Local Modulation of Host pH byColletotrichumSpecies as a Mechanism to Increase Virulence

Abstract

The phytopathogenic fungus Colletotrichum gloeosporioides produces one pectate lyase (PL) that is a key virulence factor in disease development. During growth of C. gloeosporioides, Colletotrichum acutatum, and Colletotrichum coccodes in acidified yeast extract medium, the fungus secreted ammonia and increased the medium pH. Ammonia accumulation and the consequent pH change increased as a function of initial pH and buffer capacity of the medium. PL secretion by C. gloeosporioides correspondingly increased as the pH of the medium increased. The C. gloeosporioides pelB gene-disrupted mutant was able to increase ammonia accumulation and pH of the media similarly to the wild-type isolate. C. gloeosporioides in avocado, C. coccodes in tomato, and C. acutatum in apple showed ammonia accumulation in the infected area where pH increased to 7.5 to 8 and PL activity is optima. In nonhost interactions where C. gloeosporioides was inoculated in apples, the addition of ammonia-releasing compounds significantly enhanced pathogenicity to levels similar to those caused by the compatible C. acutatum-apple interaction. The results therefore suggest the importance of ammonia secretion as a virulence factor, enhancing environmental pH and pathogenicity of the Colletotrichum species.

Plant infection and the establishment of fungal biotrophy

“…Some Colletotrichum spp. produce ammonia in response to acidic pH, which seems to increase the pH in vitro and in the host plant to the optimum level for pectate lyase activity [23].…”

Extracellular enzymes ofColletotrichum fructicola isolates associated to Apple bitter rot and Glomerella leaf spot

“…The Colletotrichum ability to change the environmental pH from acidic to neutral has been known for decades (Kubicek et al 2014). This species produces ammonia in culture medium and in infected tissues causing alkalisation of the medium and consequently favours enzymatic secretion (Prusky et al 2001; Drori et al 2003). Thus, secretion of some extracellular enzymes, i.e.…”

“…In fact, causes of the lower activity found in vivo could be associated to the alkaline environment needed for gene expression and subsequently secretion of some extracellular enzymes. Another possibility could be related to the presence of preformed or induced antifungal compounds in leaf tissues which inhibit fungal extracellular enzymes (Prusky et al 2001). To clarify such differences between ABR- and GLS-isolate, for future research it would be interesting to evaluate gene expression of extracellular enzymes involved in virulence.…”

“…On the other hand, PNL is important especially during necrotrophic phase, but is also involved in the switch from biotrophic to necrotrophic phase (Shih et al 2000). Specific conditions in the host tissue, such as, temperature and pH, are also important factors for regulation of gene expression (Prusky et al 2001; Drori et al 2003) and subsequent enzyme production (Yakoby et al 2001). …”

Cloning and characterization of an endo--1,4-xylanase gene from Colletotrichum lindemuthianum and phylogenetic analysis of similar genes from phytopathogenic fungus

“…The regulation of lytic enzymes in some fungi (St. Leger et al, 1988) and gene expression in several species of Colletotrichum can be modulated by ambient pH (Prusky et al, 2001) and this mechanism of regulation operates in a vast diversity of organisms (Denison, 2000). Therefore, we considered it necessary to determine whether the pH of the extracellular medium varied during the period of incubation under the different culture conditions.…”