This spring, as the first shoots of new plants emerge from the soil, many are bent over in a hook. This simple act protects the fragile stem tip and leaves as the sprout pokes up through the dirt. But how the plant orchestrates this feat at the molecular level is not fully understood. A recent Nature study reports how the well-studied plant hormone auxin, present outside of the cell, influences gene activity and stunts cell growth to create that crucial curved shape, known as the apical hook.
The researchers reveal a novel signaling pathway for a hormone with a long history. Auxin, the first plant hormone ever discovered, was studied by the likes of Charles Darwin and contemporaries. But even today researchers haven’t fully enumerated its role in plant growth and its various modes of action. “Auxin can control nearly all aspects,” says study senior author and plant biologist Tongda Xu of the Fujian Agriculture and Forestry University in Fuzhou, China, citing its roles in embryo formation and growth of shoots, roots, and leaves.
Auxin is known to work within cells by binding a nuclear protein called transport inhibitor response 1 (TIR1). Another protein on the plant cell’s outer membrane, called transmembrane kinase 1 (TMK1), is emerging as an important mediator of auxin effects, says Zhenbiao Yang, Xu’s former advisor and a plant biologist at the University of California at Riverside, who has worked with Xu on the kinase.
“This study again highlights the complexity of degradation and stabilization mechanisms used by plants to fine-tune cellular and subcellular auxin response, gene transcription, and ultimately cell growth,” says Jian Xu (no relation), a biologist at the National University of Singapore who was not involved in the Nature study.
In the apical hook, auxin accumulates on the inner side, where it inhibits growth. To figure out how, Tongda Xu and colleagues studied the model plant Arabidopsis thaliana. Plants with mutations in the gene for TMK1 didn’t have the proper growth inhibition on one side of the stem. While they could start to form an apical hook, they couldn’t maintain the structure.
When the researchers analyzed normal plants, they found that on the inner side of the curve, TMK1 left the cell membrane to accumulate within the cytosol and nucleus. Moreover, the newly interior protein had been sliced into a smaller 50-kDa piece, less than half its full size. While cleaving a membrane protein is a common way to transmit signals in animals, it’s quite rare in the plant world, notes Tongda Xu.
In further experiments, the researchers determined that the TMK1 fragment adds a phosphate group to transcriptional repressors in the nucleus, stabilizing them. These then act on the genome to inhibit growth, maintaining the stem’s bend.
“What this tells us is that a very simple hormone apparently has several ways cells can respond to it, through completely different biochemical mechanisms,” says Dolf Weijers, a developmental biologist at Wageningen University in the Netherlands who served as a reviewer as noted in the Nature paper. He suspects similar TMK-based pathways might operates in other parts of plants, as well.
If that’s true, the discovery could have practical implications down the road. For example, bioengineers might exploit the TMK1 pathway to make bigger fruits, Yang speculates. Tongda Xu is working to confirm that the same pathway occurs in crop plants like tomatoes and soybeans. He hopes that altering the genes in the TMK1 pathway might improve the ability of the hook to protect the fragile stem tip, which may result in higher rates of successful sprouting. “We can find some way to that helps them to germinate…to protect them,” he says.
For now, the key question is how TMK1 senses auxin in the extracellular environment. “This is a big mystery,” says Weijers. Tongda Xu is already pursuing an answer to that question as well.