DIFFERENTIAL EXPRESSION OF PEROXIDASE ISOGENES DURING THE EARLY STAGES OF INFECTION OF THE TROPICAL FORAGE LEGUME STYLOSANTHES HUMILIS BY COLLETOTRICHUM-GLOEOSPORIOIDES

Infection of Stylosanthes humilis by the fungal phytopathogen Colletotrichum gloeosporioides is associated with an increase in peroxidase enzyme activity within 24 h postinoculation. Peroxidase gene expression was investigated as a first step towards understanding the regulation and functional importance of this host response to fungal attack. Four distinct cDNAs Shpx 2, 5, 6, and 12, isolated from a cDNA library of S. humilis contained deduced amino acid (aa) sequence motifs characteristic of peroxidases. Three of these (Shpx 2, 5, and 6) were full-length and their deduced proteins each fell into a different homology group based on comparisons with other plant peroxidases. Each cDNA appeared to hybridize to only one or two genes in S. humilis. mRNAs corresponding to Shpx2, Shpx6, and Shpx12, were expressed relatively abundantly in young leaves, with lesser expression of Shpx2 and Shpx6 and no expression of Shpx12 detected in roots. No expression of these genes was detected in stems or old leaves. The mRNA of Shpx5 was relatively abundant in stems and to a lesser extent in young leaves. However, infection of young leaves with C. gloeosporioides greatly increased expression of the mRNAs of Shpx2 and Shpx6 but not Shpx5 nor Shpx12 compared to mock-inoculated controls. The mRNA of Shpx6 was strongly induced by the pathogen 4 h postinoculation, a time which precedes fungal penetration, while Shpx2 was induced to higher levels than controls at 24 h after inoculation. The mRNAs of both Shpx2 and Shpx6 but not Shpx5 and Shpx12 were also induced by wounding. These results indicate that specific host peroxidase isogenes are induced at very early stages of the interaction of C. gloeosporioides with S. humilis and that host recognition of the pathogen appears to occur prior to physical penetration of the epidermal cell wall.