FORMATION OF LATERAL ROOT-MERISTEMS IS A 2-STAGE PROCESS

In both radish and Arabidopsis, lateral root initiation involves a ser ies of rapid divisions in pericycle cells located on the xylem radius of the root, In Arabidopsis, the number of pericycle cells that divide to form a primordium was estimated to be about 11, To determine the s tage at which primordia are able to function as root meristems, primor dia of different stages were excised and cultured without added hormon es. Under these conditions, primordia that consist of 2 cell layers fa il to develop while primordia that consist of at least 3-5 cell layers develop as lateral roots, We hypothesize that meristem formation is a two-step process involving an initial period during which a populatio n of rapidly dividing, approximately isodiametric cells that constitut es the primordium is formed, and a subsequent stage during which meris tem organization takes place within the primordium.