Chitosan Biosynthesis and Degradation: A Way to Modulate Plant Defenses in Endophytic Biocontrol Agents?

Nematophagous fungi have been employed in biological control to protect crops of interest because of their ability to manage nematodes sustainably. Pochonia chlamydosporia, a fungal parasite of nematode eggs and females is present worldwide responsible for natural suppression of soils to plant parasitic nematodes. The nematode egg-shell mainly is composed of a protein matrix embedding chitin microfibrils. P. chlamydosporia is a true endophyte of both mono and dicot crop plants which modulates their local and systemic defenses. Extracellular depolymerases of nematophagous fungi reflect their parasitic, endophytic and saprophytic traits. Focusing on plant cell wall degrading enzymes which allow fungi to colonize plants, P. chlamydosporia presents fewer cellulases, xylanases and pectinases than mycoparasite and nematode-trapping fungi. P. chlamydosporia genome show genes adapted to infect hosts with external barriers based on chitin/protein. For instance, P. chlamydosporia shows a highly expanded family of hydrolases and other enzymes related with chitin modification. Chitosan is a highly deacetylated form of chitin with antimicrobial activity. Entomopathogenic and nematophagous fungi are resistant to chitosan. Genomes of most isolates of P. chlamydosporia from worldwide origin show genes similar to those encoding putative chitin deacetylases and chitosanases. Most of these isolates display high parasitism to nematode eggs and degrade chitosan. However, we found no correlation between egg-parasitism and chitosanolytic activity. Chitin perception is a key component of the Plant Immune System. Chitin shielding/deacetylation in fungi is a way to circumvent plant defenses. Plant chitinases show less affinity for chitosan than chitin. Therefore, chitosan is a less efficient plant defense elicitor than chitin. We propose that chitosan production and degradation could protect endophytic biocontrol fungi such as P. chlamydosporia from plant defenses in the rhizosphere.