Why tiny ‘water balloons’ cover quinoa plants

Quinoa and other extremely resilient plants are covered with strange balloon-like bladders that were long believed to protect them from drought and salt. New findings say otherwise.

These so-called bladder cells serve a completely different though important function. The finding makes the breeding of more resistant quinoa plants more, which could lead to wider cultivation of the sustainable crop worldwide.

Quinoa has been touted as a future-proof crop because it is rich in proteins and highly tolerant of drought and salt, and thus climate change. Scientists believed that the secret to quinoa’s tolerance was in the many epidermal bladder cells on the surface of the plant. Until now, it was assumed that they served as a kind of salt dump and to store water. But they don’t, and we have strong evidence for it,” says professor Michael Palmgren of the department of plant and environmental sciences at the University of Copenhagen.

Epidermal bladder cells are fluid-filled hair structures on the leaves, stems, and surfaces of a variety of plants. A few plants, including quinoa, are often completely covered with them.

Bladder cells are actually a form of trichomes. Trichomes are hairlike structures that most plants have. As a rule, trichomes look completely different and appear more like the hairs on the leaves of stinging nettles.

clear orbs on surface of green plant
Blader cells under a microscope. (Credit: U. Copenhagen)

Mutants without bladder cells

Three years ago, a research group led by PhD student Max Moog and his supervisor, Palmgren, began studying the epidermal bladder cells of quinoa plants in ways that had never been used before. The hope was to understand the plant’s mechanisms for making it resilient to salt and drought.

To this end, the researchers cultivated mutant plants without bladder cells to compare their reactions to salt and drought with those of wild quinoa plants covered with bladder cells.

To their surprise, the researchers discovered that bladder cells have no positive influence on the plant’s ability to tolerate salt and drought. On the contrary, they seem to weaken tolerance. Instead, bladder cells serve as a barrier against pests and disease.

“Whether we poured salt water on the mutant plants without bladder cells or exposed them to drought, they performed brilliantly and against expectations. So, something was wrong. On the other hand, we could see that they were heavily infested with small insects—unlike the plants covered with bladder cells. That’s when I realized that bladder cells must have a completely different function,” says Moog, now a postdoc at the department of plant and environmental sciences and first author of the study in the journal Current Biology.

When the researchers analyzed the contents of the bladder cells, they didn’t find salt as expected—despite having added extra salt to the plant. Instead, they found compounds that repel intruders.

“We discovered that bladder cells act as both a physical and chemical barrier against hungry pests. When tiny insects and mites trudge around on a plant covered with bladder cells, they are simply unable to get to the juicy green shoots that they’re most interested in. And as soon as they try to gnaw their way through the bladder cells, they find that the contents are toxic to them,” says Palmgren.

Among other things, the epidermal bladder cells of quinoa contain oxalic acid, a compound also found in rhubarb, which acts as a deadly poison on pests.

The experiments also demonstrated that the bladder cells even protect quinoa against one of the most common bacterial diseases in plants, Pseudomonas syringae. This probably happens because the bladder cells partially cover the stomata on the plant’s leaves, a point of entry for many bacterial invaders.

“Our hypothesis is that these bladder cells also protect against other plant diseases like downy mildew, a fungal disease which severely limits quinoa yields,” says Moog.

Is super quinoa to come?

There are thousands of varieties of the South American crop, and the density of bladder cells on the plant’s surface varies from variety to variety. But there is much to suggest that density determines how effective a safeguard the bladder cells are.

“Quinoa varieties with a higher density of bladder cells are most likely more robust against pests and diseases. On the other hand, they may be slightly less tolerant of salt and drought. And vice versa. These variations don’t change the fact that quinoa is generally very resistant to salt and drought. But the explanation must be found somewhere other than in the bladder cells,” says Moog, continuing:

“Due to efforts to expand quinoa cultivation around the world, the new knowledge can be used to adapt the crop to various regional conditions. For example, southern Europe has very dry conditions, while pests are a bigger problem than drought in northern Europe. Here in northern Europe, it would make sense to focus on quinoa varieties that are densely covered with bladder cells.”

According to Palmgren, the new results provide a concrete recipe for how to breed “super-quinoa” relatively easily:

“Thus far, these bladder cells have been ignored in the breeding of quinoa. If you want a crop that is extra resistant to pests and diseases, but is still tolerant of salt and drought, one can opt to breed varieties that are densely covered with bladder cells. So, we may now have a tool that allows us to simply cross-breed our way to an extra tolerant ‘super-quinoa,’” says Palmgren.

The research results add a new dimension to our knowledge about quinoa. Until now, very little was known about how the plant defends itself against attacks from hostile organisms.

“Now we know, quinoa isn’t just tolerant of non-biological stressors like drought and salt, but also of biological influences such as pests and pathogenic bacteria. And at the same time, we’ve found the secret of these odd-looking bladder cells. This research is an example of how what’s established doesn’t always turn out to be what’s true,” says Palmgren.

Coauthors of the study are the University of Copenhagen, the University of Amsterdam, Ishikawa Prefectural University, and the University of Nevada.

The research has funding from the Novo Nordisk Foundation and the University of Copenhagen, as well as the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Fellowship Programme.

Source: University of Copenhagen


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