Paper 4


The role of IBA induced Tubulin Synthesis during new root formation

In the hypocotyls of Phaseolus Vulgaris L;


G.R. Kantharaj, S. Mahadevan & G. Padmanabhan,

Dept. of Biochemistry, Indian Institute of Science, Bangalore 560070


(Plant Physiology, 65, (196), 36, 1980.)




Indole Butyric Acid (IBA), an auxin, initiates roots in the hypocotyl tissue of Phaseolus vulgaris (French bean). The response is dependent on the concentration of IBA and the duration of exposure to the hormone. IBA enhances the rate of total protein synthesis in about 30 min after exposure of the hypocotyl segments to the hormone. There is no detectable change in total or poly (A)-containing RNA synthesis in this period although significant increases are seen 2 hr after hormone pre-treatment. The early IBA – mediated increase in protein synthesis (30 min) is not sensitive to Actinomycin D but the antibiotic blocks the increase manifested 2 hr after hormone pre-treatment. Inhibition of early protein synthesis by cycloheximide depresses and delays root initiation.  Cytosol prepared from IBA – treated hypocotyls tissue stimulates protein synthesis in vitro to a greater extent than that of the control.


The auxin-induced formation of roots in the hypocotyls of Phaseolus vulgaris can be prevented by treatment with Actinomycin D, colchicine or cytochalasin B if applied within 24-36 hr of hormone treatment. Shortly after auxin pretreatment, there is an increase in translatable messenger RNA activity. Analysis of the labeled cell-free products indicate, among other changes, a striking increase in a protein co-migrating with tubulin dimers, in the case of RNA isolated from Indole Butyric Acid (IBA) pretreated hypocotyls. An increase in tubulin content in vivo can also be demonstrated on the basis of SDS polyacrylamide gel analysis of membrane proteins and

 Functional assays for tubulin polymerization. An increase in the synthesis of tubulin in vivo can also be demonstrated after IBA pretreatment. In addition, the auxin is also able to promote tubulin polymerization when added in vivo. It is suggested among others that tubulin synthesis and microtubule assembly are some of the early events in auxin-mediated root differentiation.  One of the early proteins synthesized in response to IBA treatment is 135KD protein that appears transitorily at a time at which root primordials can be observed