5-decades old mystery behind plant growth solved

Molecule masterminds elaborate growth process.

It’s been known that plant growth is dependent on a molecule called auxin. Yet, it remains unclear whether how the molecule sets growth in motion.

Now, scientists at UC Riverside found the mechanism used by auxin turns a single cell into something as large as a tree.

A shell-like cell wall covers plant cells. The primary layer of the wall consists of three major components: cellulose, hemicellulose, and pectin. Cellulose offers a broad base of strength. It is then reinforced by hemicellulose chains and sealed in by pectin.

All these components define the shape of plant cells, sometimes prompting surprising formations like the puzzle-piece-shaped leaf epidermis cells. Such shapes hold the cell together and provide physical strength for plants against elements such as the wind.

Plant cell components
Illustration of plant cell components. (Caroline Dahl)

If everything is locked so tightly by the cell walls, how are movement and growth possible?

According to a theory, auxin causes the cells to become acidic, especially when the plants are ready to grow. As a result, the components lose the bonds between them, allowing the wall to soften and expand. This theory was proposed half a century ago, but how auxin activates acidification remains a mystery until now.

In this new study, scientists discovered that auxin generated acidity by triggering proton pumping into the cell walls, lowering their pH levels. The lower pH activates a protein, expansin, appropriately named because it breaks down links between cellulose and hemicellulose, allowing the cells to expand.

The proton pumping within the cell wall also drives water uptake into the cell, building inner pressure. If the cell wall is loose enough and enough pressure inside the cell, it will expand. Lowering the pH in a cell wall can allow water outside a cell to move in, fueling turgor pressure and expansion.

There are two main pathways that auxin uses to trigger plant growth and development. One is, as described in this study, lowering pH. The other one involves auxin’s ability to activate gene expression in the nucleus of the plants’ cells.

UC San Diego professor of cell biology Mark Estelle said“The work represents a significant advance in our understanding of how auxin regulates cell expansion. It’s been known that acidification of the extracellular space promotes cell expansion, but it wasn’t known how this happens. It’s exciting to see an old problem being solved.”

In simple words, auxin is important for plant growth. It is essential to nearly every aspect of a plant’s growth and development, including important factors to agriculture, such as fruit, seed, and root development, shoot branching, and leaf formation. Auxin also monitors the plant’s responses to gravity and light. It ensures roots head down while the shoots grow up toward the light.

UCR botany professor and research team leader Zhenbiao Yang said, “Understanding how the basic biology works may eventually have an impact on human health. As our knowledge expands, we may learn that processes in humans are analogous.”

Journal Reference:
  1. Lin, W., Zhou, X., Tang, W. et al. TMK-based cell-surface auxin signaling activates cell-wall acidification. Nature 599, 278–282 (2021). DOI: 10.1038/s41586-021-03976-4

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