Literature data, as well as results obtained in this study, suggest that the formation of a local ARM before the first anticlinal formative divisions is the common mechanism underlying LRI in flowering plants. We propose that the mechanisms of the regulation of root branching are independent of the position of the LRI site relative to the parental root tip.
The auxin response pattern in the root apex of squash resembled that in arabidopsis roots. Composite squash plants obtained by A. rhizogenes-mediated transformation are a good tool for the investigation of root apical meristem development and root branching.
The effect of 10 -6 and 10 -4 M NiSO 4 on cell growth, proliferation, and differentiation was studied over 48 h in seminal and lateral roots of five-day-old Triticum aestivum seedlings. 10 -6 M NiSO 4 did not significantly affect the root system, whereas 10 -4 M NiSO 4 inhibited its development. However, 10 -6 M NiSO 4 disturbed the contacts between the groups of closely related cells of the rhizodermis in the meristem. In the exodermis, an additional layer of cells was formed. At the nickel concentration of 10 -4 M, cell divisions in the outer layers of the root cells and metaxylem ceased earlier than in other root tissues positioned both centripetally and acropetally. Differentiation of protophloem sieve elements was completed in the meristem but at a greater distance from the root tip. Cell elongation started at the same distance from the root tip as in control plants. The rate of elongation decreased, and acropetally it stopped. Therefore, the cells of the xylem and metaphloem started to differentiate, and primordia of lateral roots were initiated and formed closer to the root tip. At a lethal concentration ( 10 -4 M), nickel induced necroses of elongating cells of the endodermis and pericycle. Nickel is supposed to enter the tissues of the central cylinder predominantly via the protoxylem and rapidly translocate along the xylem. As a result, the incubation of the roots at this concentration for 48 h almost did not affect the development of the phloem and probably sugar unloading, that makes possible to maintain the growth of meristematic cells and the cell division of the most important tissues for longer time.
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