Abstract
BackgroundParthenocarpy is an excellent agronomic trait that enables crops to set fruit in the absence of pollination and fertilization, and therefore to produce seedless fruit. Although parthenocarpy is widely recognized as a hormone-dependent process, hormone-insensitive parthenocarpy can also be observed in cucumber; however, its mechanism is poorly understood. To improve the global understanding of parthenocarpy and address the hormone-insensitive parthenocarpy shown in cucumber, we conducted a physiological and proteomic analysis of differently developed fruits.ResultsPhysiological analysis indicated that the natural hormone-insensitive parthenocarpy of ‘EC1’ has broad hormone-inhibitor resistance, and the endogenous hormones in the natural parthenocarpy (NP) fruits were stable and relatively lower than those of the non-parthenocarpic cultivar ‘8419 s-1.’ Based on the iTRAQ technique, 683 fruit developmental proteins were identified from NP, cytokinin-induced parthenocarpic (CP), pollinated and unpollinated fruits. Gene Ontology (GO) analysis showed that proteins detected from both set and aborted fruits were involved in similar biological processes, such as cell growth, the cell cycle, cell death and communication. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that ‘protein synthesis’ was the major biological process that differed between fruit set and fruit abortion. Clustering analysis revealed that different protein expression patterns were involved in CP and NP fruits. Forty-one parthenocarpy-specialized DEPs (differentially expressed proteins) were screened and divided into two distinctive groups: NP-specialized proteins and CP-specialized proteins. Furthermore, qRT-PCR and western blot analysis indicated that NP-specialized proteins showed hormone- or hormone-inhibitor insensitive expression patterns in both ovaries and seedlings.ConclusionsIn this study, the global molecular regulation of fruit development in cucumber was revealed at the protein level. Physiological and proteomic comparisons indicated the presence of hormone-independent parthenocarpy and suppression of fruit abortion in cucumber. The proteomic analysis suggested that hormone-independent parthenocarpy is regulated by hormone-insensitive proteins such as the NP-specialized proteins. Moreover, the regulation of fruit abortion suppression may be closely related to protein synthesis pathways.
Highlights
Parthenocarpy is an excellent agronomic trait that enables crops to set fruit in the absence of pollination and fertilization, and to produce seedless fruit
Our results showed that the length and diameter of the parthenocarpic and pollinated fruits linearly increased from 0 to 6 dpa, and the natural and CPPU-induced parthenocarpic fruits showed similar growth curves, wherein the fruit size was generally larger than the pollinated fruits
Based on the evidence provided in this study, a working hypothesis for the cucumber parthenocarpic fruit set was proposed (Fig. 8), whereby parthenocarpy in cucumber may be promoted by a ‘parallel switch,’ namely, Fig. 8 A proposed model for parthenocarpy in cucumber
Summary
Parthenocarpy is an excellent agronomic trait that enables crops to set fruit in the absence of pollination and fertilization, and to produce seedless fruit. Auxin dose-response assays showed that the down-regulation of IAA9 led to auxin hypersensitivity and resulted in parthenocarpy [5] Another auxin signaling component involved in fruit set is ARF8, which was identified as a candidate gene for two parthenocarpy QTLs in tomato [6]. Based on the findings of Goetz et al [7], a model was proposed for the mechanism of parthenocarpic induction According to this model, ARF8 forms an inhibitory complex together with an AUX/IAA protein, possibly IAA9, to repress the transcription of the auxin response genes and induce parthenocarpy. ABA may acts as an antagonist of GA or auxin to induce and maintain the dormant state of ovaries, likely by repressing their transition to fruit [20] These studies demonstrate the complicated and confusing relationships among hormone responses during fruit set. The key integrating molecular players remain largely undiscovered, and a global understanding of the mechanisms underlying parthenocarpy has yet to be attained
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