzymes known as chitin deacetylases. The deacetylation of chitin produces a polysaccharide known as chitosan, which has previously been shown to accumulate specifically on infection structures in plant pathogenic fungi. However, in this study, we show that germling-localized chitosan is not required for pathogenicity, arguing against a role as a 'stealth mechanism' at this stage. Instead, chitosan is required for the development of the appressorium, a critical fungal infection structure required for the penetration of plant cells. This requirement can be attributed to chitosan mediating the adhesion of germlings to surfaces, which is required for the perception of physical stimuli.