The role of pollination in controlling Ginkgo biloba ovule development

Abstract

Summary Generally, in gymnosperms, pollination and fertilization events are temporally separated and the developmental processes leading the switch from ovule integument into seed coat are still unknown. The single ovule integument of Ginkgo biloba acquires the typical characteristics of the seed coat long before the fertilization event. In this study, we investigated whether pollination triggers the transformation of the ovule integument into the seed coat. Transcriptomics and metabolomics analyses performed on ovules just prior and after pollination lead to the identification of changes occurring in Ginkgo ovules during this specific time. A morphological atlas describing the developmental stages of ovule development is presented. The metabolic pathways involved in the lignin biosynthesis and in the production of fatty acids are activated upon pollination, suggesting that the ovule integument starts its differentiation into a seed coat before the fertilization. Omics analyses allowed an accurate description of the main changes that occur in Ginkgo ovules during the pollination time frame, suggesting the crucial role of the pollen arrival on the progression of ovule development.

Identification of key regulatory genes involved in the sporophyte and gametophyte development in Ginkgo biloba ovules revealed by in situ expression analyses

“…In situ hybridization experiments were conducted on buds enclosing ovule and leaf primordia (Figure 1A, B) and on ovules during the pollination phase (Figure 1C, D), respectively stages 4 and 8 in D'Apice et al (2021) (Figure 1). The study of these two time points in the ovule development highlights the changes of the expression domains of selected key regulatory genes during the early ovule development and during its subsequent maturation, which is represented by reaching the pollination phase.…”

“…At this stage, ovules display an integument that has completely encircled the nucellus with a completely formed micropyle; however, the three characteristic layers in which the single integument will differentiate to form the seed coat are not yet recognizable. Therefore, stage 8 is still an early stage of development (albeit advanced in respect to stage 4) because the female gametophyte has just started its mitotic divisions that four months later will form the mature female gametophyte (D'Apice et al, 2021). By studying these two time points, we have been able to obtain an overview of the changes that occur inside the buds and therefore in the developing young ovules for the selected regulatory genes.…”

“…Experiments have been designed to investigate two stages of Ginkgo ovule development: early ovules still contained inside wintering buds, and ovules at the pollination time. Early ovules (stage 4 of ovule development from D'Apice et al, 2021) display a still undifferentiated and incomplete integument—because it has not yet completely encircled the underneath nucellus—therefore representing a very early stage of development that has not been characterized so far in terms of gene expression patterns. Because most of the genes we have taken into consideration are required both for the determination and the emergence of the ovule primordia and for the integument morphogenesis in the model plant Arabidopsis thaliana , we wanted to understand if also in Ginkgo the expression of these orthologs is precocious as in Arabidopsis .…”

“…Ginkgo biloba has been chosen because of its isolated phylogenetic position and the availability of its genome. Recently, it has been demonstrated that the processes that drive the seed coat development in Ginkgo are activated upon pollination, with the activation of lignin and fatty acids biosynthesis pathways, which is necessary for the differentiation of the seed coat layers (D'Apice et al, 2021). Nevertheless, the early mechanisms and genes that drive and regulate the early ovule development are still poorly studied.…”

“…Some of these regulatory genes have already been investigated in Ginkgo , mostly regarding diverse organs (not only ovules) and stages of development (Floyd et al, 2006; Lovisetto et al, 2012; Finet et al, 2016; Zumajo‐Cardona et al, 2021). Here, we describe the expression pattern of these genes in an early stage of ovule development and during the crucial stage of pollination (respectively corresponding to stages 4 and 8 as described in D'Apice et al, 2021), highlighting the changes in their expression domains. This study provides an overview of the gene network controlling Ginkgo early ovule development, expanding the knowledge about ovule developmental programs in nonmodel seed plants.…”