Final answer:
The class of plant cell proteins that promotes growth by relaxing the cell wall in response to turgor pressure is known as expansins. They are crucial for plant cell elongation and growth as they loosen the cell wall, allowing it to expand.
Step-by-step explanation:
The class of plant cell proteins that facilitates plant cell growth by causing localized relaxation of the cell wall is known as expansins. Expansins work by breaking the hydrogen bonds between cellulose microfibrils and other cell wall components, thus allowing the cell wall to loosen.
This loosening enables the plant cell to elongate at that site in response to turgor pressure, which is generated by the osmotic flow of water into the cell, causing it to swell. The action of expansins is crucial because the strength of the cell wall normally resists cellular expansion, and growth occurs only when the internal pressure exceeds this restraint. The wall proteins allow for both material exchange through the cell wall and the controlled 'softening' which is vital to cell expansion and overall plant growth.