Progress 09/01/05 to 08/31/08
Outputs
OUTPUTS: During the funding period we made several important fundamental discoveries concerning how ethylene receptors work to control tomato fruit ripening. The work was disseminated as publications in refereed publications. We also produced transgenic plants with fruits that ripen significantly earlier than their non-transgenic parents. These materials are made available to any not-for-profit researcher who requests them. PARTICIPANTS: Brian Kevany. Received his PhD and wrote his dissertation on the work that he published on the project. Valeriano Dal Cin. Postdoctoral researcher funded by this project to identify genetic loci controlling the timing of fruit ripening. Two papers from the work are being written and are expected to be submitted for publication by the end of 2008. TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
We showed for the first time that the plant hormone ethylene causes the degradation of its own receptors. this ligand-mediated receptor destruction likely determines the timing of tomato fruit ripening. Separately, we identified several genetic loci that control the timing of tomato fruit ripening. We narrowed down the positions of these loci to short segments of chromosomes.
Publications
- Kevany B, Taylor M, Dal Cin V, Klee H. 2007. Ethylene Receptor Degradation Controls the Timing of Ripening in Tomato Fruit. Plant J. 51:458-467.
- Kevany B, Taylor M, Klee H. 2008. Fruit-specific Suppression of the Ethylene Receptor LeETR4 Results in Early Ripening Tomato Fruit. Plant Biotech. J. 6: 295-300.
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Progress 09/01/06 to 08/31/07
Outputs
For the past several years we have been working toward understanding the mechanism(s) whereby the plant hormone ethylene controls the initiation of tomato fruit ripening. It has been known for many years that ethylene exposure quantitatively accelerates the onset of ripening but the mechanism for measuring ethylene exposure has not been known. In the past year we have demonstrated that ethylene binding triggers a process of degradation of its receptor. Ethylene binding triggers a ubiquitin-mediated receptor degradation. Exposure of immature fruits to ethylene causes a drop in total receptor content and that drop in turn is associated with earlier ripening. These results confirm our original hypothesis that receptor depletion is a major point of control in the ethylene signaling pathway. This work has been published in 2007. Using the above information, we have designed a strategy for early ripening. Varieties of fruits that ripen earlier without sacrificing quality are
highly desirable in agriculture as they provide early entry into markets at a time when returns are highest. We have made transgenic tomato plants that are depleted in expression of the most abundant receptor specifically in fruits. These transgenic fruits ripen on average a full five days earlier than controls. The quality and quantity of the fruits produced from the transgenic plants are indistinguishable from controls. Thus, we have produced high quality fruits with early ripening. In a separate, non-transgenic approach to early ripening, we continued characterization of QTLs derived from Lycopersicon hirsutum that result in earlier ripening (up to 20%). We have screened the population for recombinants that contain smaller pieces of introgressed DNA. Candidates have been identified and are being made homozygous for subsequent evaluations.
Impacts
The identification of the molecular mechanism that regulates the onset of tomato fruit ripening is a major and unexpected fundamental insight. Using that insight, we have designed transgenic plants with a very highly desirable early ripening phenotype. Since there is a degree of hesitancy regarding transgenic fruit crops in the marketplace, we are proceeding with the parallel non-transgenic approach to the same end.
Publications
- Kevany B, Taylor M, Dal Cin V, Klee H. 2007. Ethylene Receptor Degradation Controls the Timing of Ripening in Tomato Fruit. Plant Journal 51:458-467.
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Progress 09/01/05 to 09/01/06
Outputs
In the past year we have made significant progress on characterization of the tomato ethylene receptors. We have produced antibodies against the two most abundant receptors, LeETR4 and LeETR6. We had previously shown that expression of the RNA for these two receptors increases very significantly at the onset of fruit ripening. Since the receptors are known to be negative regulators of ethylene responses, it did not make sense that there should be much higher rates of receptor synthesis during a time when responses to ethylene are absolutely essential to fruit ripening. To our surprise, we discovered that the protein levels of LeETR4 and LeETR6 actually drop at the same time that their corresponding RNAs are going up. This led us to examine whether exposure to ethylene has an effect on protein stability. Indeed, our data strongly suggest that ethylene promotes receptor degradation. These results have been confirmed in vegetative tissues where we can show a rapid
drop-off in receptor proteins following a brief exposure to ethylene. All of these data support our original hypothesis that cumulative ethylene exposure of fruits determines the onset of ripening by regulating receptor content. We have also made good progress on identifying quantitative trait loci (QTLs) that affect either ethylene synthesis during tomato fruit ripening or the time from fertilization to the onset of ripening. After two separate seasons of growth measurements, we have confirmed the existence of four ethylene QTLs and six early ripening QTLs. In the case of the ethylene loci, we have loci that cause synthesis of twice as much ethylene as the parental tomato variety.For the early ripening, we see up to a 20% earlier ripening in the best QTLs. We have developed a large number of molecular markers to accurately delimit the chromosomal segments containing each QTL and have initiated crosses to select for recombinants that further pare down the regions containing each
locus.
Impacts
We are close to confirming our hypothesis that the onset of fruit ripening is mediated by cumulative ethylene exposure and its effects on receptor content. The observations that ethylene promotes receptor protein turnover is unprecedented. The identification of QTLs that cause ripening to occur as much as 20% earlier have very significant potential application to the industry where earlier fruits bring the most return to the grower.
Publications
- No publications reported this period
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