The figure below shows the growth of excised hypocotyls at different pH (top in

Question

The figure below shows the growth of excised hypocotyls at different pH (top in heat inactivated hypocotyls at pH 4.5 (bottom). Based explanation for the mechanism of cell expansion. ) and on these data, provide arn 20 Acid-induced extensio of native walls pH 4.5 fer pH 7 buffer pH 7 buffer 0 30 6090 Timo (min) 0.4 Heat-inactivated walls add expansin H 4.5 ffer control 030 6090 Time (min) Arabidopsis seedlings were grown vertically on plates. Seven days after germination, stems were excised as shown in the diagram below and the di of auxin movement was measured.

Explanation / Answer

Expansion of cell is constrained by cell wall. Acid growth hypothesis gives a conceptual framework how plant cells undergo cell expansion by loosening  of cell wall. Apoplast acidification allows cell wall to loose and activate enzyme expansin. Major role is played by auxin hormone, auxin receptor is present over the plasma membrane of cell. As auxin binds over the receptor H+ pump opens allowing H+ to move from plasma membrane to apoplast. pH decreases resulting activation of expansin (max activity ~ 4 pH) and cell wall expansion. In heat inactivated cell wall when alpha expansin (extracted from growing plant cell) added, it restores the ability to cell expansion irreversly.

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