Does Ethylene Play a Role in the Release of Lateral Buds (Tillers) from Apical Dominance in Oats?

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Abstract
The growth of lateral buds (tillers), which are undergoing release from apical dominance, was measured in upright and gravistimulated intact Avena sativa L. cv. `Victory' (oat) shoots as well as in isolated Avena stem segments treated with kinetin and sucrose. During release, the tiller bud initially shows a slow rate of elongation accompanied by swelling. It is followed by a more rapid rate of elongation. Ethylene (C2H4) production in shoot segments containing a tiller bud was found to occur at the onset of tiller swelling during gravistimulation as well as during inflorescence emergence. Exogenous application of indoleacetic acid or C2H4 inhibits kinetin-induced tiller bud swelling and elongation. However, stem segments pulsed for 24 hours in C2H4 or the C2H4 biosynthesis precursor, 1-amino-cyclopropane-1-carboxylic acid (ACC) and then transferred to kinetin and sucrose, showed a significant increase in swelling elongation as compared with segments maintained under the same conditions but without C2H4 or ACC in the pulse. Segments pulsed for 24 hours with kinetin and sucrose plus the ACC biosynthesis inhibitor, aminoethoxyvinylglycine, or the C2H4 action inhibitor, CO2, then transferred to kinetin and sucrose medium, showed inhibition of tiller swelling during the pulse and of subsequent elongation. These results indicate that C2H4 plays a role in promoting tiller swelling during the onset of tiller release from apical dominance and may act as a modulator hormone in promoting tiller elongation in the presence of cytokinin.