Acid growth

Acid growth hypothesis refers to a plant growth theory that suggests cell wall loosening and expansion are induced by a decrease in pH (acidification) around the cell wall, primarily facilitated by the action of auxin, a plant hormone. This hypothesis has been instrumental in understanding how plants regulate their growth and respond to environmental stimuli.

Overview

The acid growth hypothesis was first proposed in the early 1970s as a mechanism to explain how auxins promote elongation in plant cells, particularly in the shoots and roots. According to this hypothesis, auxin stimulates the activity of proton pumps in the plasma membrane, leading to an extrusion of H+ ions into the space between the cell wall and the plasma membrane. The resulting decrease in pH activates enzymes known as expansins, which break the bonds between cellulose fibers in the cell wall, making the wall more extensible and allowing the cell to expand.

Mechanism

The acid growth hypothesis outlines a detailed mechanism involving several steps:

1. Auxin binds to its receptor on the cell surface, initiating a signal transduction pathway.
2. This signaling leads to the activation of proton pumps, which transport H+ ions from the cytoplasm to the cell wall.
3. The acidification of the cell wall environment activates expansins.
4. Expansins loosen the cell wall by breaking hydrogen bonds between cellulose microfibrils.
5. The cell can now take up water more easily, leading to cell expansion due to increased turgor pressure.

Evidence

Experimental evidence supporting the acid growth hypothesis includes observations that applying auxin to plant tissues leads to rapid cell elongation and that this elongation is accompanied by acidification of the cell wall. Additionally, inhibitors of proton pump activity can block auxin-induced growth, further supporting the role of pH in cell expansion.

Implications

The acid growth hypothesis has significant implications for understanding plant growth and development. It explains how plants can rapidly adjust their growth in response to environmental conditions, such as light and gravity, by modulating auxin distribution and activity. This mechanism also underlies the plant's ability to grow towards light (phototropism) and against gravity (gravitropism).

Criticism and Limitations

While the acid growth hypothesis is widely accepted, it has faced criticism and challenges. Some researchers argue that other factors, such as the role of calcium ions and other hormones, may also play significant roles in cell wall loosening and expansion. Additionally, the exact molecular details of how expansins work and how auxin signaling leads to their activation remain areas of ongoing research.

See Also

• Plant hormone
• Cell wall
• Auxin
• Phototropism
• Gravitropism