Signalling Sex in Plants: The Role of Peptide Signals and the Cell Wall in Fertilisation

Plant reproduction underpins seed production and is central to food security, yet our understanding of the molecular process by which ovules (the female gametes) are fertilised by pollen (the male gametes) remains incomplete. Fertilisation involves tightly regulated communication between the gametes to ensure that the pollen tube reaches the ovule and releases the sperm cells at the correct position within the ovule. At the core of this molecular conversation is a group of related receptor kinases which span the cell membrane to detect small molecules outside the cell. Signal detection by these receptors is translated into an appropriate cell response via varied downstream events. We know that this family of receptors binds small proteins (peptides) as external cell signals, however the identity of the peptides that pass between the pollen tube and ovules during pollen tube reception is unknown. Downstream events likely include changes in cell wall composition to allow degeneration of a specialised synergid cell at the entrance of the ovule, and burst of the pollen tube within the ovule, but functional data to support this idea is limited, and the range of cell wall changes unclear. Peptide-receptor-cell wall signalling during pollen tube reception is, thus, poorly understood. We will address a number of questions regarding signalling from the pollen tube to the ovule and vice versa in this project using Arabidopsis: 1) which peptides are perceived by the three receptor kinases at the ovule?; 2) which ovule cell wall composition changes during fertilisation are important and which receptors mediate these changes?; 3) are similar peptide signals passed back to the pollen tube from the ovule?; and 4) do they also result in cell wall composition changes required for fertilisation? These questions lead directly to the objectives of the project. Firstly, we will use an assay for receptor signalling in living ovules and test responses to a panel of synthesised ovule and pollen tube peptides to identify which peptides and receptors interact. Secondly, we will produce an atlas of cell well composition in ovules during pollen tube reception in normal plants and mutants lacking the key ovule receptors. Genetic and biochemical techniques will be used to test the functional significance of the interaction of peptides with the ovule receptors, and importance of the cell wall changes for fertilisation. Thirdly, we will then test if similar peptides signal from the ovule back to pollen tube receptors, and fourthly, what changes are seen in pollen tube cell wall composition during fertilisation, following up with genetic and biochemical characterisation of these functions. We will therefore make a significant advance in our understanding of the fundamental process of plant fertilisation, linking signal transduction to altered cell wall composition, and providing novel insight into how signalling controls gamete responses. Longer term, the agricultural industry will benefit from greater understanding of the mechanism behind plant fertilisation, a process important for all our major crops. Due to evolutionary conservation of this fundamental process, knowledge of how fertilisation occurs in a model plant can inform attempts to improve fertilisation (and yield) in crops. This is particularly relevant in the context of the ongoing climate emergency, with more extreme temperature events affecting fertilisation and thus yield. The knowledge gained will contribute to efforts to breed climate ready crops. Beneficiaries of this project include the scientific community who will gain from expanded understanding of plant signalling, using fertilisation as a tractable model system. A post-doctoral research associate and technician will benefit from training in genetics, biochemistry, and molecular, developmental and cell biology. Finally, by supporting a collaboration with Switzerland, the project will strengthen international ties.