Role of Nitrogen-Metabolism Genes Expressed During Pathogenicity of the Alkalinizing Colletotrichum gloeosporioides and Their Differential Expression in Acidifying Pathogens

Abstract

Pathogens can actively alter fruit pH around the infection site, signaling modulation of pathogenicity-factor expression, as found for alkalinizing (Colletotrichum and Alternaria spp.) and acidifying (Penicillium, Botrytis, and Sclerotinia spp.) fungi. The nitrogen-metabolism genes GDH2, GS1, GLT, and MEP genes are differentially expressed during colonization by Colletotrichum gloeosporioides, and a Δgdh2 strain reduces ammonia accumulation and pathogenicity. We analyzed the contribution of transporters GLT and MEPB to C. gloeosporiodes pathogenicity. Germinating spores of Δglt strains showed reduced appressorium formation; those of ΔmepB mutants showed rapid ammonia uptake and accumulation inside the hyphae, indicating deregulated uptake. Both mutants reduced pathogenicity, indicating that these transporters function during alkalinizing species pathogenicity. We compared the expressions of these genes in C. gloeosporioides and Sclerotinia sclerotiorum, and found five to 10-fold higher expression at the transcript level in the former. Interestingly, GLT and MEPB in the alkalinizing species showed no and very low sequence identity, respectively, with their counterparts in the acidifying species. Knockout analysis of GLT and MEPB and their differential transcript regulation in the alkalinizing and acidifying species suggest that the ammonia accumulation contributing to pathogenicity in the former is modulated by factors at the gene-regulation levels that are lacking in the acidifying species.

Identification of positive selection in disease response genes within members of the Poaceae

“…These transporters may play important roles in plant defense against alkalinizing pathogens as Colletotrichum spp, which secrete ammonia to increase the pH of the host tissue and infect the host. 36,37 The second cluster is 31447249-e, which contains proteins with similarity to the PRELI/MSF1 domain, and has three sites that have undergone episodic selection. The function of the PRELI/MSF1 domain is unknown, although it is conserved in lipid-binding proteins and proteins involved in vesicle transport and secretion mechanisms.…”

Virulence Regulation of Phytopathogenic Fungi by pH

“…The ability of the pathogen to alter the pH locally has been described initially for Sclerotinia sclerotiorum, but similar findings been have extended to some other pathogens, such as Colletotrichum spp., Alternaria alternata, Botrytis cinerea, Penicillium expansum, P. digitatum, P. italicum, Phomopsis mangiferae, and Fusarium oxysporum (39, 52, 53, 89,100,101,123,124,129). To simplify the terminology in the following discussion, fungi that alkalinize the environment will be referred to as ''alkaline fungi'' and those that acidify it will be called ''acidic fungi'' (Fig.…”

“…3). Ammonium accumulation has been detected in association with pathogenicity of many Colletotrichum species, including C. gloeosporioides, C. acutatum, C. higgisianum, C. graminicola, and C. coccodes (1,46,100,110); Alternaria alternata (52, 53); and Fusarium oxysporum (101). The ammonium secreted by Colletotrichum spp., F. oxysporum and A. alternata result in alkalization of the host tissue, and its concentration can reach approximately 5 mM, as found in decayed avocado, tomato, and persimmon fruits (1,52,53,100).…”

“…(1,2,100). Recent findings that F. oxysporum, C. graminicola, and C. higginsianum secrete ammonia (101,110) suggest that various fungal pathogens modulate the surrounding pH to create an optimal pH for expression and activity of their pathogenicity factors.…”

“…phaseoli to alkalinize the environment by ammonia secretion (101) or to acidify the extracellular pH by secretion of fusaric acid (18,25,26,40) indicates that a given pathogen can deploy various tools, and it may activate any of them, depending on the host conditions.…”