Effect of Auxin upon Loss of Calcium from Cell Walls

Abstract

In 1955, Bennet-Clark (2) and Carlier and Buffel (4) independently suggested that auxin induces elongation of stem and coleoptile tissues by removing calcium from the cell wall. According to this calciumbridge hypothesis, the factor which limits cellular expansion is the number of calcium ions which crosslink the pectin chains of the cell wall. Removing such cross-linkages by auxin would result in elongation. It has been suggested that auxin brings about this removal of calcium by chelation (2, 9, 19) or by methylation of the carboxyl groups to which the calcium is attached (12, 13). Considerable evidence from work with stems and coleoptiles has been used to support such a theory. Calcium increases the rigidity of cell walls (1, 15) and inhibits elongation (8, 16, 19). Ethylenediamine tetraacetic acid (EDTA), which might be expected to remove calcium from the wall by chelation, promotes elongation (2, 9, 19). Auxin induces an increase in the transfer of methyl groups from methionine to the pectin of Avena (12,13) and maize (6) coleoptile cell walls. The fact that calcium promotes root elongation suggests that calcium bridges do not control the elongation of root cells (3). If this theory is correct, adding auxin to a tissue should result in a loss of calcium from the cell walls. The loss of calcium from whole Avena coleoptile tissues has been examined by Thimann and Takahashi. Initial experiments suggested that the combination of auxin and EDTA enhanced this calcium loss (17) but subsequent experiments have indicated that the increase is not significant (Thimann, personal communic.) This calcium-bridge hypothesis has now been directly tested. The effect of auxin on the loss of Ca45 from cell walls has now been determined in Avena coleoptile and maize mesocotyl tissue.