Delayed Abscission and Shorter Internodes Correlate with a Reduction in the Ethylene Receptor LeETR1 Transcript in Transgenic Tomato

Abstract

Stable transformation of tomato (Lycopersicon esculentum cv Ailsa Craig) plants with a construct containing the antisense sequence for the receiver domain and 3Ј-untranslated portion of the tomato ethylene receptor (LeETR1) under the control of an enhanced cauliflower mosaic virus 35S promoter resulted in some expected and unexpected phenotypes. In addition to reduced LeETR1 transcript levels, the two most consistently observed phenotypes in the transgenic lines were delayed abscission and reduced plant size. Fruit coloration and softening were essentially unaffected, and all the seedlings from first generation seed displayed a normal triple response to ethylene. Two independent lines with a single copy of the transgene and reduced LeETR1 transcript accumulation were selected for detailed phenotypic analysis of second generation (R1) plants. Delayed abscission, shorter internode length, and reduced auxin movement all correlated with the presence of the transgene and the degree of reduced LeETR1 transcript accumulation. No significant differences were noted for fruit coloration or fruit softening on R1 plants and all seedlings from R1 and R2 seed displayed a normal triple response. LeETR2 transcript accumulation was only slightly reduced in the R1 plants compared with azygous plants, and LeETR3 (NR) transcript levels appeared to be unaffected by the transgene. We propose that ethylene signal transduction occurs through parallel paths that partially intersect to regulate shared ethylene responses. Chang et al. (1993) described the cloning and characterization of the gene, AtETR1, responsible for a dominant ethylene-insensitive mutant in Arabidopsis and discovered that it shared many similarities with two-component regulators in yeast (Saccharomyces cerevisiae) and bacteria. Expression of the AtETR1 gene in yeast generated ethylene-binding sites with similar affinity for ethylene to that estimated from the dose response curve for ethylene inhibition of hypocotyl growth of Arabidopsis seedlings (Schaller and Bleecker, 1995). The AtETR1 ethylene receptor has three domains: a sensor, a His kinase, and a receiver domain (response regulator). Ethylene is bound within the N-terminal sensor domain that includes three membrane-spanning helices (Schaller and Bleecker, 1995). The predicted His kinase domain of the AtETR1 gene product is autophosphorylated (Gamble et al., 1998). Mutation of the His or other putative catabolic residues in the His domainabolished autophosphorylation of AtETR1 (Gamble et al., 1998). However, a function for the His kinase has not been determined. Moreover, a function for the putative receiver domain is also unknown. Based on similarity with other two component systems, phosphorylation of the conserved Asp group in the receiver domain may serve to propagate a signal received by the sensor domain; however, an essential role for the Asp in ethylene signal transduction has not been established (Chang and Shockey, 1999).There are currently five characterized genes that make up a family of ethylene r...

Characterization of the peach homologue of the ethylene receptor, PpETR1 , reveals some unusual features regarding transcript processing

“…Based on effects observed regarding cell size in AtETR1 loss-of-function mutants, it has been suggested that ETR1 is associated with cell elongation in an ethylene-independent manner . In transgenic tomato expressing antisense LeETR1, delayed leaf and flower abscission, as well as shortened internodes, were correlated with reduced levels of LeETR1 (Whitelaw et al 2002). Although our results with PpETR1 show subtle (2-to 3-fold) differences in expression during fruit development and wounding, these differences may be biologically significant, since in the Whitelaw et al (2002) study, small differences in LeETR1 transcript abundance were often correlated with dramatic effects on abscission.…”

“…In transgenic tomato expressing antisense LeETR1, delayed leaf and flower abscission, as well as shortened internodes, were correlated with reduced levels of LeETR1 (Whitelaw et al 2002). Although our results with PpETR1 show subtle (2-to 3-fold) differences in expression during fruit development and wounding, these differences may be biologically significant, since in the Whitelaw et al (2002) study, small differences in LeETR1 transcript abundance were often correlated with dramatic effects on abscission. Furthermore, changes in the levels of PpETR1b observed for developing 'Bailey' fruit and for wounded 'Suncrest' fruit may have similar significance in view of the observations of Clark et al (1998) indicating that deletion of the AtETR1 receiver domain weakened the binding affinity between AtETR1 and AtCTR1.…”

Molecular Changes Occurring during Acquisition of Abscission Competence following Auxin Depletion in Mirabilis jalapa

“…Although most published results are in agreement with this, some inconsistencies have been reported. For instance, the increased ethylene receptor expression as tomato fruit approach maturity, a phase which is associated with their highest ethylene levels and sensitivity (Klee, 2004); suppression of LeETR1 delaying tomato leaf abscission rather than increasing it, attributed to its effect on auxin movement that led to increased auxin levels in the petiole AZ (Whitelaw et al, 2002); enhanced transcription of ERS1 in response to ethylene in delphinium (Delphinium spp.) florets associated with their increased sensitivity to ethylene during their abscission (Kuroda et al, 2003(Kuroda et al, , 2004; and the positive correlation obtained between the sensitivity of miniature rose (Rosa hybrida) varieties to ethylene and the expression of ETR gene family (RhETR1-4) resulting in flower abscission (Mü ller et al, 2000a(Mü ller et al, , 2000b.…”

Regulated Ethylene Insensitivity through the Inducible Expression of the Arabidopsis etr1-1 Mutant Ethylene Receptor in Tomato

“…Ethylene can influence germination, sex determination, organ elongation, leaf and flower senescence, fruit ripening, programmed cell death, organ abscission, and pathogen responses (Feldman, 1984;Ecker and Davis, 1987;Mattoo and Suttle, 1991;Abeles et al, 1992;Fray and Grierson, 1993;Grbic and Bleecker, 1995;John et al, 1995;Young et al, 1997;Llop-Tous et al, 2000;Ciardi et al, 2001;Tieman et al, 2001;Whitelaw et al, 2002;Kevany et al, 2007Kevany et al, , 2008. In several dicotyledonous species, exposure of etiolated seedlings to ethylene results in a triple response phenotype that includes inhibition of hypocotyl and root elongation, radial expansion of the hypocotyl and roots, and the formation of an exaggerated apical hook with unexpanded cotyledons (Neljubow, 1901).…”