Regulatory functions of soluble auxin-binding proteins

Since the effects of auxin on plant tissues are complex, the mode of action of auxin at the molecular level may not depend on a single mechanism. There may be a mechanism by which the interaction of auxin with receptors localized in the cytoplasmic membranes activates certain enzymes which are necessary to generate the putative second messengers. On the other hand, soluble auxin-binding proteins have been isolated from a variety of plant tissues. Some of these proteins have a high affinity for auxins and the binding is auxin specific, reversible, and saturable, characteristics which suggest that these proteins may be auxin receptors. Although these criteria are often used to distinguish real receptors from nonfunctional binding proteins, it is necessary to clarify the biological function of the binding proteins to classify them as putative receptors. The reported results on the function of soluble auxin-binding proteins demonstrate that, in the transcription system composed of isolated nuclei, auxin interacts with soluble auxin-binding proteins and stimulates the expression of specific genes. Thus, one of the mechanisms of action of auxin may involve a direct interaction with a soluble receptor protein, such that the resultant auxin-receptor complex, possibly together with other protein factors, can subsequently recognize the promoter region of specific gene(s) and interact with RNA polymerase II to cause a transcription of the gene(s).