Abstract
Facultative parthenocarpy is of great practical value. However, the molecular mechanism underlying facultative parthenocarpy remains elusive. Transcriptional co-repressors (TPL) act as a central regulatory hub controlling all nine phytohormone pathways. Previously, we proved that SlTPLs participate in the auxin signaling pathway by interacting with auxin/indole acetic acid (Aux/IAAs) in tomato; however, their function in fruit development has not been studied. In addition to their high expression levels during flower development, the interaction between SlTPL1 and SlIAA9 stimulated the investigation of its functional significance via RNA interference (RNAi) technology, whereby the translation of a protein is prevented by selective degradation of its encoded mRNA. Down-regulation of SlTPL1 resulted in facultative parthenocarpy. Plants of SlTPL1-RNAi transgenic lines produced similar fruits which did not show any pleiotropic effects under normal conditions. However, they produced seedless fruits upon emasculation and under heat stress conditions. Furthermore, SlTPL1-RNAi flower buds contained higher levels of cytokinins and lower levels of abscisic acid. To reveal how SlTPL1 regulates facultative parthenocarpy, RNA-seq was performed to identify genes regulated by SlTPL1 in ovaries before and after fruit set. The results showed that down-regulation of SlTPL1 resulted in reduced expression levels of cytokinin metabolism-related genes, and all transcription factors such as MYB, CDF, and ERFs. Conversely, down-regulation of SlTPL1 induced the expression of genes related to cell wall and cytoskeleton organization. These data provide novel insights into the molecular mechanism of facultative tomato parthenocarpy and identify SlTPL1 as a key factor regulating these processes.
Highlights
The great global production and consumption (Klap et al, 2017; Quinet et al, 2019) of tomato unquestionably make it the most important vegetable crop in the world
The results showed that SlTPL1 displayed a reverse trend to that of SlARF7, which decreased sharply in the ovary at anthesis and increased when fruit set was completed (Figure 1C)
We found 3689 differentially expressed genes (DEGs) related to successful fruit set in SlTPL1-RNA interference (RNAi) plants, including 3409 DEGs expressed in the SlTPL1-mediated fruit set process plus 280 DEGs common to WT and SlTPL1-RNAi plants, which were reversely expressed in SlTPL1-RNAi and WT during fruit set (Figure 5D and Supplementary Table 3)
Summary
The great global production and consumption (Klap et al, 2017; Quinet et al, 2019) of tomato unquestionably make it the most important vegetable crop in the world. Low fruit-set rates will reduce fruit production and quality, resulting in great economic losses (McAtee et al, 2013). When plants undergo unfavorable conditions, such as extreme temperature (heat or cold) or humidity, fruit set will be inhibited because of low pollen viability, which affects microsporogenesis and pollination (Picken, 1984; Sato et al, 2006; Mesihovic et al, 2016). Parthenocarpy, which is the formation of seedless fruit from the ovary in the absence of pollination and fertilization, has been recognized as an important trait to counter harsh environmental conditions (Gorguet et al, 2005). Despite the importance of the trait, the use of these parthenocarpic mutants in breeding programs remains limited
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