Flavonoids stimulate spore germination in Fusarium solani pathogenic on legumes in a manner sensitive to inhibitors of cAMP-dependent protein kinase

Many soilborne fungal plant pathogens remain as resting propagules until the appearance of a potential host stimulates their germination. The plant-derived stimulus for germination has generally been assumed to be nutrients exuded from roots. We show that certain flavonoids, including defense-related isoflavonoid phytoalexins, stimulate spore germination of Fusarium solani formae speciales pathogenic on pea or bean. The stimulatory action of specific flavonoids are consistent with the flavonoids previously identified in root exudates of these two hosts and with the levels of flavonoids reported to be exuded by bean roots. Inhibitors of cAMP-dependent protein kinase (PKA) prevented flavonoid-responsive germination, but not nutrient-responsive germination. Thus, these two stimuli, flavonoids and nutrients, appear to utilize separate signal pathways to initiate germination. Germination of macroconidia in root exudates was significantly inhibited by a PKA inhibitor, indicating that flavonoids present in root exudates may be at least as active as nutrients in stimulating germination. These results suggest that flavonoids in legume root exudate may be perceived as a signal in a number of plant-microbe interactions, not only for initiating symbiotic rhizobial interactions but also for initiating pathogenic fungal interactions