Boudjeko Thaddée, Andème-Onzighi Christine, Vicré Maïté, Balangé Alain-Pierre, Ndoumou Denis Omokolo, Driouich Azeddine
UMR CNRS 6037, IFRMP 23. Centre Commun de Microscopie Electronique, Université de Rouen, 76821 Mont Saint Aignan, France.
Planta. 2006 Jan;223(2):271-82. doi: 10.1007/s00425-005-0090-2. Epub 2005 Sep 14.
Cocoyam (Xanthosoma sagittifolium) is an important tuber crop in most tropical zones of Africa and America. In Cameroon, its cultivation is hampered by a soil-borne fungus Pythium myriotylum which is responsible for root rot disease. The mechanism of root colonisation by the fungus has yet to be elucidated. In this study, using microscopical and immunocytochemical methods, we provide a new evidence regarding the mode of action of the fungus and we describe the reaction of the plant to the early stages of fungal invasion. We show that the fungal attack begins with the colonisation of the peripheral and epidermal cells of the root apex. These cells are rapidly lost upon infection, while cortical and stele cells are not. Labelling with the cationic gold, which binds to negatively charged wall polymers such as pectins, is absent in cortical cells and in the interfacial zone of the infected roots while it is abundant in the cell walls of stele cells. A similar pattern of labelling is also found when using the anti-pectin monoclonal antibody JIM5, but not with anti-xyloglucan antibodies. This suggests that early during infection, the fungus causes a significant loss of pectin probably via degradation by hydrolytic enzymes that diffuse and act away from the site of attack. Additional support for pectin loss is the demonstration, via sugar analysis, that a significant decrease in galacturonic acid content occurred in infected root cell walls. In addition, we demonstrate that one of the early reactions of X. sagittifolium to the fungal invasion is the formation of wall appositions that are rich in callose and cellulose.
芋（黄肉芋）是非洲和美洲大部分热带地区的一种重要块茎作物。在喀麦隆，其种植受到一种土传真菌——繁茂腐霉的阻碍，这种真菌会引发根腐病。该真菌定殖于根部的机制尚未阐明。在本研究中，我们使用显微镜和免疫细胞化学方法，提供了关于该真菌作用方式的新证据，并描述了植物对真菌早期入侵的反应。我们发现真菌的攻击始于根尖外周细胞和表皮细胞的定殖。感染后这些细胞会迅速死亡，而皮层细胞和中柱细胞则不会。用阳离子金标记时，皮层细胞和受感染根的界面区域没有标记，而阳离子金会与带负电荷的壁聚合物（如果胶）结合，中柱细胞的细胞壁中则有大量标记。使用抗果胶单克隆抗体JIM5时也发现了类似的标记模式，但使用抗木葡聚糖抗体时未发现。这表明在感染早期，真菌可能通过水解酶扩散并在远离攻击部位起作用而导致果胶大量损失。通过糖分析表明感染的根细胞壁中半乳糖醛酸含量显著降低，这进一步支持了果胶损失的观点。此外，我们证明了黄肉芋对真菌入侵的早期反应之一是形成富含胼胝质和纤维素的壁沉积物。
