Fruit surface topography, timely transcriptional responses, and reduced susceptibility factors enhance anthracnose resistance in papaya

Anthracnose disease, caused by Colletotrichum spp., leads to significant postharvest losses in papaya fruit. In this study, we identified a papaya genotype with quantitative resistance to anthracnose and investigated the mechanisms underlying this resistance using microscopy, fruit physicochemical analyses, and transcriptomics. The resistant genotype exhibited several beneficial traits compared to a susceptible genotype, including a thicker cuticular layer, lower stomatal density, greater firmness, and lower total soluble sugars. These characteristics can be considered preformed barriers or constitutive defenses or correlate with reduced susceptibility factors. We also found that the resistant genotype responds more rapidly to fungal presence by synthesizing components that maintain surface and cell wall integrity, such as cutin and cuticular elements, and by depositing callose. In contrast, the susceptible genotype had accumulated more susceptibility factors, including elevated sugar content and activation of CW-degrading enzymes. While the susceptible genotype had a more robust immune response at the later stages of infection, the combination of susceptibility factors and a delayed response to the pathogen was insufficient to control the disease. This study highlights the need to characterize potential susceptibility factors and physicochemical traits to better understand fruit-pathogen interactions. Such knowledge can provide breeding programs with strong targets for developing crop varieties that are less susceptible to fungal diseases yet maintain quality traits that consumers expect. Overall design: Comparative transcriptomic analysis of natural anthracnose infections (Colletotrichum gloeosporioides, Colletotrichum truncatum) of Carica papaya at 5 and 6 days post harvest.

由炭疽菌属（Colletotrichum spp.）病原菌引发的炭疽病，会造成番木瓜果实严重的采后损失。本研究中，我们筛选得到一份对炭疽病具有定量抗性的番木瓜基因型，并通过显微镜观察、果实理化分析及转录组学技术，解析了该抗性背后的分子机制。与感病基因型相比，该抗病基因型具备多项优良性状：角质层更厚、气孔密度更低、果实硬度更高，且总可溶性糖含量更低。此类性状可被视为预形成屏障或组成型防御机制，或是与病原菌侵染易感性降低相关的关联因素。我们还发现，抗病基因型可通过合成维持表皮与细胞壁完整性的组分（如角质及表皮相关成分）、沉积胼胝质，更快地响应真菌侵染。与之相反，感病基因型则积累了更多易感性因子：糖含量升高，且激活了细胞壁降解酶。尽管感病基因型在侵染后期可触发更为强烈的免疫响应，但易感性因子与对病原菌的延迟响应共同作用，仍不足以抑制病害发生。本研究强调，需对潜在易感性因子及理化性状进行系统表征，以更深入地解析果实-病原菌互作机制。此类研究成果可为育种项目提供明确的靶标，助力培育出既抗真菌病害，又保留消费者所期望的优良品质性状的作物品种。实验设计：对采后第5天和第6天的番木瓜（Carica papaya）自然感染胶孢炭疽菌（Colletotrichum gloeosporioides）与平头炭疽菌（Colletotrichum truncatum）的样本开展比较转录组分析。