水稻徒長病菌 Fusarium fujikuroi 致病過程探討

Abstract

Bakanae disease of rice is caused by Fusarium fujikuroi Nirenberg. The disease occurs worldwide in rice-growing areas. The infected seedlings are thinner, taller, and paler than healthy ones. The infected adult plants show adventitious roots on the node or empty panicles, leading to yield losses ranging from 30-95%. Due to the lack of knowledge on the pathogenesis of bakanae disease of rice plant caused by F. fujikuroi, this study aims to explore the infection and colonization patterns of F. fujikuroi during different rice growth stages. Artificially inoculated rice seedlings [a resistant cultivar Tainung 67 (TNG67) and a susceptible cultivar Zerawchanica karatals (ZK)] and naturally infected adult plants [Kaohsiung 139 (KH139), Taikeng 2 (TK2), and Tainan 11 (TN11)] were investigated by hand sectioning and microscopic observation as well as real-time quantitative PCR (qPCR). A green fluorescent protein (GFP)-tagged F. fujikuroi transformant generated by protoplast-polyethylene glycol (PEG) transformation method was used for artificial inoculation. By fluorescence microscopy, we observed direct penetration of F. fujikuroi hyphae through the epidermis of stem and roots. F. fujikuroi infected intra- and intercellularly from the embryo to the basal part of rice stems and roots during early infection stage. qPCR analysis of consecutive 3-cm segments of the whole seedlings revealed that F. fujikuroi was mainly detected in the embryo, basal stem, and basal roots. Since 14 days post inoculation, the biomass of F. fujikuroi in the embryo became much higher than in the stem and roots. TNG67 and ZK contained similar amounts of F. fujikuroi in the stem and root tissues, implying that the macroscopic bakanae phenotypes were not necessarily correlated with the pathogen quantity. Treatment of germinated seeds with 0.5 mg/L of gibberellic acid-3 (GA3) resulted in significantly more elongated seedlings of ZK than TNG67, suggesting that the bakanae susceptibility in ZK is likely associated with its higher sensitivity to GA3. In the infected plants at the tillering and booting stage, the hyphae were found heavily colonizing almost entirely in vascular bundles in cultivars KH139 and TK2 but in TN11 hyphae colonizing less abundantly in vascular bundles as well as the surrounding parenchyma cells. Moreover, F. fujikuroi was mainly detected in the stem below 50 cm (~60% of plant height), but undetectable in the leaves. F. fujikuroi showed no preference for nodes or internodes, and the emergence of adventitious roots from a certain node did not indicate node invasion of F. fujikuroi. In addition, the appearance of adventitious roots did not reflect abundant colonization of F. fujikuroi in the plant. We hope this study can provide new insights into the interactions between F. fujikuroi and rice.