Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2006
Publication Date: 7/12/2006
Citation: Whitaker, B.D., Lester, G.E. 2006. Cloning of phospholipase da and lipoxygenase genes CmPLDa1 and CmLOX1 and their expression in fruit, floral, and vegetative tissues of ‘Honey Brew’ hybrid honeydew melon (cucumis melo l. var. inodorus). Journal of the American Society for Horticultural Science. 131(4):544-550. Interpretive Summary: Melon fruit are highly perishable. To maximize shelf-life and avoid damage to softened fruit during transport, growers harvest prior to full maturity. The problem with this practice is that the melons do not have a chance to develop their full sweetness, flavor, and nutritional value. It is known that water loss resulting from breakdown of tissue just beneath the rind is a primary cause of short shelf-life in melons. This study was conducted to begin to elucidate the basic biological mechanism involved in breakdown of the barrier tissue. Two genes thought to act in tandem and play a key role in the process were identified and isolated (cloned). Further study and manipulation of these genes by plant scientists could lead to development of new lines of popular melon varieties with improved eating quality and shelf- life. This will promote increases in sales and consumption of nutritious melon fruit, thus benefiting both the growers and consumer health.
Technical Abstract: Increases in phospholipase D (PLD; EC 126.96.36.199) and lipoxygenase (LOX; EC 188.8.131.52) activities are thought to play a critical role in senescence of mesocarp tissues in netted and nonnetted muskmelon fruits. We have cloned and characterized two full-length cDNAs, CmPLDa1 and CmLOX1, encoding PLDa and LOX proteins in honeydew melon (Cucumis melo L. var. inodorus). Relative levels of expression of the corresponding genes were determined by quantitative RT-PCR in developing and mature fruit mesocarp tissues (20-60 days after pollination; DAP), as well as in roots, leaves, stems, and flowers from young, actively growing plants. The coding regions of CmPLDa1 and CmLOX1 cDNAs are, respectively, 2427 and 2634 nucleotides long, encoding proteins 808 and 877 amino acids in length. CmPLDa1 is very similar to PLDa genes from castor bean, cowpea, strawberry, and tomato (77% nucleotide identity), and is the first PLD gene cloned from a cucurbit species. CmLOX1 has 94% nucleotide identity to a cucumber LOX gene expressed in roots and 80% identity to cucumber cotyledon lipid body LOX. In general, transcript of CmPLDa1 was much more abundant than that of CmLOX1, but relative levels of transcript in the various organs and tissues were similar for the two genes. Expression was highest in roots, flowers, and fruit mesocarp tissues. CmPLDa1 expression in fruit was essentially constitutive throughout development, although maximum levels occurred at 50 and 55 DAP, respectively, in middle and hypodermal mesocarp. CmLOX1 expression was generally higher in middle than in hypodermal mesocarp with maximum transcript levels occurring at 55 and 50 DAP, respectively. Overall, the patterns of expression of CmPLD'1 and CmLOX1 are consistent with a model in which their encoded enzymes act in tandem to promote or accelerate senescence in fruit mesocarp tissues.