Expression of beta-1,3-glucanase and chitinase in healthy, stem-rust-affected and elicitor-treated near-isogenic wheat lines showing Sr5-or Sr24-specified race-specific rust resistance

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 genes Sr5 and Sr24. Complete resistance (infection type 0), mediated by the Sr5 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 II 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 mediating Sr5- and Sr24-specified resistance in wheat.