Department of Organismic Interactions, Max Planck Institute for Terrestrial Microbiology, Karl-von-Frisch-Straße 10, Marburg, 35043, Germany.
New Phytol. 2021 Jul;231(1):399-415. doi: 10.1111/nph.17377. Epub 2021 May 2.
Ustilago maydis is a biotrophic fungus causing smut disease in corn. The infectious forms are dikaryotic hyphae. Here we analyze mutants lacking the nlt1 transcription factor and investigate why these mutants are unable to induce leaf tumors. The study involved reverse genetics, complementation, epistasis analysis, microscopy, gene expression analysis by quantitative reverse transcriptase PCR and virulence assays. We show that nlt1 mutants colonize maize leaves efficiently but fail to undergo karyogamy and are attenuated in late proliferation. Nlt1 activates transcription of ros1, a transcription factor controlling karyogamy, and represses see1, an effector previously shown to contribute to leaf tumor induction. In mononuclate solopathogenic strains, nlt1 mutants cause attenuated leaf tumor formation. In actively dividing maize organs, nlt1 mutants undergo karyogamy and induce tumor formation. Sporisorium reilianum, a smut fungus unable to induce leaf tumors, possesses an ortholog of nlt1 that controls the fusion of dikaryotic nuclei late in infection during cob colonization. Our results have established a regulatory connection between nlt1, ros1 and see1 and suggest the existence of two stages contributing to leaf tumor formation, one before nuclear fusion and involving nlt1 and one after karyogamy that is nlt1 independent.
玉米黑粉菌是一种生物寄生性真菌，可引起玉米黑粉病。其感染形式为双核菌丝。在这里，我们分析了缺乏 nlt1 转录因子的突变体，并研究了这些突变体为何无法诱导叶片肿瘤。本研究涉及反向遗传学、互补、上位性分析、显微镜检查、定量逆转录 PCR 基因表达分析和毒力测定。我们表明，nlt1 突变体能有效地定殖玉米叶片，但不能进行核融合，并且在晚期增殖中减弱。Nlt1 激活控制核融合的转录因子 ros1 的转录，并抑制 see1 的表达，后者先前被证明有助于诱导叶片肿瘤。在单核 solopathogenic 菌株中，nlt1 突变体导致叶片肿瘤形成减弱。在活跃分裂的玉米器官中，nlt1 突变体进行核融合并诱导肿瘤形成。黑粉菌，一种不能诱导叶片肿瘤的黑粉菌，拥有 nlt1 的同源物，该同源物控制在 cob 定殖过程中感染后期双核核的融合。我们的研究结果建立了 nlt1、ros1 和 see1 之间的调控联系，并表明存在两个促进叶片肿瘤形成的阶段，一个是在核融合之前，涉及 nlt1，另一个是在核融合之后，与 nlt1 无关。
