Cloning, disruption, and expression of two endo-beta 1, 4-xylanase genes, XYL2 and XYL3, from Cochliobolus carbonum

Abstract

In culture, the filamentous fungus Cochliobolus carbonum, a pathogen of maize, makes three cationic xylanases, XYL1, which encodes the major endoxylanase (Xyl1), was earlier cloned and shown by gene disruption to encode the first and second peaks of xylanase activity (P. C. Apel, D. G. Panaccione, F. R. Holden, and J. D. Walton, Mol. Plant-Microbe Interact. 6:467-473, 1993). Two additional xylanase genes, XYL2 and XYL3, have now been cloned from C. carbonum. XYL2 and XYL3 are predicted to encode 22-kDa family G xylanases similar to Xyl1. Xyl2 and Xyl3 are 60% and 42% identical, respectively, to Xyl1, and Xyl2 and Xyl3 are 39% identical. XYL1 and XYL2 but not XYL3 mRNAs are present in C. carbonum grown in culture, and XYL1 and XYL3 but not XYL2 mRNAs are present in infected plants. Transformation-mediated gene disruption was used to construct strains mutated in XYL1, XYL2, and XYL3. Xyl1 accounts for most of the total xylanase activity in culture, and disruption of XYL2 or XYL3 does not result in the further loss of any xylanase activity. In particular, the third peak of cationic xylanase activity is still present in a xyl1 xyl2 xyl3 triple mutant, and therefore this xylanase must be encoded by yet a fourth xylanase gene. A minor protein of 22 kDa that can be detected immunologically in the xyl1 mutant disappears in the xyl2 mutant and is therefore proposed to be the product of XYL2. The single xylanase mutants were crossed with each other to obtain multiple xylanase disruptions within the same strain. Strains disrupted in combinations of two and in all three xylanases were obtained. The triple mutant grows at the same rate as the wild type on xylan and on maize cell walls. The triple mutant is still fully pathogenic on maize with regard to lesion size, morphology, and rate of lesion development.