Pathogenesis-related expression of the two antifungal hydrolases β-1,3-glucanase (EC 3.2.1.39) and chitinase (EC 3.2.1.14) was studied in wheat (Triticum aestivum L.) as part of the defence response to stem rust (Puccinia graminis f.sp.tritici; Pgt), mediated by the semi-dominantly acting resistance genesSr5 andSr24. Complete resistance (infection type 0), mediated by theSr5 gene in cultivar Pre-Sr5, closely correlates with the hypersensitive response of penetrated cells at early stage of the interaction, when the first haustorium is formed. In contrast, cultivar Pre-Sr24 shows intermediate resistance (infection type 2–3) which is not directly linked to cell death. In both cases, the plant response included a rapid increase in β-1,3-glucanase activity between 24 and 48 h after inoculation. One main extracellular 30-kDa isoform of β-1,3-glucanase was present in both lines, as shown by polyacrylamide-gel electrophoresis. Two additional minor isoforms (32 and 23 kDa) were detected only in Pre-Sr24, and only at later time points. Increased enzyme activity and the appearance of new isoforms in the resistant lines was preceded by accumulation of mRNAs encoding β-1,3-glucanases and chitinases. However, there were no changes in chitinase activity or isoforms. A high constitutive level of chitinase activity was observed in all wheat genotypes. Serological studies indicated the presence of a class 11 chitinase of 26 kDa. Accumulation of β-1,3-glucanase and chitinase transcripts was detected before the pathogen penetrated the leaves through stomata and approximately 16 h before the typical hypersensitive response was observed, indicating that signal(s) for defense gene activation were recognised by the host plant long before a tight contact between the pathogen and a host cell is established. A glycoprotein (Pgt elicitor) derived from hyphal walls, strongly induced β-1,3-glucanase. We discuss the possible role of the elicitor in the early signalling mediatingSr5 andSr24-specified resistance in wheat.
