Mechanisms of host-recognition by plant pathogens mediated by signaling factors for pathogenicity

真菌的致病性被预测为在感染过程中释放致病性所必需的信号分子。尽管在许多植物病原真菌中真菌致病的必要组分仍然是未知的，但已知某些真菌病原体，特别是链格孢属和旋孢属，产生宿主特异性毒素（HST）作为相容剂。目前已发现的链格孢菌有10余种，主要是A. alternata致病型，已知产生HST，和大多数毒素的结构已被阐明。根据HST的化学结构，将其分为聚酮化合物、环肽和癸三烯酸酯三大类。最近，分别参与AAL毒素和AM毒素生物合成的聚酮合成酶基因和环肽合成酶基因已被克隆和鉴定。在这两种情况下，有针对性的基因破坏导致消除毒素的生产和致病性的损失，表明HST生物合成基因是真菌致病性决定因素。这些HST生物合成基因也存在于条件性染色体上，仅在致病性和HST产生菌株中发现，并且在非致病性菌株中未检测到该基因的同源物。这表明，这些基因可能是通过水平基因转移获得的，并提供了一种可能的机制，使新的致病型可能在自然界中出现。

Fungal pathogenicity are predicted to release signaling molecules essential for path ogenicity during the process of infection. Although essential components for fungal pathogenesis are still unknown in many plant pathogenic fungi, certain fungal pathogens, especially the genera Alternaria and Cochliobolus, are known to produce host-specific toxins (HSTs) as compatibility agents. There are now ten or more Alternaria pathogena, mainly A. alternata pathotypes, known to produce HSTs, and the structures of most of the toxins have been elucidated. HSTs from A.alternata pathotypes have been classified into three groups based on their chemical structures, I.e.polyketides, cyclic peptides and decatrienoic acid esters. Recently a polyketied synthetase gene and a cyclic-peptide synthetase gene, involved in AAL-toxin and AM-toxin biosynthesis, respectively, have been cloned and characterized. In both cases, targeted gene disruption lead to elimination of toxin production and loss of pathogenisity indicating that HST biosynthetic genes are fungal pathogenicity determinants. These HST biosynthetic genes also reside on conditionally dispensable chromosomes found only in the pathogenic and HST-producing strains of A.alrernata and homologues of the genes were not detected in nonpathogenic strains of A.alrernata. This suggests that these genes may have been acquired by horizontal gene transfer and provides a possible mechanism whereby new pathotypes could arise in nature.