Roles for Auxin, Cytokinin, and Strigolactone in Regulating Shoot Branching      

Abstract

Many processes have been described in the control of shoot branching. Apical dominance is defined as the control exerted by the shoot tip on the outgrowth of axillary buds, whereas correlative inhibition includes the suppression of growth by other growing buds or shoots. The level, signaling, and/or flow of the plant hormone auxin in stems and buds is thought to be involved in these processes. In addition, RAMOSUS (RMS) branching genes in pea (Pisum sativum) control the synthesis and perception of a long-distance inhibitory branching signal produced in the stem and roots, a strigolactone or product. Auxin treatment affects the expression of RMS genes, but it is unclear whether the RMS network can regulate branching independently of auxin. Here, we explore whether apical dominance and correlative inhibition show independent or additive effects in rms mutant plants. Bud outgrowth and branch lengths are enhanced in decapitated and stem-girdled rms mutants compared with intact control plants. This may relate to an RMS-independent induction of axillary bud outgrowth by these treatments. Correlative inhibition was also apparent in rms mutant plants, again indicating an RMS-independent component. Treatments giving reductions in RMS1 and RMS5 gene expression, auxin transport, and auxin level in the main stem were not always sufficient to promote bud outgrowth. We suggest that this may relate to a failure to induce the expression of cytokinin biosynthesis genes, which always correlated with bud outgrowth in our treatments. We present a new model that accounts for apical dominance, correlative inhibition, RMS gene action, and auxin and cytokinin and their interactions in controlling the progression of buds through different control points from dormancy to sustained growth.

Strigolactone Acts Downstream of Auxin to Regulate Bud Outgrowth in Pea and Arabidopsis

“…6). This nicely fits the classical second messenger theory of apical dominance (Snow, 1929(Snow, , 1937 and is consistent with our previous models of shoot branching (Beveridge, 2000;Ferguson and Beveridge, 2009). Indeed, this apical dominance experiment showed that any changes in auxin level, movement, or signaling caused by removal of the main auxin supply in the shoot in no way prevented the inhibition of bud outgrowth by strigolactone application to buds.…”

“…As shown previously for pea, this treatment led to a wave of IAA that moved down the stem at the speed of polar auxin transport (1 cm h 21 ; Fig. 4, A-E; Morris et al, 2005;Ferguson and Beveridge, 2009). NPA applied in a ring around the stem of pea plants prevented the movement of this wave, indicating that the IAA applied moved in the polar auxin transport stream (Fig.…”

“…In addition, the application of an auxin transport inhibitor to pea buds slowed their outgrowth, but only after several days, whereas the response to strigolactone application was rapid. Taken together with auxin transport experiments, our results suggest that auxin transport from buds is not the initial trigger of bud release, although it may be crucial for ongoing bud outgrowth, as we have suggested previously Ferguson and Beveridge, 2009). Rather, it seems likely that auxin promotes strigolactone biosynthesis in the main stem, implying that strigolactone acts as the classical second messenger in apical dominance.…”

“…In particular, the exact role of auxin content and transport in the regulation of bud outgrowth has been questioned Dun et al, 2006;Ferguson and Beveridge, 2009). Indeed, big and bud1 mutants in Arabidopsis have reduced auxin transport and enhanced branching (Gil et al, 2001;Dai et al, 2006), in contrast to Arabidopsis SMS mutants, which have increased auxin transport and enhanced branching (Bennett et al, 2006).…”

“…Indeed, big and bud1 mutants in Arabidopsis have reduced auxin transport and enhanced branching (Gil et al, 2001;Dai et al, 2006), in contrast to Arabidopsis SMS mutants, which have increased auxin transport and enhanced branching (Bennett et al, 2006). Additionally, depletion of auxin content is not always sufficient to induce bud outgrowth Ferguson and Beveridge, 2009). The recent identification of SMS provides the crucial missing link to assess branching control in plants.…”