Mol Plant Microbe Interact. 13(2): 238-41.(Feb 2000)
Is a Fully Established Arbuscular Mycorrhizal Symbiosis Required for Bioprotection of Pisum sativum Roots against Aphanomyces euteiches?
Sophie Slezack, Eliane Dumas-Gaudot, Michel Paynot, and Silvio Gianinazzi

Bioprotection of pea roots against Aphanomyces euteiches by the arbuscular mycorrhizal fungus G. mosseae was demonstrated to depend on a fully established symbiosis. This was related with induction of mycorrrhiza-related chitinolytic enzymes. Possible mechanisms implicated in bioprotection are discussed.

Pea (Pisum sativum L.) root rot caused by Aphanomyces euteiches is one of the most serious pea diseases in several countries (Mauffras et al. 1997; Papavizas and Ayers 1974). The difficulties in controlling this disease have prompted a search for biological alternatives (Parke et al. 1991; Rao et al. 1995). Mycorrhizal plants are able to overcome biotic stresses (Azcón-Aguilar and Barea 1996; St. Arnaud et al. 1995). Rosendahl (1985) reported first that A. euteiches infection of pea roots was reduced in plants colonized by the arbuscular mycorrhizal (AM) fungus Glomus fasciculatum. However, molecular mechanisms governing bioprotection by AM fungi are poorly understood (Azcón-Aguilar and Barea 1996; Hooker et al. 1994; St. Arnaud et al. 1995). Furthermore, the conditions allowing its expression are ambiguous (Caron 1989; St. Arnaud et al. 1995). While several studies reported a reduction in damages caused by a pathogenic fungus after coinoculation, or post-inoculation, with an AM fungus (Caron et al. 1986; Krishna and Bagyaraj 1983), others showed that only a well-established AM infection could protect plants (Cordier et al. 1996,, 1998; Dassi et al. 1998; Rosendahl 1985). Among molecular changes related to AM symbiosis formation, the induction of additional chitinase and chitosanase isoforms was reported (Dumas-Gaudot et al. 1994; Pozo et al. 1996, 1998; Slezack et al. 1996). Both categories of enzymes are generally known to be involved in plant resistance (El Ouakfaoui and Asselin 1992; Graham and Sticklen 1994).

By using the Myc1 pea mutant on the one hand and hormonal treatment on the other, we clearly demonstrate that bioprotection by G. mosseae against A. euteiches is dependent on a fully established symbiosis with the presence of arbuscules. Furthermore, our results give some information on the mechanisms that could be implicated in this bioprotection. Modifications of mycorrhizosphere were suggested to play a role in bioprotection by AM fungi (Azcón-Aguilar and Barea 1996). Here, the relationship described between AM colonization, reduction of disease symptoms, and pathogen amount strongly suggests that colonization by itself is an essential requirement for bioprotection. Bioprotection of peas against A. euteiches could result from pre-activation of plant defense responses by AM fungi, as suggested by Gianinazzi (1991). Gollotte et al. (1993) showed the activation of several plant defense responses in G. mosseae-inoculated Myc1 pea mutants. How ever, these defense responses are not sufficient to protect plants against the pathogen, as reported here. Cordier et al. (1998) demonstrated that induced resistance against Phytophthora parasitica in mycorrhizal tomato roots resulted from both localized defense responses in arbuscule-containing cells and systemic defense responses in non-mycorrhizal parts of mycorrhizal roots. By using Ga3 treatment, we show the connection between the suppression of arbuscules, the nonexpression of mycorrhiza-related chitinolytic isoforms, and the suppression of bioprotection. It is tempting to speculate on the possible involvement of the mycorrhiza-related chitinolytic isoforms in bioprotection. Recently, one of these chitinase isoforms has been partially sequenced, confirming its plant origin (Slezack 1999). To make a full assessment of the involvement of these enzymes in bioprotection, it remains to isolate mycorrhiza-related chitinase genes to see whether inhibition of the expression of such genes will have any effect on bioprotection.

Mycorrhizal Fungi