2.8.2S
SOURCES AND SINKS: THE IMPACT OF BIOTROPHIC FUNGAL PATHOGENS ON HOST PHYSIOLOGY AND GENE EXPRESSION

JD SCHOLES and SA ROLFE

Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK

Background and objectives
Biotrophic foliar pathogens, the rusts, downy and powdery mildews are important pathogens of cereal and broadleaf crops, often causing severe losses in yield. These parasites lower the yield of their hosts by two major processes: First, they lower the rate of photosynthesis of infected leaves and second, as these fungi are completely reliant on host nutrients for their growth and reproduction, they compete very effectively with the host for host carbon and nitrogen. I examine the mechanisms underlying both the low rate of photosynthesis and the redirection of carbon to the fungus.

Results and conclusions
Leaves infected with biotrophic pathogens are extremely heterogeneous; they consist both of cells directly invaded by the fungus and cells that are not invaded but which may be modified by its presence. When examining changes in the regulation of host photosynthesis it is essential to understand the way in which photosynthesis changes in the different regions of an infected leaf. Chlorophyll fluorescence imaging allows us to nondestructively quantify changes in the rate of photosynthesis across a leaf with high resolution [1]. Using this technique I will present images that show that infection of Arabidopsis thaliana leaves with white blister rust (Albugo candida) lowers the rate of photosynthesis in discrete areas of the leaf.

Mechanisms underlying changes in host metabolism have also been studied in this host-pathogen system. We have shown that following infection host apoplastic invertase activity increases in areas invaded by the fungus. In addition, there is a change in the isoform pattern of the soluble acid invertase. As invertase activity increases there is an accumulation of glucose and fructose in the infected region of the leaf [2]. Since hexose sugars cannot be loaded into the phloem less carbon will be transported out of the infected leaf. Our studies have led to the hypothesis that the lower rate of photosynthesis in infected leaves is due, at least in part, to the accumulation of soluble carbohydrates following the increase in host invertase activity. We propose that the accumulation of carbohydrates initiates a signal transduction pathway leading to a coordinated repression of the transcription of photosynthetic genes e.g. Rubisco and chlorophyll a/b-binding protein (CAB). The experimental data which has led to this hypothesis will be presented.

References
1. Scholes JD, Rolfe SA, 1996. Planta 199, 573-582.
2. Tang X, Rolfe, SA, Scholes, JD, 1996. Plant Cell and Environment 19, 967-975.