Cell-layer interactions influence the number and position of lateral shoot meristems in Nicotiana

In higher plants the formation of lateral shoot meristems (i.e., axillary meristems) in the axils of leaves establishes potential growth centers along the principal axis of the stem. The position and number of lateral buds in Nicotiana glauca, two genotypes of Nicotiana tabacum, and a series of interspecific periclinal chimeras composed of these species was studied to establish the role of position (location of the node along the main axis), flowering, and cell-layer interactions on the pattern of lateral meristem initiation. In N. glauca, both the number of nodes generated and the transition to flowering influenced the number and position of lateral meristems. A short-day mutant of N. tabacum grown under long days remained vegetative and never produced multiple lateral buds per node, indicating that attaining a certain node number was not sufficient to cause the formation of multiple buds. Yet, flowering plants of both short-day and day-neutral N. tabacum possessed multiple buds in their upper nodes. An analysis of the number of buds per node and bud position along the main axis in periclinal chimeras indicated that the genotype of the third apical layer (LIII) of the meristem had the greatest influence on the pattern of lateral shoot meristems in both vegetative and flowering plants. The lineage of the three apical layers (LI, LII, and LIII) of the terminal shoot meristem was preserved in primary (1 degree) lateral meristems but minor deviations in lineage were observed in secondary (2 degrees) buds (i.e., those formed later but in the same node as the 1 degree bud). An analysis of the phenotype of 2 degrees shoots that displayed deviations from expected lineage indicated that in most cases the periclinal cell divisions that destabilized the lineage occurred at the flanks of the meristem and began before the most basal node, indicating that periclinal cell divisions most likely occurred prior to the inception of the 2 degrees lateral meristem. Based on our studies, we conclude that both 1 degree and 2 degrees lateral meristems in Nicotiana ultimately descend from derivatives of all three apical layers of the terminal shoot meristem.