Acid growth refers to the ability ofplant cells andplant cell walls to elongate or expand quickly at low (acidic)pH. The cell wall needs to be modified in order to maintain the turgor pressure. This modification is controlled by plant hormones like auxin. Auxin also controls the expression of some cell wall genes.[1] This form of growth does not involve an increase in cell number. During acid growth, plant cells enlarge rapidly because the cell walls are made more extensible byexpansin, a pH-dependent wall-loosening protein. Expansin loosens the network-like connections between cellulosemicrofibrils within the cell wall, which allows the cell volume to increase byturgor andosmosis. A typical sequence leading up to this would involve the introduction of aplant hormone (auxin, for example) that causesprotons (H+ ions) to bepumped out of the cell into the cell wall. As a result, the cell wall solution becomes moreacidic. It was suggested by different scientist that the epidermis is a unique target of the auxin but this theory has been disapproved over time. This activates expansin activity, causing the wall to become more extensible and to undergowall stress relaxation, which enables the cell to take up water and to expand.[2] The acid growth theory has been very controversial in the past.
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