BIOLOGY Volume 2 - A Guide to General Biology - 2004

16. PLANT COORDINATION AND REGULATION

16.2. Plant Growth Substances

16.2.3. Mechanism of Auxin Action

The Effect of Auxins on Cell growth is fairly well understood. Cell expansion by elongation implies that the cellulosic framework of The Cell wall has not yet acquired its final rigidity. The increase in cell size occurs both through osmotic Swelling (stretching of the protoplast by Water entering it) and through the deposition of new wall material. The direction of stretching is apparently determined by the orientation of Cellulose microfibrils already incorporated into the wall. Wall 'softening' is maintained by an acidic environment and auxins. In 1973, four independent research groups demonstrated that auxins stimulate proton secretion from the Cytoplasm into The Cell wall. This leads to a drop in its pH (increased acidity) and, consequently, to its 'softening'. It is possible that an enzyme with a low pH optimum breaks bonds within the wall-forming Polysaccharides, thereby enabling the overall framework to stretch. In addition, it is necessary for the cell to maintain a low osmotic potential and have an adequate supply of water entering the cell to generate high turgor pressure. According to more recent data, the primary action of auxins is probably not due to cell wall acidification; they may bind to receptors in the Plasmalemma of epidermal Cells, leading to changes in Gene activity. These changes result in the synthesis of new Enzymes or other Proteins directly involved in growth.