Root factors stimulate ³²P uptake and plasmalemma ATPase activity in vesicular‐arbuscular mycorrhizal fungus, Gigaspora margarita

Abstract

S!_! M M A R YRoot-inducing (Ri) T-DKA transformed roots of carrot were used as the plant partner in a study of ^^P absorption and plasmalemma ATPase activity in the hyphae of the vesicular-arbuscular mycorrhizal fungus Gigaspora margarita Becker & Hall. Hyphae from germinating spores were grown in the presence or absence of root exudates and volatiles. In the presence of these root factors, ^-P isotope labelling occurred in hyphae, auxiliary cells and spores, while in the absence of these factors, the labelling only occurred in fungal spores. Similarly, ATPase activity appeared on the fungal plasmalemma only in the presence of root factors. The use of the .ATPase inhibitor, diethylstilbestrol, demonstrated the importance of plasmalemma ATPase in the stimulation of hyphal growth of this obligately biotrophic fungus.

Transmembrane electric potential difference of germ tubes of arbuscular mycorrhizal fungi responds to external stimuli

“…They suggested that the high affinity system was likely to be a 2H + :H # PO % − co-transporter while the low affinity system could be facilitated diffusion of P (Smith & Smith, 1990). By contrast, Lei et al (1991) found, using microautoradiography, that germ tubes of Gi. margarita were only able to absorb $#P if root exudates and volatiles were present.…”

“…In the absence of a suitable host, hyphal elongation ceases after a few days and the cytoplasm is withdrawn from the hyphae. Cessation of growth is thought to occur before spore reserves are completely exhausted (Lei et al, 1991). We do not know if the lack of continuing growth indicates that some substance is lacking in the medium or whether the germ tube is unable to take up nutrients until it has been stimulated by a signal from a host.…”

“…Using cytochemical methods, Lei et al (1991) showed that ATPase activity only occurred in germ tubes which had been stimulated by root extracts and volatiles. The increase in p.d.…”

“…margarita was chosen for these experiments because its spores are large ( 100 µm diameter) and because it has been used in other studies of germtube physiology (Thomson et al, 1990 ;Lei et al, 1991 ;Berbara et al, 1995).…”

“…card & Piche! , 1990 ; Lei et al, 1991) measurement of p.d. allows us to show that the fungus responds almost instantaneously to stimulation.…”