Could higher temperatures break pollen?

Green Innovation 12. dec 2024 3 min Doctoral researcher Donam Sebuliba Tushabe, Tenure Track Assistant Professor Sergey Rosbakh Written by Eliza Brown

Global warming is challenging agricultural norms, with soaring temperatures exposing pollen as a fragile link in plant reproduction. Although leaves tolerate extreme climates, pollen often fails above 45°C, risking crop failure in heat-prone regions. Researchers urge a focus on the heat-tolerant traits of tropical plants to engineer resilient crops. Without swift action, agriculture may struggle to adapt to rising global temperatures and shifting climate realities.

Since the first seeds were sown, farmers have worked to bring the bounty of the tropics to places with milder temperatures and shorter growing seasons. “Historically, we humans have been trying to advance our agriculture to colder climates,” says Sergey Rosbakh, a plant ecologist at the University of Copenhagen.

But global warming has flipped this on its head: now researchers are racing to develop climate-resilient crops that can withstand unprecedented heat.

Scientists have identified pollen as the weak link in the plant life cycle, Rosbakh explains. It is exposed to the elements, and its short lifespan gives temperature spikes outsized impact. But pollen’s thermal limits have been surprisingly inadequately studied given its importance to agriculture. “It has not received nearly enough attention,” Rosbakh says.

Rosbakh and a colleague at the University of Regensburg, Germany analysed temperature data on 191 plant species to pin down a temperature range at which pollen can function. Their findings, published in Plant, Cell & Environment, include worrisome evidence of a temperature ceiling beyond which most pollen seems to fail.

And according to Donam Tushabe, a plant scientist at the University of Regensburg and lead author of the new article, planet Earth is already too close to that ceiling for comfort.

“With climate change, we are getting more hot weather,” Tushabe says.

Scarce data and a pollen cookbook

For scientists studying a plant’s response to temperature, leaves are the low-hanging fruit. “You can collect leaves and they remain viable for experiments for a few days,” Rosbakh says.

But pollen works on an unforgiving timescale, being viable for just hours or minutes. Miss that window and you have to wait six months for your plants to germinate again.

Even then, pollen must be coaxed and cajoled to germinate in experimental settings. Germination is the process by which a grain of pollen activates when it contacts a receptive flower.

“Pollen will not germinate in water in a petri dish,” Rosbakh explains. “You need to create a special recipe. You need to add some sugar, some chemicals, some iron, some growth factors. Donam Tushabe spent six months just collecting data to make an effective pollen cookbook.”

All the effort studying pollen takes produces little information. Even for the most common crops, such as wheat and barley, “there are no data – zero,” on pollen’s thermal limits, Rosbakh says.

Rosbakh and Tushabe combed the scientific literature going back to the 1930s and found studies on just 191 species out of the tens of thousands of wild plants and cultivars known to science. “Nearly 60% of these data come from my experimental work,” Rosbakh adds.

The researchers hoped to compare temperature tolerance for the plants’ vegetative parts – their stems and leaves – with the resilience of their pollen. They found complementary data on the thermal limits for the vegetation of 30 species.

Armed with these historical data, Tushabe and Rosbakh set out to look for common threads. Are wild species comfortable in a wider range of temperatures than cultivars? Are certain clades better at cold?

A narrow Goldilocks zone

It was immediately clear that the “pollen temperature range is much smaller than that of the leaves,” Tushabe says.

Vegetative plant parts could tolerate between –10°C and 60°C, whereas most pollen would only germinate between about –5°C and 40°C.

Among the 30 species for which they had both leaf and pollen data, Tushabe found that the bottom end of the range coordinated fairly well. Plants that fared well in the cold had pollen that germinated at lower temperatures. But the high end of the range was a different story.

Tushabe identified a magic threshold at about 45°C beyond which almost no pollen germinated. Only a handful of species that evolved to live in extreme environments, such as African violets and eucalyptus, could continue their life cycle above 45°C.

The researchers found no difference in the pollen of cultivated strains versus wild species. “We have bred the leafy parts of plants to tolerate high temperatures, but pollen is still as it used to be 500 years ago,” Rosbakh says.

Some like it hot

Although additional study will be needed to identify the mechanism responsible, Tushabe suspects that the hard stop at 45°C could be caused by failure at the cellular or protein level. Under high enough temperatures, proteins can denature – essentially unravel – to the point that they can no longer perform their roles in the cell. This challenge is among the most difficult for an organism to adapt around in a short time frame, Rosbakh adds.

A temperature of 45°C is too close for comfort for agriculture in many countries. Rosbakh points to South and South-East Asia, which are already experiencing these temperatures today – with disastrous effects on crops. “Last year, we had scorching heat in Pakistan with temperatures above 50°C,” Rosbakh says. As much as half the mango crop has failed in recent years, which researchers attribute to more intense heat-waves caused by climate change.

Rosbakh and Tushabe agree that studying tropical plants that can germinate in those punishing temperatures – such as the African violet, which can germinate at 66.9°C, and eucalyptus, which can germinate at 70°C – should be a priority. “Perhaps we could identify the genes that help pollen to tolerate higher temperatures,” Rosbakh says. With gene editing, these special traits could be spliced into other strains to help them better weather climate change.

Despite the painstaking work, the researchers think that the agricultural sector must start to measure the thermal limits of pollen as part of the standard tests in developing new strains. Otherwise, we risk breeding seemingly hardy plants that are completely unable to reproduce.

Given the lead time required to develop a new strain, we are already several steps behind. “Developing a new variety takes up to 20 years,” Rosbakh says. “To have plants resilient to the climate change we are seeing now, we should have started in the early 2000s.”

“Patterns and drivers of pollen temperature tolerance” has been published in Plant, Cell & Environment. The research was funded by the German Research Foundation, the Bavarian Programme for the Realisation of Equal Opportunities for Women in Research and Teaching and the Novo Nordisk Foundation.

The Institute of Plant Sciences at the University of Regensburg conducts advanced research in plant biology, focusing on areas such as plant reproduct...

The Department of Plant and Environmental Sciences at the University of Copenhagen conducts research across plants, microorganisms, soil, and ecosyste...

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