A deep dive into wheat’s heat-responsive biochemical processes is being undertaken to identify new opportunities  to relay heat tolerance traits to breeding programs

Heat has complex impacts on plant biology, requiring molecular, metabolic and physiological responses if the plant is to retain the ability to maintain itself, grow and reproduce under such stress.

While studies have characterised some of these heat-sensitive responses, they have not been systematically studied in the context of improving agricultural cereal production.

With GRDC investment, a project  was launched in 2023 to subject  heat-responsive biochemical pathways in wheat to rigorous analysis. The aim is to scan for new opportunities to provide wheat breeders with tools to select for more heat-tolerant wheat germplasm.

In all, three biochemical pathways are being analysed at the University of Adelaide in collaboration with Dr Michael Haydon  at the University of Melbourne and  Dr Sunita Ramesh at Flinders University.

1. The heat shock proteins

The heat shock proteins (HSPs) are encoded by a family of genes that are expressed in response to stressful conditions. They were first described in relation to heat shock, but are now known to respond to other stresses such as cold, ultraviolet light and wound healing.

The genes are common across species, highlighting their importance. They play a range of roles, all related to maintaining the integrity and functionality of a plant cell’s molecular machinery.

Of particular interest to this project is the way HSPs protect against the tendency of heat stress to damage proteins, which in turn can cause enzymes to malfunction or stop functioning all together.

This pathway is being analysed primarily at the University of Adelaide.

2. The circadian clock

The circadian clock genes provide plants with information on daily and seasonal changes in light and temperature. They directly control many developmental processes and daily metabolism. Research across plant species has found that genetic variation in the circadian clock genes affects the plant’s level of tolerance to heat. However, less is known specifically about the wheat circadian genes.

This pathway is being analysed primarily at the University of Melbourne.

3. Gamma-aminobutyric acid levels

Gamma-aminobutyric acid (GABA) is a chemical messenger that functions as a neurotransmitter in humans but is also found in plants. A high concentration of GABA is known to elevate plant stress tolerance by improving photosynthesis, inhibiting reactive oxygen species generation, activating antioxidant enzymes and regulating stomatal opening in drought stress.

This pathway is being analysed primarily at Flinders University.

Figure 1: Workflow used to analyse biomarkers from three heat-responsive biochemical pathways – the gamma aminobutyric acid (GABA), the heat shock proteins (HSP) and the circadian clock (Clock) pathway.

Source: Scott Boden, University of Adelaide

Heat-responsive biomarkers

In the first year of experimental work, 10 to 15 genes within each of the three pathways were selected for analysis in wheat.

This entailed selecting 30 diverse wheat lines from a broader pool of  530 lines assembled for this project. The majority of the collection is landraces and cultivars sourced from around the world and includes material identified by the Industry Research Hub in Hot Dry Climates (now completed). However, elite cultivars recommended by global breeding programs have also been included.

The 30 lines were grown in temperature-controlled rooms at 17ºC until the plants reached development stages that correspond to when crops may be exposed to temperature stress in the paddock. Plants were then subjected to either 22ºC or 27ºC for either two or 26 hours.

Changes in expression of the targeted genes for each pathway were measured, with the Flinders University team directly measuring GABA metabolite levels.

While the data is still being processed, the expression of upward of four genes in each pathway have been found to respond to changes in temperature. Furthermore, genetic variation in the responses of these genes has been detected within the pool of 30 wheat lines.

This type of screening can detect genetic diversity associated with two types of heat responses.

There is genetic diversity causing differences in gene expression when grown at 22ºC and 27ºC, with reductions in expression indicating the plant may be better equipped to deal with heat stress. There is also genetic diversity that maintains basal expression of genes through heat stress, which better protects the plant. Both types are discernible in this study.

Moving forward, two of the most promising heat-responsive biomarkers from each pathway will be analysed across the remaining 500 wheat lines. This will eventually come to include yield and fertility data.

In the last phase, the 60 to 80  lines with the largest and smallest changes in gene expression will undergo field-based heat trials. Australian Grains Technologies will assist and advise in the running of these trials.

The ideal outcome is the identification of markers that cut through the complexity of biochemical responses to heat stress and present an economically feasible way for breeders to select for heat tolerance in wheat.

More information: Scott Boden, scott.boden@adelaide.edu.au

Read also: Re-imagining heat tolerance traits in wheat.