The old pea model in a new light: power of auxin over photoassimilates

Breaking of apical dominance by removing the shoot apex initiates competition between the remaining buds to assume the dominant position. Cotyledonary buds located closest to the stored photoassimilates also participate in this race, as was demonstrated a century ago in a pea model. Using pea (Pisum sativum L.) plantlets we showed that sucrose feeding through the stem stump enhances the outgrowth rate of initiated buds. However, this cannot overwrite the axillary bud competition pattern, during which the upper axillary bud outcompetes all the lower buds. Further, disruption of polar auxin flow by stem wounding triggers cotyledonary bud outgrowth regardless of assimilates supplied by the cotyledons. Similarly, replacement of the cotyledons with a sucrose-containing gel did not influence the cotyledonary bud outgrowth scheme, whilst in contrary, an auxin-containing gel did. Using PIN1 immunolocalization assay, we demonstrated that the cotyledonary buds are located at a confluence of two auxin flows originating from the apex and the cotyledons. Formation of a PIN1-labelled auxin channel leading to the stem polar auxin flow is a prerequisite for cotyledonary bud outgrowth.