Leaf initiation and de novo pattern formation in the absence of an apical meristem and pre-existing patterned leaves in watercress (Nasturtium officinale) axillary explants

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Abstract
The production of new buds was followed in five explants from leaf axils of watercress (Nasturtium officinale R. Br.). Watercress axillary tissue normally gives rise to a primary axillary bud and numerous roots which arise from conical organ primordia adaxial to the bud. The primary bud was excised from the explants but roots and organ primordia were left intact. A new secondary bud frequently formed within several days at a predictable position on the explant. In three cases, a set of approximately spirally arranged new leaf primordia formed in a roughly triangular region bounded by the cut edge of the explant and by a pair of existing organ primordia. Where there was variation in the shape of the triangular space, there was a corresponding variation in the pattern of leaves. Leaves formed in a closely packed cluster, but leaves originally in a single group could become recruited for different secondary buds. The first two to three leaves of a bud usually formed before an apical meristem dome was visible, but the dome could also be the first structure formed for the new bud. Prior to the appearance of the first sign of bulging for a new leaf primordium, cells at the leaf site divided and grew in a new orientation. The pattern of new leaf positioning is interpreted as arising from spatial constraints imposed by existing boundary structures and the newly formed primordia. Keywords: apical meristem, axillary buds, bud initiation, cytokinin, division orientation, leaf initiation, sequential scanning electron microscopy, phyllotaxis, watercress.