Does Methyl Salicylate, A Component of Herbivore-induced Plant Odour, Promote Sporulation of the Mite-pathogenic Fungus Neozygites tanajoae?

Abstract

Blends of volatile chemicals emanating from cassava leaves infested by the cassava green mite were found to promote conidiation of Neozygites tanajoae, an entomopathogenic fungus specific to this mite. Methyl salicylate (MeSA) is one compound frequently present in blends of herbivore-induced plant volatiles (HIPV) as well as that of mite-infested cassava. Here, we investigated the effect of methyl salicylate in its pure form on the production of pre-infective spores (conidia), and the germination of these spores into infective spores (capilliconidia), by a Brazilian isolate and a Beninese isolate of N. tanajoae. Mummified mites previously infected by the fungal isolates were screened under optimal abiotic conditions for sporulation inside tightly closed boxes with or without methyl salicylate diffusing from a capillary tube. Production of conidia was consistently higher (37%) when the Beninese isolate was exposed to MeSA than when not exposed to it (305.5 +/- 52.62 and 223.2 +/- 38.13 conidia per mummy with and without MeSA, respectively). MeSA, however, did not promote conidia production by the Brazilian isolate (387.4 +/- 44.74 and 415.8 +/- 57.95 conidia per mummy with and without MeSA, respectively). Germination of the conidia into capilliconidia was not affected by MeSA for either isolate (0.2%, 252.6 +/- 31.80 vs. 253.0 +/- 36.65 for the Beninese isolate and 4.2%, 268.5 +/- 37.90 vs. 280.2 +/- 29.43 for the Brazilian isolate). The effects of MeSA on the production of conidia were similar to those obtained under exposure to the complete blends of HIPV for the case of the Beninese isolate, but dissimilar (no promoting effect of MeSA) for the case of the Brazilian isolate. This shows that MeSA, being one compound out of many HIPV, can be a factor promoting sporulation of N. tanajoae, but it may not be the only factor as its effect varies with the fungal isolate under study.

Inhibitory and stimulatory effects of essential oils and individual monoterpenoids on growth and sporulation of four soil-borne fungal isolates of Aspergillus terreus, Fusarium oxysporum, Penicillium expansum, and Verticillium dahliae

“…Calvo et al (2002) speculated that seed fatty acids, like linoleic acid, can increase conidial development in Aspergillus by mimicking and/or interfering with signals that regulate fungal sporogenesis. There is evidence that abiotic stresses, such as excess of light, chemicals or mechanical damage, also increase fungal sporulation and/or conidial germination (Hountondji et al 2006). In our results, whenever a stimulatory effect on sporulation was apparent, the corresponding inhibitory effect on growth was absent, negligible or very low (0 to 9%).…”

Fungal entomopathogens in a tritrophic context

“…This suggests that the fungus is able to time its production of conidia to the presence of actively feeding hosts and not just solely plant cues. Follow up studies demonstrated that a common component of herbivore-induce plant volatiles, methyl salicylate promoted the production of primary conidia but not their germination into secondary capilliconidia (Hountondji et al 2006). This response was isolatedependent and differed from the earlier study, indicating that methyl salicylate is unlikely to be the only factor involved.…”

“…A chemical factor that facilitates fungal germination and increases insect mortality should promote plant fitness. One potential route already discussed is the role of plant volatiles, particularly those produced as a result of herbivore damage (Hountondji et al 2005(Hountondji et al , 2006. If a fungus can respond to herbivore-induced plant volatiles in a way that enhances the infection rate, such as by increased germination in the presence of the host, then fungal-induced mortality should also increase.…”

Plant-mediated effects on entomopathogenic fungi: how the olive tree influences fungal enemies of the olive moth, Prays oleae

“…The reductions in P. formosa spore viability and in B. bassiana sporulation in the presence of leaves were mainly due to the release of volatile compounds from these organs. Previous studies have reported contradictory effects of plant volatiles, showing either an inhibitory (Hountondji et al 2006) or no effect (Baverstock et al 2005) on fungal sporulation. This inconsistency was also found in the present study.…”