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Applied and Environmental Microbiology, February 2006, p. 1034-1039, Vol. 72, No. 2
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.2.1034-1039.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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

H. Kramer-Haimovich,^1, E. Servi,^1, T. Katan,² J. Rollins,⁴ Y. Okon,³ and D. Prusky^1*

Department of Postharvest Science of Fresh Produce,¹ Department of Plant Pathology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel,² Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Faculty of Agricultural, Food and Environmental Quality Sciences, Rehovot 76100, Israel,³ Department of Plant Pathology, University of Florida, Gainesville, Florida 32611⁴

Received 27 April 2005/ Accepted 10 November 2005

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 KNO₃ 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 KNO₃ 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.

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* Corresponding author. Mailing address: Department of Postharvest Science of Fresh Produce, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel. Phone: 972-3-9693880. Fax: 972-3-9683622. E-mail: dovprusk{at}agri.gov.il.

H. Kramer-Haimovich and E. Servi contributed equally to this work.

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Applied and Environmental Microbiology, February 2006, p. 1034-1039, Vol. 72, No. 2
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.2.1034-1039.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.