Figure 4.

The P. ultimum genome contains genes encoding enzyme activities necessary for the degradation of plant cell wall polysaccharides and storage sugars (blue ticks indicate the polysaccharides targeted by P. ultimum enzymes). Some activities are equally relevant for P. ultimum's own cell wall metabolism. Degradation of the plant cell wall relies essentially on the action of cellulases and pectinases. Significantly, the absence of identified enzymes with xylanase, pectin methylesterase or cutinase activities is in agreement with previous studies of P. ultimum and other Pythium spp. [70,104,144]. For Pythium's pathogenic action, penetration is primarily limited to wounded tissue, or to young roots and germinating seedlings with little or no suberized tissue. Penetration and root rot, for some Pythium spp., is limited to the first layers of cells (RC and EC) [104]. Other genes, including those coding for transporters, elicitin-like, and stress proteins, were upregulated when P. ultimum was grown in contact with A. thaliana seeds. CC, cortical cells; EC, epidermal cells; RC, root cap; H, hyphae. (Figure adapted from [104,144-146].)