Fungal cell walls represent the frontline contact with the host and play a prime role in pathogenesis. While the roles of the cell wall polymers like chitin and branched β-glucan are well understood in vegetative and pathogenic development, that of the most prominent galactose-containing polymers galactosaminogalactan and fungal-type galactomannan is unknown in plant pathogenic fungi. Mining the genome of the maize pathogen Colletotrichum graminicola identified the single-copy key galactose metabolism genes UGE1 and UGM1, encoding a UDP-glucose-4-epimerase and UDP-galactopyranose mutase, respectively. UGE1 is thought to be required for biosynthesis of both polymers, whereas UGM1 is specifically required for fungal-type galactomannan formation. Promoter:eGFP fusion strains revealed that both genes are expressed in vegetative and in pathogenic hyphae at all stages of pathogenesis. Targeted deletion of UGE1 and UGM1, and fluorescence-labeling of galactosaminogalactan and fungal-type galactomannan confirmed that Δuge1 mutants were unable to synthesize either of these polymers, and Δugm1 mutants did not exhibit fungal-type galactomannan. Appressoria of Δuge1, but not of Δugm1 mutants, were defective in adhesion, highlighting a function of galactosaminogalactan in the establishment of these infection cells on hydrophobic surfaces. Both Δuge1 and Δugm1 mutants showed cell wall defects in older vegetative hyphae and severely reduced appressorial penetration competence. On intact leaves of Zea mays, both mutants showed strongly reduced disease symptom severity, indicating that UGE1 and UGM1 represent novel virulence factors of C. graminicola.

中文翻译：

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半乳糖代谢基因UGE1和UGM1是玉米炭疽病菌Colletotrichum graminicola的新型毒力因子

真菌细胞壁代表与宿主的前线接触，并在发病机制中发挥主要作用。虽然细胞壁聚合物如甲壳素和支链β-葡聚糖在营养和病原发育中的作用已被充分了解，但最重要的含半乳糖聚合物半乳糖氨基半乳聚糖和真菌型半乳甘露聚糖在植物病原真菌中的作用尚不清楚。通过挖掘玉米病原体炭疽菌的基因组，鉴定出单拷贝关键半乳糖代谢基因UGE1和UGM1，分别编码 UDP-葡萄糖-4-差向异构酶和 UDP-吡喃半乳糖变位酶。UGE1被认为是两种聚合物的生物合成所必需的，而UGM1是真菌型半乳甘露聚糖形成所特别需要的。启动子：eGFP融合菌株显示这两个基因在发病机制的各个阶段的营养菌丝和致病菌丝中表达。UGE1和UGM1的靶向删除以及半乳糖氨基半乳聚糖和真菌型半乳甘露聚糖的荧光标记证实了 Δ uge1突变体无法合成这些聚合物中的任何一种，并且 Δ ugm1突变体不表现出真菌型半乳甘露聚糖。 Δ uge1的附着胞（而非 Δ ugm1突变体）在粘附方面存在缺陷，这突出了半乳糖氨基半乳聚糖在疏水表面上建立这些感染细胞中的功能。 Δ uge1和 Δ ugm1突变体均在较老的营养菌丝中表现出细胞壁缺陷，并且附着渗透能力严重降低。在玉米的完整叶子上，两种突变体都表现出明显降低的疾病症状严重程度，表明UGE1和UGM1代表了C. graminicola的新毒力因子。