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, c hitinase, beta-glucanase, and peroxidase, and phenylalanine ammonia lyase ( PAL) transcripts was studied in five winter wheat cultivars differing in re sistance to snow mould, under controlled conditions following growth at 20 degrees C, during hardening at 2 degrees C and dehardening at 20 degrees C, The expression of these transcripts was also studied in two cultivars of w inter wheat under natural conditions during autumn, winter, and early sprin g of 1997-98 at a field site in Lethbridge. The relative abundance of trans cripts was similar in both the field and controlled environment studies: ch itinase was the most abundant transcript followed by PAL, beta-1.3-glucanas e, PR-1, and peroxidase. Under field conditions, all PR-protein transcripts exhibited the same basic pattern of expression during the autumn, winter a nd 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 decrea sed during the spring. Under controlled environment conditions, transcripts encoding for PAL and the PR-proteins, except chitinase, were constitutivel y expressed to varying extents In the unhardened treatments. The general pa ttern of expression fell into two groups: (1) transcripts of chitinase, bet a-1,3-glucanase, and PAL: were weakly expressed in the unhardened treatment s, strongly up-regulated following exposure to hardening conditions, and di sappeared or remained stable following dehardening; and, (2) PR-1 and perox idase transcripts were down-regulated upon initial exposure to hardening, i ncreased slightly following prolonged exposure to hardening conditions, and remained stable during dehardening. Under controlled conditions, the patte rn 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, u nder field conditions, expression levels were generally higher for all PR-p roteins in the snow mould resistant cultivar, C114106 than in the moderatel y susceptible cultivar, Norstar. These results demonstrate that different P R-protein and PAL transcripts in winter wheat are expressed in the field th roughout the winter and are differentially induced in response to exposure of plants to low, hardening temperatures under both field and controlled en vironment conditions.