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Title: Transcriptome profiling of grapefruit following exposure to low temperature and conditioning treatments uncovers principal molecular components involved in chilling tolerance and susceptibility.

Author
item Maul, Dora
item McCollum, Thomas
item POPP, MICK - UNIV. OF FLORIDA
item GUY, CHARLES - UNIV. OF FLORIDA
item PORAT, RON - ARO, VOLCANI, ISRAEL

Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2008
Publication Date: 6/1/2008
Citation: Maul, D.P., Mccollum, T.G., Popp, M., Guy, C.L., Porat, R. 2008. Transcriptome profiling of grapefruit following exposure to low temperature and conditioning treatments uncovers principal molecular components involved in chilling tolerance and susceptibility.. Plant Cell and Environment.31(6):752-768.

Interpretive Summary: Chilling injury is a physiological disorder that results in from holding of citrus fruit during handling and storage. Fruit with chilling injury symptoms are unmarketable and as a consequence, there is economic loss. Conditioning grapefruit by holding them at 16C for 1 week improves their tolerance to low temperature and reduces the incidence of chilling injury symptoms. We hypothesized that the conditioning treatment causes changes in gene expression that confer the increase in chilling tolerance. To test this hypothesis we determined how conditioning and chilling affect levels of more than 30,000 genes in grapefruit peel. We found that 1,762 genes were differentially affected by the conditioning and chilling treatments. Some of the genes were unique to the chilling response and some of the genes were unique to the conditioning response, and others common to both responses. Our results reveal new information on how temperature affects gene expression in citrus fruit.

Technical Abstract: In citrus, a pre-storage conditioning (CD) treatment at 16'C for 7 d enhanced chilling tolerance and reduced the development of chilling injuries (CI) during storage at 5'C. To gain a better understanding of the molecular mechanisms involved in the responses of citrus fruit to low temperatures, we performed genome wide transcriptional profiling analysis using the newly developed Affymetric Citrus GeneChip® microarray. Overall, out of 30,171 probe sets, representing citrus transcripts present on the microarray, we observed 22,766 present calls, and found that the expression of 9,127 (40%) of them was significantly (P ' 0.001) affected by either the conditioning or the chilling treatment. To discriminate between different patterns of gene expression following a 2-week chilling treatment at 5'C, we performed one-way ANOVA pair-wise comparisons and identified 1,762 probe sets (505 up-regulated and 1,260 down-regulated) that were differentially expressed by at least 4-fold in chilling-sensitive control fruits, and 1,854 probe sets (549 up-regulated and 1,305 down-regulated) that were differentially expressed in CD-treated chilling-tolerant fruits. Comparing these lists of probe sets, revealed that 1,345 probe sets were affected by chilling in both control and CD-treated fruits (chilling-response regulon), 509 probe sets were unique to the CD-treated fruits (chilling tolerance regulon), and 417 probe sets were unique to the chilling-sensitive control fruits (chilling stress regulon). Overall, exposure to chilling led to transcription-governed arrest of general cellular metabolic activity, including massive down-regulation of cell wall, pathogen defence, photosynthesis, respiration, and protein, nucleic acid and secondary metabolism transcripts. On the other hand, chilling enhanced transcriptional adaptation processes that involve changes in the expression of transcripts related to membranes, lipid, sterol and carbohydrate metabolism, stress stimuli, hormone biosynthesis, and modifications in DNA binding and transcription factors. Further examination of the defined chilling-response, chilling-tolerance and chilling-stress regions sheds light onto the molecular mechanisms and biochemical pathways involved in the acquisition of chilling tolerance and susceptibility in citrus fruits.