Fig. 5

Cotyledonary bud outgrowth control by a two-directional auxin flow model. (a) Above: Scheme of intact plantlet. The shoot apex supplies main stem with auxin that is translocated polarly towards the root tip. Cotyledons are also source of auxin, which flows through the cotyledonary petioles towards the main stem where it joins the apex-derived polar auxin flow. Cotyledonary buds are unable to join the flow; they are not connected with PIN1-marked auxin channel, and are therefore trapped in inhibition. Below: Image of 7-day-old intact plantlet with inhibited cotyledonary buds. (n = 45; all plants behaved the same). (b) Above: After disruption the apex-derived auxin flow by stem shortening the possibility of joining opens. The cotyledonary buds start to form PIN1-marked channels and export their auxin to the stem auxin flow enabling outgrowth of both buds. Below: Image of plant captured 7 days after removal of the apex and axillary buds containing stem part. Both cotyledonary buds started to grow out and compete for replacement the lost main shoot. (n = 45; all plants behaved the same). (c) Above: If in addition to disruption the apex-derived auxin flow one of the cotyledonary auxin flows is also disrupted by removal of this source, auxin export will be realised only from the bud on the side of the removed cotyledon. Consequently, only this cotyledonary bud will form a shoot. Below: Image of plant captured 7 days after removal of the apex and axillary buds containing stem part together with one of the cotyledons. Shoot was formed from the cotyledonary bud of the removed cotyledon only, whilst bud of the preserved cotyledon remained in inhibition. (n = 45; all plants behaved the same). Green oval bodies represent shoot apex, axillary and cotyledonary buds; green-white cylinder represents the main stem with hypocotyl. Yellow hemispheres represent the cotyledons with the petioles. Red arrows represent auxin flow and its direction; dashed black arrows represent possible but not initiated auxin flow. Green arrows mark bud outgrowth; red crosses mark bud inhibition. Red arrows represent auxin flow and its direction; dashed black arrows represent possible but not initiated auxin flow.