Sugar versus Auxin: which is dominant?

Of the plethora of aspects of plant growth and development that the hormone (OK, plant growth regulator…) auxin is implicated in/involved with (e.g. embryo development, leaf formation, phototropism, gravitropism, fruit development, abscission, root initiation and development…), surely one of the most enduring is apical dominance.

Apical dominance is the phenomenon whereby the outgrowth of buds on the side of a shoot is suppressed in favour of growth by the apical bud (hence its name…). Maintenance of this suppression has long been assumed to be due to the production of auxin by the apical bud and its transport down the stem, which effectively keeps the lateral buds in check. Understandably, outgrowth of lateral buds upon removal of the apical bud – and its associated auxin-production and outflow – is a key bit of evidence for the role of auxin in this phenomenon.

Just as you should never (ever…) take anything for granted in science (or anything else), it’s rather satisfying to note that work by Michael Mason et al. – and rather pleasingly from ‘down under’ – has seemingly burst that little bubble of plant physiological certainty.  The primarily Australia-based team show that bud outgrowth following apical bud removal takes place >24 hours before changes in auxin content in the adjacent stem, i.e. ‘initiation of bud growth after shoot tip loss cannot be dependent on apical auxin supply’. However, upon removal of the shoot tip, sugars not only accumulate in axillary buds, but do so within a timeframe that correlates with bud release. Rather than auxin being the main lateral-growth suppressant, the team conclude that enhancement in sugar supply is both necessary and sufficient for suppressed buds to be released from apical dominance. Ah, the sweet smell of success? G’day Bruces, Sheilas… and ‘possums’ everywhere!

[And if this item has initiated a craving for more sugar-based botanical items, may I recommend Winnie Lin et al.’s Letter investigating nectar secretion and the role of the sugar transporter, aptly named SWEET9? – Ed.]