Characterization of heat shock protein expression in apple peel under field and laboratory conditions

Western immunoblot analyses showed that small heat shock proteins (smHSPs) are low or undetectable in the peel of 'Fuji', 'Jonagold', 'Criterion', 'Ga la', and 'Delicious' apples [(Malus sylvestris (L.) Mill var. domestica (Bo rkh.) Mansf.] growing shaded within the tree canopy (shade apples), but are high in apples growing exposed to direct sunlight (sun apples). 'Fuji', 'J onagold', and 'Gala' sun apples sampled biweekly between 1 July and 21 Oct. 1997 were highest in content of smHSPs on 31 July, 13 Aug., and 10 Sept., corresponding to some of the warmest periods of the sampling period. The sm HSPs started to disappear first in 'Gala', the earliest maturing cultivar, and last in 'Fuji', the latest maturing cultivar indicating that maturity m ight play a role in regulating smHSP accumulation. In sun apple fruit left on trees for 60 to 120 days beyond commercial maturity and exposed to field temperatures as low as -4 degreesC, a 71.7 ku (u = unified atomic mass uni t) polypeptide was detected with a polyclonal antiwheat (Triticum aestivum L.) HSP70 in the peel and cortex of all five cultivars. While no smHSPs wer e detected in these apples, three smHSPs, as detected by antibodies against pea (Pisum sativum L.) cytosolic HSP18.1, could be induced in the same fru it 24 hours after heating to 45 degreesC for 4 hours. In 'Fuji' shade apple s heated at 40 degreesC, smHSP accumulation was detected after the second h our of a 4-hour heat treatment and continued to increase over the next 48 h ours at 22 degreesC. Levels of HSP70 did not change in 'Fuji' shade apples heated at 45 degreesC for 2, 4, or 6 hours, but smHSPs became detectable im mediately after each of these heat treatments and further increased over th e next 24 hours at 22 degreesC. Accumulation of smHSPs was maximal in the 4 -hour heat treatment. After a 4-hour heat treatment at 45 degreesC, smHSPs increased during the next 48 hours at 22 IC and then declined by 72 hours. Using two-dimensional electrophoretic analysis, as many as 17 proteins rang ing from 15 to 29 ku were found to accumulate in the peel 48 hours after a 4-hour heat treatment. Thus, apples can respond rapidly to high temperature stress, even at advanced stages of maturity, by synthesizing smHSPs, which likely play an important role in protecting cellular biochemical processes during these periods of stress.