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Pumping up cells

Plants lift up their leaves in crowded environments, so they can capture more light. While researchers know a lot of how plants do this, what causes cells to grow, which is needed for this response, is unclear. Now a group of international researchers show that the potassium channel AKT5 pumps in K+ ions into the cell, leading subsequently to an ion disbalance, and uptake of water, which increases the internal pressure and promotes the cell to expand.

To lift up their leaves plants stimulate only the cells at the bottom (abaxial) side of the petiole, which connects the leaf with the stem to elongate. To ensure that the cells all grow in the same direction, cell walls are remodelled in such a wat that only stretching along the length of the petiole is possible. But what drives the cells to growth itself was up till now unknown.

Characterising the pump

However, this study did not start out with trying to find an answer to that question. Instead, the researchers wanted to get to know more about a potassium channel called AKT5. AKT5 was a bit of a mystery compared to its siblings. It had no visible K⁺ ion uptake activity. So, the researchers wanted to know why.

First, they took a look at its structure, that showed a close resemblance to the closed state of its siblings that pump in K⁺ into the cell. One of AKT5 close siblings, AKT1, is forced into an open state by disruption of either one of two its amino acids. When the researchers replaced the equivalent amino acids in AKT5, they found that replacing one of them indeed resulted in forcing AKT5 into an open state. And in this open state AKT5 pumped K⁺ ions into the cell.

Next up the researchers checked if AKT5 like AKT1 needs to be phosphorylated by an CBL-CIPK pair for it switch to an open state. Out of the 260 possible pairs three pairs stood out, of which the researchers studied the effects of one, CBL9-CIPK10, in more detail. When only both CBL9 and CIPK10 where there they could phosphorylate and activate AKT5.

Lifting leaves

Having that sorted out, the researchers looked at where in the plant AKT5 was located. Finding that it made its home in the cell membranes of young petiole cells. Subsequently the researchers studied plants without AKT5. While under uncrowded environments these plants grew just as much as plants with AKT5, in crowded environments they did not, there they grew less big. They also failed to lift their leaves. Suggesting that the pumping in K⁺ ions by AKT5 is what is driving the cells at the bottom of the petiole to elongate in crowded environments.

So, this study ties up two lose ends. First now with AKT5 characterised, all nine of the so-called Shaker-type K⁺ channels are characterised. And second, that it is the influx of K⁺ ions that is the driver of the petiole cell elongation needed for the lifting up of the leaves.

Literature

Yuki Muraoka et al., AKT5 is a bona fide potassium channel and controls petiole growth in Arabidopsis. Sci. Adv. 12, eaeh7630 (2026) https://www.science.org/doi/10.1126/sciadv.aeh7630

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