Plant and Environmental Protection Sciences, University of Hawaii at Manoa, 3190 Maile Way, St John 317, Honolulu, HI 96822, USA.
Plant J. 2013 Aug;75(3):498-514. doi: 10.1111/tpj.12217. Epub 2013 May 24.
Alternaria brassicicola is a successful saprophyte and necrotrophic plant pathogen. To identify molecular determinants of pathogenicity, we created non-pathogenic mutants of a transcription factor-encoding gene, AbPf2. The frequency and timing of germination and appressorium formation on host plants were similar between the non-pathogenic ∆abpf2 mutants and wild-type A. brassicicola. The mutants were also similar in vitro to wild-type A. brassicicola in terms of vegetative growth, conidium production, and responses to a phytoalexin, reactive oxygen species and osmolites. The hyphae of the mutants grew slowly but did not cause disease symptoms on the surface of host plants. Transcripts of the AbPf2 gene increased exponentially soon after wild-type conidia contacted their host plants . A small amount of AbPf2 protein, as monitored using GFP fusions, was present in young, mature conidia. The protein level decreased during saprophytic growth, but increased and was located primarily in fungal nuclei during pathogenesis. Levels of the proteins and transcripts sharply decreased following colonization of host tissues beyond the initial infection site. When expression of the transcription factor was induced in the wild-type during early pathogenesis, 106 fungal genes were also induced in the wild-type but not in the ∆abpf2 mutants. Notably, 33 of the 106 genes encoded secreted proteins, including eight putative effector proteins. Plants inoculated with ∆abpf2 mutants expressed higher levels of genes associated with photosynthesis, the pentose phosphate pathway and primary metabolism, but lower levels of defense-related genes. Our results suggest that AbPf2 is an important regulator of pathogenesis, but does not affect other cellular processes in A. brassicicola.
链格孢菌 Alternaria brassicicola 是一种成功的腐生菌和植物坏死性病原菌。为了鉴定致病性的分子决定因素，我们创建了一个转录因子编码基因 AbPf2 的非致病性突变体。非致病性 ∆abpf2 突变体与野生型 A. brassicicola 之间，在宿主植物上的萌发和附着胞形成的频率和时间相似。在体外，突变体在营养生长、分生孢子产生以及对植物抗毒素、活性氧和渗透物的反应方面与野生型 A. brassicicola 相似。突变体的菌丝生长缓慢，但在宿主植物表面不会引起疾病症状。AbPf2 基因的转录本在野生型分生孢子接触其宿主植物后很快呈指数级增长。使用 GFP 融合物监测到，在年轻、成熟的分生孢子中存在少量 AbPf2 蛋白。在腐生生长过程中，蛋白质水平下降，但在发病过程中增加，并主要位于真菌核中。当转录因子在野生型中被诱导时，106 个真菌基因也在野生型中被诱导，但在 ∆abpf2 突变体中没有被诱导。值得注意的是，这 106 个基因中的 33 个编码分泌蛋白，包括 8 个推定的效应蛋白。用 ∆abpf2 突变体接种的植物表达了与光合作用、戊糖磷酸途径和初级代谢相关的基因水平较高，但与防御相关的基因水平较低。我们的结果表明，AbPf2 是发病机制的一个重要调控因子，但不影响 A. brassicicola 中的其他细胞过程。
