The Role of Auxins and Cytokinins in the Release of Buds From Dominance

Abstract

The paper deals with the general problem of the physiological basis of branching, and the roles of known and unexplored factors in sensitivity to apical dominance. It is shown that when pea seedling shoots are completely or partially inhibited by other shoots on the same plant auxin can promote their elongation, even though it does not have this effect on inhibited buds. This influence of auxin is only exerted on internodal elongation and not on apical growth. When kinetin in a solution of alcohol and carbowax is applied directly to the lateral buds of pea seedlings, it releases them from inhibition by the growing apex. It is shown that the role of alcohol in this solution is to act as a surfactant, permitting good contact with the buds, while that of carbowax, being hygroscopic, is to maintain a thin film of solution over the buds. Buds thus released from apical dominance by kinetin do not elongate as much as do uninhibited control buds. Such kinetin‐treated buds can, however, be made to elongate normally by the application of auxin locally to their apices. It is concluded that growing shoots are relatively insensitive to correlative inhibition because they synthesize two types of growth substances, namely, auxin, which antagonizes the inhibitory effect on internodal elongation, and cytokinins, which permit the apex itself to develop. In the discussion it is brought out that many cases of branching, which appear at first to bear little relation to one another, can be understood on the basis of two principles, namely: (1) Any reduction in the growth rate of a dominant apex reduces its inhibitory effect on other apices, and (2) once an apex starts growing it becomes less sensitive to inhibition by other apices These generalizations and the experimental results are tentatively interpreted in terms of an interaction between the syntheses of auxin and of cytoldnin.

A Proteomic Approach to Analyze Differential Regulation of Proteins During Bud Outgrowth Under Apical Dominance Based on the Auxin Transport Canalization Model in Rice (Oryza sativa L.)

“…According to the second hypothesis, called the second messenger hypothesis, auxin in the stem regulates the production of a second messenger. This messenger can enter the bud and directly regulate its activity (Snow 1929;Sachs and Thimann 1967;Bangerth 1994). However, neither of these two hypotheses has been conclusively supported, and both of them have limitations.…”

“…Cytokinins and strigolactones seem to be able to promote or inhibit bud activation directly. Consistent with this role, the direct application of cytokinins to buds in the presence of apical auxin promotes bud outgrowth (Sachs and Thimann 1967). Conversely, direct application of strigolactones to buds can inhibit bud outgrowth.…”

The interaction between nitrogen availability and auxin, cytokinin, and strigolactone in the control of shoot branching in rice (Oryza sativa L.)

“…The second hypothesis, also called the second messenger hypothesis, posits that auxin in the stem regulates the production of a second messenger. This messenger can enter the bud and directly regulate its activity (Snow 1929;Sachs and Thimann 1967;Bangerth 1994).…”

The metabolism of N6(Δ2-isosopentenyl) [3H]adenine by different stem sections of Pisum sativum

“…The demonstration of a gradient of cytokinin metabolic activity in the stem and its possible modulation by IAA allow a different interpretation on the theory of Sachs and Thiamann [22] that inhibited buds lack the capacity to synthesize cytokinins or are deficient in endogenous cytokinins [20] . It is more likely that the release of lateral buds from dominance represents an active process resulting from the ability of the stem and possibly the buds to metabolise and utilise certain growth regulators when signalled by removal of the shoot apex .…”

“…Substantial evidence suggests that apically synthesized indole-3-acetic acid (IAA) is a controlling factor in apical dominance [22,26] . Release of lateral buds can be achieved by decapitation of the shoot apex or application of a cytokinin to the inhibited bud, implicating an interaction between auxin and cytokinin in apical dominance control [21] .…”