A small-RNA pathway that regulate male fertility under heat stress in grasses

Unlike in animals, plants become extremely vulnerable during sexual reproduction as they undergo genetic reduction (meiosis) and large-scale developmental and whole-genome reprogramming to produce specialized haploid gametes (e.g. male sperm cells and female egg cells). Due to their sessile nature, plants are thought to have evolved coping mechanisms to be able to withstand fluctuations in ambient temperature during these critical times, however, the molecular details of such mechanisms are far from clear. We recently uncovered the presence of a novel class of small RNAs (Hphasi) acting with specific Argonaute proteins found exclusively in male germ cells and their surrounding somatic cells to regulate male fertility during heat stress conditions. We also have various lines of evidence suggesting that this pathway is conserved in the grasses. In this proposal, we therefore propose to use maize as a model crop system to determine exactly how this sRNA pathway regulates male fertility under environmental constrains, by using a variety of experimental approaches (eg. developmental genetics, biochemical, and molecular techniques) to uncover the various components and regulatory factors involved. We anticipate that this work will not only advance our basic understanding of how cereal plants safeguard their male germline but will also provide important know-how to engineer new breeding tools in cereal crops to make them more resilient to temperature stress brought on by climate change.

Sexually reproducing organisms must undergo reduction through meiosis to generate the male a female germ lineages. As sessile organisms, plants are highly vulnerable to environmental perturbations occurring during this phase and so have evolved mechanisms to help safeguard germ cells from environmental damage - the details of which remain elusive to date. The aim of this proposal is to characterise a novel phased secondary small interfering RNA (Hphasi) pathway that we have recently discovered operating in the grasses. Interestingly, the Hphasi pathway is activated in somatic neighbouring cells of male germ cells in plants exposed to heat stress, where it is necessary to sustain male fertility. Our recent work has uncovered a discrete class of Argonaute proteins (named MAGO) that act as gatekeepers of these Hphasi to prevent damage caused by the activation of transposons, and ultimately ensure fertility under temperature restrictive conditions. This proposal focuses on elucidating how the Hphasi pathway is:
1. activated and regulated in response to heat stress (by identifying upstream components and key regulatory factors),
2. able to act in conjunction with other canonical epigenetic pathways to protect genome integrity of the male germline under heat stress conditions.
3. functionally conserved in wheat and other cereals to enhance or maintain male fertility and seed productivity.
Collectively, this work will advance our basic understanding of how the male germline is protected from environmental damage, and will provide essential know-how to create novel strategies and tools to mitigate the effects of climate change in agronomically important cereal species.