Phosphonate transport in Phytophthora capsici

Abstract

Phosphate was shown to reverse the in vitro activity of phosphonate ions against P. capsici (strain 375) more efficiently in liquid than in solid media. Phosphonate transport by mycelia incubated in aqueous solutions was enhanced by a previous phosphate starvation and the presence of K þ cations. The intracellular phosphonate concentration reached a constant level and this concentration, which is a function of the external concentration, fits a hyperbolic relationship. Phosphonate transport was greatly stimulated when mycelia were incubated in modified Ribeiro's medium. In the absence of any phosphorus source in the growth medium, the phosphate content of mycelia at stationary phase of growth decreased. This sign of phosphate deficiency was intensified in the presence of phosphonate. INTRODUCTIONPhosphonate is known to be the systemicallymobile degradation product of the anti-oomycete fungicide fosetyl-Al (Bompeix & Saindrenan, 1984;Fenn & Coffey, 1984;Darakis et al., 1985). This anion was shown to be the active metabolite of the fungicide, since in both soil and plant tissues, fosetyl-Al breaks down rapidly to phosphonate (Smilie et al., 1988;Fenn & Coffey, 1989). Contrary to other systemic fungicides, phosphonate was shown to possess only weak direct activity against some species of Phytophthora despite its ability to control these pathogens in vivo (Guest, 1984(Guest, , 1986. In fact, phosphonate reduces the mycelial growth of several species of Phytophthora when applied directly during in vitro tests, but at concentrations generally higher than those achieved in phosphonate-or fosetyl-Al-treated plants Nemestothy & Guest, 1990). However, in Phytophthora species that are more sensitive in vitro, the phosphonate content of plant tissues could account for direct inhibition of fungal growth (Bompeix & Saindrenan, 1984;Darakis et al., 1985;Guest & Grant, 1991).Recent studies on the mode of action of phosphonate are orientated towards the possibility that the fungus is the primary target of the compound, whereas the plant's natural defence mechanisms have the main role in the inhibition of fungal growth, particularly in cases where species are more sensitive in vivo Guest & Grant, 1991;Barchietto et al., 1992). A number of studies have identified alterations to fungal metabolism Niere et al., 1990Niere et al., , 1994Barchietto et al., 1992) and changes of morphology (Jiang, 1990;Jiang & Grossmann, 1992). These changes might account for both direct fungal inhibition and modification of the interactions between host and parasite. It was suggested, therefore, that a perturbation of phosphorus metabolism may also affect the virulence mechanisms of the pathogen (Saindrenan et al., 1990;Smillie et al., 1990;Guest, 1995). However, little is known about the relationships between the direct and the indirect action. Furthermore, it was established that there is a phosphonate/phosphate antagonism within the fungal cell, since phosphate (Pi) sometimes reversed the antifungal action of phosphonate in in vitro tests (Bompeix & ...

Suppression of the in vitro growth and development of Microdochium nivale by phosphite

“…Darakis et al . () concluded the presence of K facilitated phosphite uptake into Phytophthora capsici hyphae. If mycelial growth suppression is used as an indicator of increased phosphite assimilation, then this enhanced assimilation of phosphite in the presence of K may have occurred, as KH 2 PO 3 produced statistically significantly ( P < 0.05) greater inhibition compared to H 3 PO 3 .…”

“…B€ ucking & Heyser (1999) stated that the presence of K facilitates the uptake of mobile polyphosphate into fungal cells, maintaining that it helps retain the charge balance and pH of the fungal cell and is the counter ion to the transport of polyphosphates into the vacuole. Darakis et al (1997) concluded the presence of K facilitated phosphite uptake into Phytophthora capsici hyphae. If mycelial growth suppression is used as an indicator of increased phosphite assimilation, then this enhanced assimilation of phosphite in the presence of K may have occurred, as KH 2 PO 3 produced statistically significantly (P < 0.05) greater inhibition compared to H 3 PO 3 .…”

Field Evaluation of Integrated Management for Mitigating Citrus Huanglongbing in Florida

“…Fosetyl-Al (ALI) is a phosphonate that demonstrated antimicrobial effects when used to treat infections caused by various microorganisms (Darakis et al, 1997; Panicker and Gangadharan, 1999). Several studies demonstrated that the mode of action of phosphonates can alter fungal metabolism (Guest and Grant, 1991) and cell morphology (Jiang, 1990; Jiang and Grossmann, 1992).…”

Effectiveness of AG3 phosphonate formulation in suppressing phytophthora blight in cucumber and bell pepper plants under growth room conditions

“…In our study, increasing concentrations of AG3 phosphonate inhibited the growth of P. capsici on V8 agar medium but not in solutions. These results may be partly explained by the fact that P. capsici is known as one of the phosphonate-tolerant species in the genus Phytophthora (Bompeix & Saindrenan, 1984;Darakis et al, 1997). The concentration of phosphate in the medium can also have an effect on in vitro toxicity of phosphonate (Bompeix & Saindrenan, 1984;Darakis et al, 1997).…”

“…These results may be partly explained by the fact that P. capsici is known as one of the phosphonate-tolerant species in the genus Phytophthora (Bompeix & Saindrenan, 1984;Darakis et al, 1997). The concentration of phosphate in the medium can also have an effect on in vitro toxicity of phosphonate (Bompeix & Saindrenan, 1984;Darakis et al, 1997). Whether the effect of AG3 phosphonate against the P. capsici is direct or indirect, control of damping-off or root rot of cucumber or phytophthora blight of bell peppers was evident in the peat-based substrate or sandy-loam soil artificially infested with the soil inoculum of the pathogen.…”