Lateral root initiation by asymmetrical transverse divisions of pericycle cells in adventitious roots ofAllium cepa

Summary In onion adventitious roots cellular events have been identified that indicate that lateral root initiation occurs earlier and nearer the apex than previously documented. Lateral roots are not initiated when a pericycle cell divides periclinally but earlier, when a pair of neighbouring pericycle cells in the same column divide transversely and asymmetrically, with both mitoses close to the end towards the neighbouring pericycle cell. Each cell therefore produces two cells of unequal length. The shorter cells produced by the mother pericycle cells are adjacent, while the longer cells are located above and below the shorter cells. This objective morphological criterion allows clear identification of the site of lateral root initiation. Subsequent to these asymmetric divisions, both the longer pericycle cells again divide transversely and asymmetrically producing more short cells adjacent to the previous ones. The first periclinal division occurs in one of these short pericycle cells.