Case study on Ustilaginoidea virens, the causal agent of rice false smut in India

Abstract

Rice false smut, caused by the non-obligate biotrophic fungus Ustilaginoidea virens, is an emerging threat in major rice-growing regions worldwide. The pathogen primarily infects floral organs, leading to the formation of characteristic smut balls. However, the early infection process and disease cycle remain poorly understood, particularly regarding pathogen colonization during vegetative growth stages. This study aimed to elucidate the early stages of U. virens infection from germination to the late booting stage. Field-emission scanning electron microscopy revealed that conidia and chlamydospores of U. virens colonized various rice plant parts prior to floral infection. During early infection (seed to 35-day-old seedlings), the seedlings turned yellow and subsequently became blighted. In contrast, colonization from 60-day-old plants to the late booting stage occurred without visible symptoms. Molecular detection using U. virens-specific primers (US1-5/US3-3) confirmed the presence of U. virens sampled growth stages and plant parts, suggesting continuous asymptomatic colonization before floral infection. Mating-type analysis of 22 U. virens isolates collected from Telangana, India, revealed three isolates with the MAT1-1-1 idiomorph, seven with MAT1-2-1, and twelve containing both MAT1-1-1 and MAT1-2-1 idiomorphs, indicating the presence of homothallic isolates. Simple sequence repeat analysis grouped the 22 isolates into four clusters, with approximately 60% of isolates clustering according to their geographic origin. These findings provide novel insights into the early colonization behavior and reproductive diversity of U. virens, including the first report of homothallic isolates in India.