The Role of Auxin in the Polar Organisation of Apical Meristems

Abstract

Auxin is a correlative signal, coordinating leaf development with vascular differentiation and other developmental processes throughout the plant. It has a unique influence on the orientation of the differentiation of the cambium and its products. The problem considered was whether auxin has similar correlative roles in the development of meristematic stems. Seedlings of Pisum sativum L. were decapitated and the buds in the axil of the lower bract were used in all experiments. The lower stem internodes of these buds were ≤ 2 mm long and grew to about 50 mm in 6 d. The elongation of a stem internode continued even in the absence of all young leaves. However, vascular differentiation and transverse parenchyma growth correlated with the presence of developing leaves. Auxin replaced leaf effects on all stem tissues. The influence of both leaves and auxin were limited to the direction of the roots and to the sectors of the stem below the point of auxin application. This polarity differed from that of more mature tissues in requiring a direct contact with the roots. Another characteristic of minute stem internodes was that changes of orientation, expressed by cell shape and the axis of vascular differentiation, did not occur readily. However, at a narrow competence window local hormone applications did cause the formation of new stem-like axes. It is concluded that auxin is a correlative signal even within shoot apices and that the information it carries has an essential directional com- ponent. This directionality has not been studied at the cellular level.