Comparative analysis of "chromosome for pathogenicity" in phytopathogenic fungi

我们发现，在alternnaria alternata中，宿主特异性毒素（host-specific toxin， HST）的产量控制着植物的致病性，目前列出的基因专门参与每种或一组HST合成的代谢途径，而参与HST产生的基因仅位于一条有条件可有可无的小染色体上。在本研究中，我们重点比较分析了黄菖蒲病型间条件可有性染色体（CD染色体）的结构和功能，以了解植物病原菌进化和分化的分子基础。我们构建了三套CD染色体特异性BAC文库和三套苹果、草莓和番茄病型的cDNA文库。对感兴趣的克隆进行DNA测序，鉴定出参与HST生物合成的基因。共鉴定出16个与草莓病原菌HST生物合成有关的基因，分布在约300kb的区域。每个HST.3在苹果型中分别鉴定出2个基因，在番茄型中分别鉴定出13个位于150-kb区域的基因。苹果病型和草莓病型在CD染色体上保守一个醛酮还原酶基因，苹果病型产生含环肽的HST，草莓病型产生含环氧十三烯酸（EDT）的HST。引入的草莓病原型同源物可以在功能上弥补苹果病原型同源物的敲除功能。在苹果和/或草莓致病型和番茄致病型的CD染色体中，任何HST生物合成基因都不保守。这些结果表明，苹果和草莓的CD染色体可能来源于一条古老的染色体，而番茄的CD染色体可能不是来源于一条古老的染色体，尽管三种病原菌在同一种a.m alternata中，这为植物病原真菌的分化提供了一个新的例子。

We have found, in Alternaria alternata, productivity of host-specific toxin(HST) controls pathogenicity on plants, genes listed so far were specifically involved in metabolic pathway of synthesis for each of or a group of HST, and genes involved in HST production located only on a small chromosome that was conditionally dispensable. In this research, we focused on comparative analysis of structure and function of conditionally dispensable chromosome(CD chromosome) among pathotypes of A.alternata to understand molecular basis for evolution and differentiation of plant pathogen.1.We constructed three sets of CD chromosome-specific BAC library and cDNA library from three pathotypes of A.alternata (apple, strawberry and tomato pathotype). Clones of interests were subjected to DNA sequencing and genes involved in HST biosynthesis were identified.2.Sixteen genes located around 300-kb region were identified for HST biosynthesis of strawberry pathotype. Two genes were identified in apple pathotype and thirteen genes located around 150-kb region were also identified in tomato pathotype, respectively for each HST.3.One gene for aldo-keto reductase was conserved between CD chromosomes of apple and strawberry pathotype while apple pathotype produces a HST containing cyclic peptide and strawberry pathotype produces HSTs containing epoxy-decatrienoic acid (EDT). The introduced homolog of strawberry pathotype could functionally complement knocked out function of the other homolog in apple pathotype.4.Any genes for HST biosynthesis were not conserved between CD chromosomes of apple and/or strawberry pathotype and tomato pathotype that produces a HST containing polyketide moiety.These results revealed CD chromosomes of apple and strawberry may be derived from an ancient chromosome but that of tomato may be not although three pathotypes are in the same species A.alternata and suggest a novel example of differentiation of phytopathogenic fungi.