ABSTRACT The contribution of
gluconic acid secretion to the colonization of apple tissue by Penicillium expansum was analyzed by modulation (increase or decrease) of
gluconic acid accumulation at the
infection court. P. expansum isolates that express the most gox2 transcripts and concomitant
glucose oxidase (GOX) activity and that secrete the most
gluconic acid cause disease of apple at the fastest rate. Cultures grown under reduced
oxygen concentration generated fewer gox2 transcripts, produced less
gluconic acid, and led to a 15% reduction in disease. Furthermore, the detection of significantly high levels of transcripts of gox2 and GOX activity at the edge of the decaying tissue emphasize the involvement of GOX in tissue acidification of the decaying tissue. Taken together, these results emphasize the importance of GOX in the production of the
gluconic acid that leads, in turn, to host tissue acidification. This acidification enhanced the expression of pectolytic
enzymes and the establishment of conditions for necrotrophic development of P. expansum.