Differential Activation of Ammonium Transporters During the Accumulation of Ammonia by Colletotrichum gloeosporioides and Its Effect on Appressoria Formation and Pathogenicity

Abstract

Ammonium secreted by the post-harvest pathogen Colletotrichum gloeosporioides during host colonization accumulates in the host environment due to enhanced fungal nitrogen metabolism. Two types of ammonium transporter-encoding genes, AMET and MEP, are expressed during pathogenicity. Gene disruption of AMET, a gene modulating ammonia secretion, showed twofold reduced ammonia secretion and 45% less colonization on avocado fruit, suggesting a contribution to pathogenicity. MEPB, a gene modulating ammonium transport, is expressed by C. gloeosporioides during pathogenicity and starvation conditions in culture. Gene disruption of MEPB, the most highly expressed gene of the MEP family, resulted in twofold overexpression of MEPA and MEPC but reduced colonization, suggesting MEPB expression's contribution to pathogenicity. Analysis of internal and external ammonia accumulation by ΔmepB strains in mycelia and germinated spores showed rapid uptake and accumulation, and reduced secretion of ammonia in the mutant versus wild-type (WT) strains. Ammonia uptake by the WT germinating spores but not by the ΔmepB strain with compromised ammonium transport activated cAMP and transcription of PKA subunits PKAR and PKA2. ΔmepB mutants showed 75% less appressorium formation and colonization than the WT, which was partially restored by 10 mM exogenous ammonia. Thus, whereas both AMET and MEPB genes modulate ammonia secretion, only MEPB contributes to ammonia accumulation by mycelia and germinating spores that activate the cAMP pathways, inducing the morphogenetic processes contributing to C. gloeosporioides pathogenicity.

Simultaneous transcriptome analysis of Colletotrichum gloeosporioides and tomato fruit pathosystem reveals novel fungal pathogenicity and fruit defense strategies

“…Fungal appressoria are characterized by melanization of the cell wall and concomitant accumulation of glycerol to generate a system that provides highly localized turgor pressure focused to puncture cellular barriers (de Jong et al ., ). Cyclic AMP (cAMP; Lee & Dean, ; Shnaiderman et al ., ) and the MAPK signaling pathways are known to play a role in inducing appressoria formation and melanin biosynthesis in Colletotrichum (Takano et al ., ), Magnaporthe (Lee & Dean, ; Xu & Hamer, ; Hamer & Talbot, ) and Botrytis (Liu et al ., ). Putative transcripts encoding cAMP‐dependent kinases (3/6 of all expressed cAMP‐dependent kinases) were identified as appressoria‐specific and are candidates for activation of melanin biosynthesis (Fig.…”

Genome sequence of Valsa canker pathogens uncovers a potential adaptation of colonization of woody bark

“…4). In addition, three of the five ammonia transporters (Amt) of V. mali that contribute to nitrogen uptake (Garnica et al, 2013) and necrotrophic colonization (Shnaiderman et al, 2013) are significantly up-regulated in planta (Table S5), while only three of the 57 sugar transporters are up-regulated (Fig. 4b).…”

Carbon regulation of environmental pH by secreted small molecules that modulate pathogenicity in phytopathogenic fungi

“…Indeed, ammonia accumulation has been shown to depend on the initial environmental pH and, at pH 4, Colletotrichum coccodes showed a 3.5-fold higher ammonia production than at pH 7 (Alkan et al, 2008). The accumulation of ammonia has been shown to further contribute to necrotrophic colonization (Prusky and Yakoby, 2003;Prusky et al, 2013;Shnaiderman et al, 2013) through the activation of host RBOH (NADPH oxidase) to generate reactive oxygen species, thereby accelerating localized host cell death (Alkan et al, 2009). Hence, the modulation of host pH by fungi enhances their virulence in ripening fruit (Prusky, 1996).…”