CLASSIFICATION OF AUXIN PLANT HORMONES BY INTERACTION PROPERTY SIMILARITY INDEXES

Although auxins were the first type of plant hormone to be identified, little is known about the molecular mechanism of this important class of plant hormones. We present a classification of a set of about 50 c ompounds with measured auxin activities, according to their interactio n properties; Four classes of compounds were defined: strongly active, weakly active with weak antiauxin behaviour, inactive and inhibitory. All compounds were modeled in two low-energy conformations, 'P' and ' T', so as to obtain the best match to the 'planar' and 'tilted' confor mations, respectively, of indole 3-acetic acid. Each set of conformers was superimposed separately using several different alignment schemes . Molecular interaction energy fields were computed for each molecule with five different chemical probes and then compared by computing sim ilarity indices. Similarity analysis showed that the classes are on av erage distinguishable, with better differentiation achieved for the T conformers than the P conformers. This indicates that the T conformati on might be the active one. Further, a screening was developed which c ould distinguish compounds with auxin activity from inactive compounds and most antiauxins using the T conformers. The classifications ratio nalize ambiguities in activity data found in the literature and should be of value in predicting the activities of new plant growth substanc es and herbicides.