Scoparone eliciting activity released by phosphonic acid treatment of Phytophthora citrophthora mycelia mimics the incompatible response of phosphonic acid-treated Citrus leaves inoculated with this fungus

Abstract

Citrus leaves floating on H<inf>3</inf>PO<inf>3</inf> (10 μg/ml) were protected from infection by a strain of Phytophthora citrophtora sensitive in vitro to H<inf>3</inf>PO<inf>3</inf> (S strain). Protection was associated with significant scoparone accumulation at the infection site. On the contrary, an H<inf>3</inf>PO<inf>3</inf> relatively insensitive (RI) mutant of the fungus infected leaves floating on H<inf>3</inf>PO<inf>3</inf> (10 μg/ml), and did not produce scoparone accumulation. S and RI strains were infectious in leaves floating on control buffer devoid of H<inf>3</inf>PO<inf>3</inf>. Both scoparone accumulation and inhibition of the S strain in Citrus leaves floating on H<inf>3</inf>PO<inf>3</inf> (10 μg/ml) were reversed by pretreatment with AOA. In vitro incubation of P. citrophthora mycelia in H<inf>3</inf>PO<inf>3</inf> (10 μg/ml) released a significant scoparone eliciting activity from the S strain, but not from the RI strain, as tested in Citrus leaves. Application of the scoparone eliciting preparation of S strain towards Citrus leaves protected them against infection by either S or RI strain; both scoparone accumulation and leaf protection were suppressed by pretreatment of the leaves with AOA. Thus, scoparone elicitors released in vitro by H<inf>3</inf>PO<inf>3</inf> (10 μg/ml) treatment of mycelia of the S strain (but not of the RI strain) of P. citrophthora, mimic the incompatible response observed in H<inf>3</inf>PO<inf>3</inf>-treated Citrus leaves inoculated with the S strain (but not with the RI strain) of the fungus. © 1993.