Molecular mechanisms underlying thermal sensitivity of male reproduction

Like humans, most flowering plants reproduce sexually. Sexual reproduction in flowering plants is important, as it produces the seeds that comprise 60% of our food.

Sexual reproduction, especially on the male side, is the stage of plant life most vulnerable to temperature damage. For example, heat stress causes a dramatic decrease in the yield of major crops such as wheat and rice, primarily due to impaired male (pollen) development. As temperature extremes caused by climate change become more frequent, understanding temperature's impact on plant fertility is crucial for feeding the world.

Recently, we discovered that defects in a male tissue called tapetum are the likely cause of thermal sensitivity during male development. Tapetum provides nutrition to the developing pollen, and under high temperature the tapetum becomes abnormally vacuolated and the pollen grains are unable to mature. We also developed state-of-the-art methods for tapetal cell isolation and gene expression studies, and found that a gene regulatory mechanism called DNA methylation is important for tapetal thermotolerance. Employing a genetic screen, we identified two mutant plants with enhanced fertility at high temperature.

Our findings and technical advances put us in a unique position to understand the molecular mechanisms underlying male thermosensitivity. Our specific objectives are to: 1) reveal the molecular basis of tapetal sensitivity to heat, 2) understand how the DNA methylation pathway mediates heat tolerance in the tapetum, and 3) uncover novel genes involved in reproductive thermotolerance. We believe knowledge generated from this work will lay a solid foundation for understanding heat sensitivity during male reproduction, and can be exploited to improve the resilience of crops to heat stress, or engineer conditional male sterility lines valuable for hybrid breeding.

Male reproductive development is the most sensitive stage to heat stress during the life cycle of flowering plants. Understanding the molecular basis of thermosensitivity during male reproductive development is exceedingly important to sustain crop yield as extreme temperatures are forecast due to climate change.

Although the temperature sensitivity of male reproduction has been recognised as an important issue for many years, its molecular basis is unclear. Recently, we have: 1) developed Arabidopsis thaliana as a model for understanding this issue, and identified a heat treatment that is easily realised in our growth chambers and captures substantial and reproducible male sensitivity to heat; 2) discovered that a specific cell type in the male organ, called the tapetum (or tapetal cells), likely underlies the thermosensitivity during male development; 3) established the first protocol for pure tapetal cell isolation via fluorescence-activated cell sorting, and state-of-the-art methods for single-cell-type RNA and DNA methylome sequencing; 4) found that the RNA-directed DNA methylation pathway (RdDM) specifically methylates genes in the tapetum and mediates the tolerance of tapetum to heat; and 5) isolated mutants with enhanced heat resistance during male reproduction.

These findings and technical advances put us in a unique position to carry out this proposed work, which will elucidate the mechanisms of male heat sensitivity. Our specific objectives are to: 1) reveal the molecular basis of tapetal sensitivity to heat, 2) understand how RdDM mediates heat tolerance in the tapetum, and 3) uncover novel genes involved in reproductive thermotolerance. Through these objectives, we will discover the genetic and epigenetic mechanisms of male thermosensitivity, which can be exploited to improve the resilience of crops to temperature stress.

The impact of temperature stress on crop yield is enormous. For example, heat stress during the flowering of rapeseed brassica causes a decrease in seed yield by 52%. For rice, each 1C increase in growing-season night-time temperature leads to a grain yield loss by 10%. As temperature extremes caused by climate change become more frequent, understanding temperature's impact on crop fertility is therefore crucial for feeding the world.

The proposed project aims to reveal the mechanisms underlying heat sensitivity of male reproductive development. The knowledge gained from this research will be of significant benefit to plant breeders and biotech companies as it directly relates to crop yield and therefore to the UK public in general on an economic and social scale, providing food security for the future.

Collaboration between fundamental research, plant breeding, and biotechnology plays a major role in agricultural improvement. We aim to identify the genes and processes that are required for thermotolerance during male reproduction through the proposed research. Given the conservation of reproductive genetics across plant species, knowledge gained from this work in Arabidopsis thaliana can be exploited to generate heat-tolerant lines of Brassica and rice, or heat-sensitive (i.e. conditional) male sterile lines important for hybrid breeding. Our current explorative work on Brassica and collaboration on rice, will allow the translation of gained knowledge to agronomy improvement. Besides explicit findings of new genes that are involved in the heat sensitivity of reproductive development, this proposed research will also generate a wealth of DNA methylomic and transcriptomic data on an important cell type (the tapetal cell) in two temperatures (21C and 29C). These data are not only invaluable for scientists, but may also be used by breeders and agrobiotech industry as markers for breeding or research, which in the long term will impact positively on agricultural productivity and the UK's economy.

I will engage with the public via the Norfolk Teacher Scientist Network (TSN) to teach school children, focusing on the specifics of our work in relation to the national curriculum, including topics such as specialised plant cells and their function in development. I will make use of the External Relations Director and the Communications team at JIC to issue press releases, in an accessible format, to publicise the potential of epigenetics in crop enhancement and the outcomes of this Grant. Through meeting with scientists from breeding and agro-biotech companies and intellectual property (IP) experts, we will identify potential applications and IP targets, thus applying this research quickly and directly to crop yield improvement. The proposed project therefore has strong economic impacts that will add to the UK's economic competitiveness in addition to the social implications of maintaining a sustainable level of food production in the future.