Ovule formation is a complex developmental process in plants with a strong impact on the production of seeds. Ovule primordia initiation is controlled by a gene network, including components of the signaling pathways of auxin, brassinosteroids and cytokinins. In contrast, gibberellins (GAs) and their signaling negative regulators DELLA proteins have never been shown to be involved in ovule initiation. Here we provide molecular and genetic evidences that point to DELLA proteins as novel players in the determination of ovule number in Arabidopsis and in species of agronomic interest like tomato and rapeseed, adding a new layer of complexity to this important developmental process. DELLA activity correlates positively with ovule number, acting as a positive factor for ovule initiation. In addition, ectopic expression of a dominant DELLA in the placenta is sufficient to increase ovule number. The role of DELLA proteins in ovule number seems not to be related to auxins transport or signaling in the ovule primordia. A possible crosstalk of DELLA proteins with the molecular and hormonal network controlling ovule initiation is discussed.

This review focuses on the hormonal regulation of ovule development, especially on ovule initiation, patterning, and morphogenesis. Understanding of the genetic and molecular basis of ovule development is essential from both the scientific and economic perspective. The ovule represents an attractive system to study lateral organ development in plants, and, since ovules are the precursors of seeds, full comprehension of this process can be the key to the improvement of crops, especially those depending on high production of seeds and grains. Ovule initiation, patterning, and morphogenesis are governed by complex genetic and hormonal networks involving auxins, cytokinins, brassinosteroids, and gibberellins. These coordinate the determination of the ovule number, size, and shape through the regulation of the number of ovule primordia that arise from the placenta and/or ensuring their correct development into mature functional ovules. Here we summarize the current knowledge of how ovules are formed, paying special attention to the roles of these four plant hormones.

Human and animal nutrition is mainly based on seeds. Seed size is a key factor affecting seed yield and has thus been one of the primary objectives of plant breeders since the domestication of crop plants. Seed size is coordinately regulated by signals of maternal and zygotic tissues that control the growth of the seed coat, endosperm and embryo. Here, we provide previously unreported evidence for the role of DELLA proteins, key repressors of gibberellin responses, in the maternal control of seed size. The gain-of-function della mutant gai-1 produces larger seeds as a result of an increase in the cell number in ovule integuments. This leads to an increase in ovule size and, in turn, to an increase in seed size. Moreover, DELLA activity promotes increased seed size by inducing the transcriptional activation of AINTEGUMENTA, a genetic factor that controls cell proliferation and organ growth, in the ovule integuments of gai-1. Overall, our results indicate that DELLA proteins are involved in the control of seed size and suggest that modulation of the DELLA-dependent pathway could be used to improve crop yield.

Ovule development is essential for plant survival, as it allows correct embryo and seed development upon fertilization. The female gametophyte is formed in the central area of the nucellus during ovule development, in a complex developmental programme that involves key regulatory genes and the plant hormones auxins and brassinosteroids. Here we provide novel evidence of the role of gibberellins (GAs) in the control of megagametogenesis and embryo sac development, via the GA-dependent degradation of RGA-LIKE1 (RGL1) in the ovule primordia. YPet-rgl1Δ17 plants, which express a dominant version of RGL1, showed reduced fertility, mainly due to altered embryo sac formation that varied from partial to total ablation. YPet-rgl1Δ17 ovules followed normal development of the megaspore mother cell, meiosis, and formation of the functional megaspore, but YPet-rgl1Δ17 plants had impaired mitotic divisions of the functional megaspore. This phenotype is RGL1-specific, as it is not observed in any other dominant mutants of the DELLA proteins. Expression analysis of YPet-rgl1Δ17 coupled to in situ localization of bioactive GAs in ovule primordia led us to propose a mechanism of GA-mediated RGL1 degradation that allows proper embryo sac development. Taken together, our data unravel a novel specific role of GAs in the control of female gametophyte development.

ISHS XXIX International Horticultural Congress on Horticulture: Sustaining Lives, Livelihoods and Landscapes (IHC2014): International Symposia on Abscission Processes in Horticulture and Non-Destructive Assessment of Fruit Attributes Secondary abscission zones: understanding the molecular mechanisms triggering stylar abscission in citrus