Germination and invasion of Paraphoma radicina on roots of a susceptible and a resistant alfalfa cultivar (Medicago sativa)

Abstract

Alfalfa Paraphoma root rot (APRR) (Paraphoma radicina) is a new recently described alfalfa disease widely distributed in China, first reported in 2020. So far, the resistance levels of 30 alfalfa cultivars to APRR have been characterized. However, the resistance mechanisms among these cultivars remain unknown. To understand the resistance mechanism against APRR, we investigated the response of susceptible (Gibraltar) and resistant (Magnum Ⅱ) alfalfa cultivar roots to infection by P. radicina under the light microscope (LM) and scanning electronic microscope (SEM). In addition, we compared the conidial germination and germ tube growth in the root exudates of different resistant cultivars. The results revealed that conidial germination, germ tube development, and P. radicina penetration into root tissues of resistant plants were delayed. In susceptible and resistant cultivars, P. radicina infected roots by penetrating epidermal cells and the intercellular space between epidermal cells. During the infection process, germ tubes penetrated the root surface directly or formed appressoria through which they infected the root. However, the penetration percentage on the susceptible cultivar was significantly higher than on the resistant cultivar, irrespective of the infection route. Moreover, disintegrated conidia and germ tubes were observed on resistant cultivar roots at 48 hour post-inoculation The conidia germination and germ tube growth in root exudates of susceptible cultivars were significantly higher than in resistant cultivars. Therefore, our findings imply that the resistance differences between alfalfa cultivars might be related to root exudates. These findings provide insights into the alfalfa resistant mechanism following P. radicina infection.