MC SILVA¹, M NICOLE², L GUERRA-GUIMARAES¹, E BRESSON², L RIJO¹, JP GEIGER² and CJ RODRIGUES JR¹

¹IICT - Centro de lnvestigacao das Ferrugens do Cafeeiro (CIFC), Quinta do Marques, 2780 Oeiras, Portugal; ²ORSTOM, Plant Pathology - BP 5045, 34032, Montpellier, France

Background and objectives
Resistance of coffee plants (Coffea spp.) to races of orange rust (Hemileia vastatrix) is conditioned by at least nine major dominant genes (SH1-5H9), singly or associated [1]. A microscopical and biochemical study of compatible and incompatible C. arabica-H. vastatrix interactions was undertaken aimed at a better understanding of what differentiates both interactions in fungal growth and host responses.

Results and conclusions
Fungal growth inside leaf tissues of resistant or susceptible plants differed significantly only from 72 h after inoculation. In resistant leaves, in contrast to susceptible ones, the fungus ceased its growth, with higher frequency after the formation of at least one haustorium. The first signs of incompatibility were expressed (i) histologically and cytochemically, by haustoria encasement with callose and cellulose and by rapid host cell death (hypersensitive reaction - HR) at sites of infection and penetration; and (ii) biochemically, by a rise in peroxidase activity around 40 h after the inoculation. Histologically the HR was observed 72 h after inoculation in more than 65% of the infection sites.

Around 5 days after inoculation, another increase in peroxidase activity and stimulation of enzymes of the phenylpropanoid pathway (PAL and TAL) was found, which could be responsible for the lignification observed histologically in the host cell walls. From 7 days after inoculation, the accumulation of an acicular material was seen in the intercellular spaces around the senescent hyphae, next to the dead host cells and in close association with the middle lamella. That material was autofluorescent, stained positively for lignin and polysaccharides and labelled for pectins. Cellulose, hemicellulose, extensins, hydroxyproline-rich glycoproteins and proteins were not detected. Hypertrophy and wall thickness of the host cells in the infection area were also observed around 12 days after inoculation, corresponding macroscopically to the reaction flt. Thus in resistant coffee plants the HR was associated with an increase in peroxidase activity, precocious haustorium encasement, early arrest of fungal growth, stimulation of the phenylpropanoid pathway, accumulation of an acicular material of heterogeneous composition, and by cell hypertrophy.

References
1. Beftencourt AJ, Rodrigues Jr CJ, 1988. In Clarke RJ, Macrae R, eds, Coffee Agronomy. Vol. 4. Elsevier Applied Science, London/New York, pp. 199-234.