How do auxin plants grow

In plants, rigid cell walls form a stable framework that supports and holds the cells together and enables plants to withstand weather conditions such as wind. But how can you grow in spite of this solid structure? It has long been known that the growth hormone auxin can lower the pH value of cell walls and thus make them softer. Now researchers have shown exactly how to do just that. According to this, the protein that activates the proton pump plays an important role in this. Ensures that cells can expand and divide.

Plant hormones from the auxin group enable plants to grow in a variety of ways: they ensure the reproduction of genetic material in the cell nucleus, provide the impetus for cell division and control plants’ switch to light. In addition, they create an important prerequisite for plant growth: they make the rigid cell walls of plants soft enough and thus create the possibility for cells to expand and divide. According to the so-called acid growth theory, they activate proton pumps that lower the pH value on the cell walls and thus loosen the bonds. The exact mechanism, however, remains unclear.

Activate the proton pump

A team led by Wenwei Lin of the University of California at Riverside has solved this mystery. “Our results show that auxin is a rapid and direct interaction between a protein called transembrin kinase 1 (TMK1) and the proton pump H.⁺-ATPase (AHA1) in the cells’ plasma membrane. AHA1 is usually inactive. However, if TMK1 touches AHA1, it attaches a phosphate group to a certain point and in this way activates the pump. This then pumps protons into the cell wall region.

As a result, the pH value decreases and the stabilizing connections between cellulose molecules in the cell wall partially dissolve. This allows cells that were previously in a kind of tough corset to expand. Additional protons in the cell wall also drive water uptake into the cell, increasing the internal pressure. Lin Zhenbiao Yang’s colleague explains, “As with a balloon, the expansion depends on how thick the outer sides are and how much air you blow into.” “When the pH value of the cell wall drops, water can penetrate the cell from the outside and promote the pressure of expansion and expansion.”

Mutants with stunted growth

Lin and colleagues conducted various experiments to reach these results. Among others, they bred transgenic samples of cress (Arabidopsis) in which transmembrane kinase 1 and kinase 4 were switched off, which had the same effect. These plants exhibited significant growth disturbances, and molecular analyzes showed that phosphorylation of the proton pump by auxin no longer functions without transmembrane kinases. On the other hand, other materials were also able to activate the pump in the mutants with the help of the phosphate group. The authors conclude that “TMK1 and TMK4 are selectively required for the auxin-induced increase in phosphorylation.”

When the researchers added TMK1 to the genetically modified plants, they were able to grow normally again. Without transmembrane kinases, lower soil pH could mitigate growth disturbances. “The present results support and thus explain the acid growth theory,” the authors wrote. “Together with the latest findings on other auxin signaling pathways mediated by transmembrane kinases, it becomes clear that auxin regulates plant growth and development processes through the coordinated action of intracellular and surface signaling pathways.”

Quelle: Wenwei Lin (University of California, Riverside, USA) et al, Nature, doi: 10.1038/s41586-021-03976-4

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