Temporal changes in colonization levels of Ustilago esculenta in different parts of Zizania latifolia plants competent and incompetent for stem gall formation

Zizania latifolia plants infected with the basidiomycete fungus Ustilago esculenta develop an edible stem gall, called makomotake in Japan. In our paddy fields, we often find the appearance of Z. latifolia plants that grow only vegetatively without forming stem galls. In this study, we performed PCR-based quantitative assays to investigate the temporal changes in the colonization levels of U. esculenta in various parts of Z. latifolia plants that formed and those that did not form stem galls. Throughout the cultivation period, the colonization levels of U. esculenta in plants that formed stem gall were always higher at almost all plant parts examined than those in plants that did not form stem galls. In particular, after the vegetative stage, the plants that formed stem galls showed an increasing pattern of the fungal colonization in the culm and leaf, a decreasing pattern in the culm base, and an unchanged pattern in the runner. In contrast, the fungal colonization levels in the plants that did not form stem galls consistently remained very low at all plant parts from the seedling stage and beyond. In addition, histological observation of plant tissues showed that the hyphal structures and clusters were found extensively in plants with stem gall formation but very occasionally in plants without stem gall formation. Our results suggest that high colonization levels of U. esculenta during the seedling stage and its subsequent efficient systemic proliferation in the plant are strongly associated with successful stem gall formation.