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., axill ary meristems) in the axils of leaves establishes potential growth cen ters 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 sp ecies was studied to establish the role of position (location of the n ode along the main axis), flowering, and cell-layer interactions on th e pattern of lateral meristem initiation. In N. glauca, both the numbe r of nodes generated and the transition to flowering influenced the nu mber and position of lateral meristems. A short-day mutant of N. tabac um grown under long days remained vegetative and never produced multip le lateral buds per node, indicating that attaining a certain node num ber was not sufficient to cause the formation of multiple buds. Yet, f lowering 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 me ristem had the greatest influence on the pattern of lateral shoot meri stems in both vegetative and flowering plants. The lineage of the thre e apical layers (LI, LII, and LIII) of the terminal shoot meristem was preserved in primary (1-degrees) lateral meristems but minor deviatio ns in lineage were observed in secondary (2-degrees) buds (i.e., those formed later but in the same node as the 1-degrees bud). An analysis of the phenotype of 2-degrees shoots that displayed deviations from ex pected lineage indicated that in most cases the periclinal cell divisi ons that destabilized the lineage occurred at the flanks of the merist em and began before the most basal node, indicating that periclinal ce ll divisions most likely occurred prior to the inception of the 2-degr ees lateral meristem. Based on our studies, we conclude that both 1-de grees and 2-degrees lateral meristems in Nicotiana ultimately descend from derivatives of all three apical layers of the terminal shoot meri stem. (C) 1994 Academic Press, Inc.