Expression of plant defence-related (PR-protein) transcripts during hardening and dehardening of winter wheat

The pattern of expression of the pathogenesis-related (PR) proteins PR-1, chitinase, β-glucanase, and peroxidase, and phenylalanine ammonia lyase (PAL) transcripts was studied in five winter wheat cultivars differing in resistance to snow mould, under controlled conditions following growth at 20°C, during hardening at 2°C and dehardening at 20°C. The expression of these transcripts was also studied in two cultivars of winter wheat under natural conditions during autumn, winter, and early spring of 1997-98 at a field site in Lethbridge. The relative abundance of transcripts was similar in both the field and controlled environment studies: chitinase was the most abundant transcript followed by PAL, β-1,3-glucanase, PR-1, and peroxidase. Under field conditions, all PR-protein transcripts exhibited the same basic pattern of expression during the autumn, winter and spring sampling dates; transcripts were expressed during the late autumn, reached high levels by mid-winter, then decreased before reaching maximum levels during the spring. Conversely, PAL expression was low or absent in autumn, reached the highest levels by mid-winter, and then gradually decreased during the spring. Under controlled environment conditions, transcripts encoding for PAL and the PR-proteins, except chitinase, were constitutively expressed to varying extents in the unhardened treatments. The general pattern of expression fell into two groups: (1) transcripts of chitinase, β-1,3-glucanase, and PAL, were weakly expressed in the unhardened treatments, strongly up-regulated following exposure to hardening conditions, and disappeared or remained stable following dehardening; and, (2) PR-1 and peroxidase transcripts were down-regulated upon initial exposure to hardening, increased slightly following prolonged exposure to hardening conditions, and remained stable during dehardening. Under controlled conditions, the pattern of expression of any of the PR-proteins did not appear to be associated with known genotypic levels of snow mould resistance among cultivars but, under field conditions, expression levels were generally higher for all PR-proteins in the snow mould resistant cultivar, CI14106 than in the moderately susceptible cultivar, Norstar. These results demonstrate that different PR-protein and PAL transcripts in winter wheat are expressed in the field throughout the winter and are differentially induced in response to exposure of plants to low, hardening temperatures under both field and controlled environment conditions.