Effect of Ammonia Production by Colletotrichum gloeosporioides on pelB Activation, Pectate Lyase Secretion, and Fruit Pathogenicity

Abstract

The accumulation of ammonia and associated tissue alkalinization predispose avocado fruit to attack by Colletotrichum gloeosporioides. Secretion of ammonia by C. gloeosporioides in the presence of KNO 3 was induced by decreasing the pH from 7.0 to 4.0. When the fungus was grown at pH 4.0 or 6.0 in the absence of a nitrogen source, ammonia did not accumulate, and neither pelB (encoding pectate lyase) transcription nor pectate lyase secretion was detected. Under these nitrogen starvation conditions, only transcriptional activation of areA, which encodes the global nitrogen regulator, was detected. pelB transcription and pectate lyase secretion were both detected when C. gloeosporioides was grown at pH 6.0 in the presence of ammonia accumulated from different nitrogen sources. The early accumulation of ammonia induced early pelB expression and pectate lyase secretion. As the external pH increased from 4.0 to 6.0, transcripts of pac1, the C. gloeosporioides pacC homolog, also could be detected. Nit mutants of C. gloeosporioides, which cannot utilize KNO 3 as a nitrogen source, did not secrete ammonia, alkalinize the medium, or secrete pectate lyase. If Nit mutants were grown at pH 6.0 in the presence of glutamate, then pectate lyase secretion was induced. Infiltration of 0.1 M ammonium hydroxide at pH 10 into ripening avocado fruits enhanced the activation of quiescent infection and symptom development by C. gloeosporioides. These results suggest that ambient pH alkalinization resulting from ammonia accumulation and the availability of ammonia as a nitrogen source independently regulate pelB expression, pectate lyase secretion, and virulence of C. gloeosporioides. These data suggest that alkalinization during C. gloeosporioides infection is important for its transformation from the quiescent biotrophic stage to the necrotrophic stage of fungal colonization in the fruit host.Colletotrichum gloeosporioides is a filamentous ascomycete plant pathogen that attacks the fruits of a variety of pre-and postharvest hosts. C. gloeosporioides germinates on avocado (Persea americana Miller var. drymifolia [Schlechtendal and Chamisso] S. F. Blake) fruit peel and forms appressoria. Infection hyphae from the appressoria penetrate the epidermal cells of the avocado exocarp but remain quiescent until the fruit ripens. The resistance of unripe fruit to fungal attack during quiescence is related to the presence of preformed or inducible antifungal compounds and the lack of secretion of fungal pathogenicity factors. The activation of quiescent infection results from a decline in the concentration of preformed antifungal compounds followed by the activation of fungal pathogenicity factors (27, 28). Pectate lyase (PL) has been implicated as a virulence factor of C. gloeosporioides in avocado fruit (26), and its expression is strongly affected by alkalinization (8). Alkalinization of the tissue occurs naturally during fruit ripening, where the pH of the pericarp increases from 5.2 to 6.1. However, the pathogen also helps increase the amoun...

Virulence Regulation of Phytopathogenic Fungi by pH

“…The extent of pH change in the plant tissue depends on host buffer capacity and initial pH (1,76,121,123). 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).…”

“…Indeed, fruits differ in their buffer capacity and pH (52, 53), and low pH was found to activate ammonia production and secretion in Colletotrichum spp. (1,76).…”

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

“…In the latter, alkalinization during fruit infection is necessary for the conversion of the biotrophic stage into the necrotrophic stage (Kramer-Haimovich et al, 2006). Also, ambient pH has been described as a regulatory factor related to the pathogenesis of S. sclerotiorum (Rollins and Dickman, 2001), C. gloesporoides (Alkan et al, 2013) and C. acutatum (You et al, 2007).…”

[No Title Available]

“…Colletotrichum is a filamentous fungus that affects fruit of many varieties of botanical species in pre and post-harvest, causing quiescent infections particularly in tropical and subtropical regions, resulting in losses of billions of dollars annually (Korsten and Jeffries, 2000;Kramer-Haimovich et al, 2006). Several factors are responsible for the post-harvest losses, including post-harvest fungal diseases, which cause considerable losses on the products collected, fruits and vegetables.…”