Nodule-Like Structures Induced on the Roots of Rice Seedlings by Addition of the Synthetic Auxin 2,4-Dichlorophenoxyacetic Acid

Abstract

Nodule-like structures were induced on rice varieties Calrose and Pelde using 2,4-dichlorophenoxyacetic acid (2,4-D) in a concentration range of 2.5 × 10^-6 M to 1 × 10^-5M. The nodule-like structures appeared to result from the fusion of multiple meristems induced in very close proximity to each other. The internal tissue differentiation of some of the nodule-like structures was found to be similar to that of both Parasponia nodules and some kinds of determinate legume nodules. These spheroid nodule-like structures, similar in external morphology to determinate legume nodules, showed no internal infection or colonisation by Rhizobium strain NGR 234 or the Azorhizobium strain ORS 571. However, on the rice variety Calrose, the strain ORS 571 sometimes colonised extensively the surface of the nodule- like structures, in comparison to levels found on emerging lateral roots and to the root surface of the same plants. The strain NGR 234 did not colonise the surface of the nodule-like structures. The synthetic phytohormone 2,4-D also induced a range of other phenomena, including various levels of fusion of primordia and structures with a callus-like surface instead of an epidermis, but with a differentiated internal anatomy. These experiments indicate that tissue development in some types of nodule is the result of a number of meristems fusing together at a single site, and at a very early stage in development.