Brassinolide‐induced elongation and auxin

Abstract

Segments from the hook and subhook zone of the stem of 6‐day‐old etiolated Pisum sativum L. cv. Victory Freezer seedlings were used to study the relationship between brassinolide and auxin in the promotion of elongation. Minor changes in exogenous indole‐3‐acetic acid or4‐chloroindole‐3‐acetic acid concentration affected the kinetics markedly and the ethylene generator ethephon overcame brassinolide‐induced elongation in an antagonistic interaction. Brassinolide‐induced elongation was markedly inhibited by low concentrations of the cellulose biosynthesis inhibitor 2,6‐dichlorobenzonitrile, and diagnostic concentrations of the antiauxin 2‐(p‐chlorophenoxy) isobutyric acid did not affect brassinolide‐induced elongation. As the characteristics of auxin‐induced growth are not displayed in brassinolide‐induced elongation of the upper stem segment, it is proposed that brassinolide does not depend on auxin as a mediator in the promotion of elongation of younger tissues but that it can interact in a very complex manner with auxin. In the elongation of more mature tissues, and in bending responses, brassinolide probably accelerates auxin effects. When split, the upper stem segment was unusual in its lack of specific response to growth regulators, and the slight relief of epidermal tension.