Abstract
A methanesulfonate-generated mutant has been identified in Cucurbita pepo that alters sex determination. The mutation converts female into hermaphrodite flowers and disrupts the growth rate and maturation of petals and carpels, delaying female flower opening, and promoting the growth rate of ovaries and the parthenocarpic development of the fruit. Whole-genome resequencing allowed identification of the causal mutation of the phenotypes as a missense mutation in the coding region of CpACO1A, which encodes for a type I ACO enzyme that shares a high identity with Cucumis sativus CsACO3 and Cucumis melo CmACO1. The so-called aco1a reduced ACO1 activity and ethylene production in the different organs where the gene is expressed, and reduced ethylene sensitivity in flowers. Other sex-determining genes, such as CpACO2B, CpACS11A, and CpACS27A, were differentially expressed in the mutant, indicating that ethylene provided by CpACO1A but also the transcriptional regulation of CpACO1A, CpACO2B, CpACS11A, and CpACS27A are responsible for determining the fate of the floral meristem toward a female flower, promoting the development of carpels and arresting the development of stamens. The positive regulation of ethylene on petal maturation and flower opening can be mediated by inducing the biosynthesis of JA, while its negative control on ovary growth and fruit set could be mediated by its repressive effect on IAA biosynthesis.
Highlights
The cultivated species of the Cucurbitaceae family are a group of monoecious plants that have been utilized as a model for the study of the genetic control of sex determination in plants (Martínez and Jamilena, 2021)
We establish that CpACO1A is a key regulator in sex determination and female flower development of C. pepo
The expressions of these three ethylene biosynthesis genes were not altered by aco1a (Figure 7)
Summary
The cultivated species of the Cucurbitaceae family are a group of monoecious plants that have been utilized as a model for the study of the genetic control of sex determination in plants (Martínez and Jamilena, 2021). The gene that regulates abortion of stamens during the formation of a female flower in all studied cucurbits encodes for an ethylene biosynthesis enzyme: cucumber ACS2 and its orthologs (Boualem et al, 2008, 2009, 2016; Martínez et al, 2014; Ji et al, 2016; Manzano et al, 2016). This female-forming gene is negatively regulated by the transcription factor WIP1, which is responsible for the arrest of carpels in the formation of male flowers (Martin et al, 2009; Hu et al, 2017; Zhang et al, 2020). The increased indole-3-acetic acid (IAA) and the reduced abscisic acid (ABA) and jasmonic acid (JA) contents in the aco1a mutant may be responsible for the parthenocarpic fruit development and the delayed flower opening of the mutant female flower
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