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Title: Postharvest heat and conditioning treatments activate different molecular responses and prevent chilling injuries in grapefruit.

Author
item SAPITNITSKAYA, MARGARITA - ARO, ISRAEL
item Maul, Dora
item McCollum, Thomas
item GUY, CHARLES - UNIVERSITY OF FLORIDA
item WEISS, BATIA - ARO, ISRAEL
item SAMACH, ALON - HEBREW UNIV., ISRAEL
item PORAT, RON - ARO, ISRAEL

Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/16/2006
Publication Date: 8/14/2006
Citation: Sapitnitskaya, M., Maul, P., McCollum,T.G., Guy,C.L, Weiss, B., Samach, A., Porat, R. 2006. Postharvest heat and conditioning treatments activate different molecular responses and reduce chilling injuries in grapefruit. Journal of Experimental Botany. 57(12):2943-2953.

Interpretive Summary: Grapefruit develop a disorder referred to as chilling injury if exposed to low, but non-freezing temperatures. Chilling injury symptoms include rind pitting and discoloration as well as increased susceptibility to decay, and as a consequence result in economic loss. It is possible to increase the tolerance of grapefruit to low temperature by treatments such as a brief rinse in heated water (62'C for 20 s) or conditioning the fruit at 16'C for 7 d. We have found that combining the hot water rinse with conditioning results in greater chilling tolerance than either treatment alone, suggesting that the treatments may activate different chilling tolerance responses. We are interested in understanding how these treatments lead to increased chilling tolerance and have conducted experiments to identify changes in gene expression that result from them. We have identified 17 new chilling-responsive and heat- and conditioning-induced genes, and characterized the expression patterns of 11 more stress-related genes, antioxidant defensive genes, and genes encoding enzymes involved in membrane lipid modifications. We found that nine genes, including thioredoxin (TRX), lipase, catalase (CAT) and a dehydration-induced protein (DIP), were up- or down-regulated by exposure to chilling alone. In contrast, eight more genes, including HSP19-I, HSP19-II, dehydrin, universal stress protein (USP), EIN2, 1,3;4-beta-D-glucanase, and superoxide dismutase (SOD), were specifically regulated by the heat treatment followed by cold storage; and four genes, including fatty acid disaturase2 (FAD2) and lipid transfer protein (LTP), were specifically regulated by the conditioning treatment followed by cold storage. Furthermore, we identified four more genes, including a translation initiation factor (SUI1), a chaperonin, and alcohol dehydrogenase (ADH), that were commonly regulated by both heat and conditioning treatments followed by cold storage. Our results suggest that pre-storage heat and conditioning treatments enhance fruit chilling tolerance by activating different molecular mechanisms. The hot-water treatment activates mainly the expression of various stress-related genes whereas the conditioning treatment activates mainly the expression of lipid membrane modification enzymes. These results provide new insights regarding mechanisms plants use to cope with temperature stress.

Technical Abstract: A combination of hot water (a rinse at 62'C for 20 s) and conditioning (pre-storage at 16'C for 7 d) treatments synergistically reduced chilling injury development in grapefruit (Citrus paradisi, cv. ‘Star Ruby’) during cold storage at 2'C, suggesting that the treatments may activate different chilling tolerance responses. To study the molecular mechanisms involved: firstly we identified chilling- and conditioning-responsive genes by PCR cDNA subtraction analysis; and, secondly we constructed grapefruit hot water- and conditioning-treated cDNA libraries and used cDNA sequencing to identify putative stress-responsive and chilling-tolerance genes. PCR cDNA subtraction revealed 17 new chilling-responsive and heat- and conditioning-induced genes, and we characterized the expression patterns of 11 more stress-related genes, antioxidant defensive genes, and genes encoding enzymes involved in membrane lipid modifications. RNA gel blot hybridizations revealed that nine genes, including thioredoxin (TRX), lipase, catalase (CAT) and a dehydration-induced protein (DIP), were up- or down-regulated by exposure to chilling alone. In contrast, eight more genes, including HSP19-I, HSP19-II, dehydrin, universal stress protein (USP), EIN2, 1,3;4-beta-D-glucanase, and superoxide dismutase (SOD), were specifically regulated by the heat treatment followed by cold storage; and four genes, including fatty acid disaturase2 (FAD2) and lipid transfer protein (LTP), were specifically regulated by the conditioning treatment followed by cold storage. Furthermore, we identified four more genes, including a translation initiation factor (SUI1), a chaperonin, and alcohol dehydrogenase (ADH), that were commonly regulated by both heat and conditioning treatments followed by cold storage. We suggest that pre-storage heat and conditioning treatments enhance fruit chilling tolerance by activating different molecular mechanisms. The hot-water treatment activates mainly the expression of various stress-related genes whereas the conditioning treatment activates mainly the expression of lipid membrane modification enzymes.