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 Colletot richum gloeosporioides is associated with an increase in peroxidase en zyme 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 character istic of peroxidases. Three of these (Shpx 2, 5, and 6) were full-leng th and their deduced proteins each fell into a different homology grou p based on comparisons with other plant peroxidases. Each cDNA appeare d to hybridize to only one or two genes in S. humilis. mRNAs correspon ding to Shpx2, Shpx6, and Shpx12, were expressed relatively abundantly in young leaves, with lesser expression of Shpx2 and Shpx6 and no exp ression of Shpx12 detected in roots. No expression of these genes was detected in stems or old leaves. The mRNA of Shpx5 was relatively abun dant in stems and to a lesser extent in young leaves. However, infecti on of young leaves with C. gloeosporioides greatly increased expressio n 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 t he pathogen 4 h postinoculation, a time which precedes fungal penetrat ion, while Shpx2 was induced to higher levels than controls at 24 h af ter inoculation. The mRNAs of both Shpx2 and Shpx6 but not Shpx5 and S hpx12 were also induced by wounding. These results indicate that speci fic host peroxidase isogenes are induced at very early stages of the i nteraction of C. gloeosporioides with S. humilis and that host recogni tion of the pathogen appears to occur prior to physical penetration of the epidermal cell wall.