BREAKTHROUGH research has shown that plants use an animal neurotransmitter as a chemical and electrical signal to regulate growth when stressed by harsh conditions.
Published today in Nature Communications, the University of Adelaide research shows the mechanism by which plants use gamma-aminobutyric acid (GABA) as a signal, despite the lack of a nervous system.
GABA is the chief inhibitory neurotransmitter in the mammalian central nervous system. Many sedatives and anti-anxiety drugs such as Xanax and Valium work by interacting with proteins in the human GABA-signalling system.
“We’ve known for a long-time that GABA is produced by plants under stress, for example when they encounter drought, salinity, viruses, acidic soils or extreme temperatures,” says senior author Associate Professor Matthew Gilliham, ARC Future Fellow in the ARC Centre of Excellence in Plant Energy Biology at the University’s School of Agriculture, Food and Wine.
“But it was not known whether GABA was a signal in plants. We’ve discovered that plants bind GABA in a similar way to animals, resulting in electrical signals that ultimately regulate plant growth when a plant is exposed to a stressful environment.”
The researchers confirmed GABA's signalling properties through an interesting interplay between aluminium toxic soils and acidic conditions while studying plant root growth under stressful conditions.
Eventually they discovered that there wasn't just one GABA responsive protein, but many.
"We found that different types of plants that survived aluminium toxicity in acidic soils regulated their GABA concentration differently. We began to probe at a known mechanism of aluminium tolerance in plants, the release of an anion into the soil that binds with aluminium to prevent toxicity."
Associate Professor Gilliham and his team found that the protein in the plant that confers aluminium tolerance essentially acts as a GABA sensor.
Eventually they discovered that there wasn't just one GABA responsive protein, but many. These proteins are involved in a number of processes in plants - including the opening and closing of pores in leaves that absorb carbon dioxide and emit oxygen and the regulation of pollen tube growth that underpins how many seeds plant produce..
"By understanding something, you have scope for manipulating it so you change how a plant responds to stress and how it regulates growth."
" Under stressful conditions GABA concentration can increase within seconds, the response is rapid," Associate Professor Gilliham says.
"In a lot of these stressful conditions, the first response of plants is to stop growth. We haven't known how a plant signals for this to happen. This finding is a mechanism by which this can happen. By understanding something, you have scope for manipulating it so you change how a plant responds to stress and how it regulates growth."
Evidence suggests that plants and animals co-opted GABA in their signalling systems quite separately, and fairly early in their development. The system is widespread through the plant kingdom.
"We've surveyed things like rice, wheat, barley and grapevine, and they all respond. We've looked through the genomes of other plants and the molecular databases. We have no reason to think it isn't very broadly present."
The real potential for these findings is with plant scientists and breeders developing more stress tolerant traits in crops, helping with food production and lowering the risks of food shortages around the globe.
Around 40% of soils in Australia have issues with aluminium and acidity. Acidity is also prevalent around the world.
Interestingly, the protein which confers tolerance to acidic soil also confers tolerance to alkaline soils through a slightly different mechanism. Approximately 80% of cereal growing soils in South Australia are alkaline.
"Sometimes when breeding in stress tolerant traits, you can reduce the growth of the plants in ideal conditions. We don't want to do that. By understanding these mechanisms, we can get the plant to only respond strongly under stressful conditions, but not reduce yield in a good season," Associate Professor Gilliham says.
"The holy grail is to impart these stress-resilient traits and not reduce plant growth."
Their initial area of research will be in aluminium and acid soils, but the School of Agriculture, Food and Wine is also opening up some collaborations in the US and Portugal, looking in to how GABA signalling regulates fertilisation and seed number.
"I think we’ve found ourselves a research area for the next 30 years," Associate Professor Gilliham says.
"We've opened up the doors to a whole new field of study."