Arabidopsis root system responds to phosphorus (P) deficiency by decreasing primary root elongation and developing abundant lateral roots. Feeding plants with ascorbic acid (ASC) stimulated primary root elongation in seedlings grown under limiting P concentration. However, at high P, ASC inhibited root growth. Seedlings of ascorbate-deficient mutant (vtc1) formed short roots irrespective of P availability. P-starved plants accumulated less ascorbate in primary root tips than those grown under high P. ASC-treatment stimulated cell divisions in root tips of seedlings grown at low P. At high P concentrations ASC decreased the number of mitotic cells in the root tips. The lateral root density in seedlings grown under P deficiency was decreased by ASC treatments. At high P, this parameter was not affected by ASC-supplementation. vtc1 mutant exhibited increased lateral root formation on either, P-deficient or P-sufficient medium. Irrespective of P availability, high ASC concentrations reduced density and growth of root hairs. These results suggest that ascorbate may participate in the regulation of primary root elongation at different phosphate availability via its effect on mitotic activity in the root tips. 1. Introduction It has been reported that phosphorus (P) availability affects root architecture in Arabidopsis. Seedlings of this plant, exposed to low P concentration (1？μM), formed a highly branched root system with abundant lateral roots and a short primary root. Under these growth conditions primary, and secondary roots had an abundance of long root hairs. Under high P concentrations (1？mM), the root system was composed of a long primary root with few lateral roots and short root hairs [1–3]. P deficiency responses of the root system are dependent on changes in cell proliferation. Under low P conditions, the reduction in primary root growth is due to inhibition of cell division and cell differentiation within the primary root meristem. The mitotic activity is relocated to the sites of lateral root formation, resulting in increased lateral root density. However, similar to the primary root tip, cell differentiation in older lateral roots occurs within the apical root meristem, which results in the arrest of lateral root elongation [4]. Besides the reduction in cell division rate, low P treatment inhibits cell elongation and reduces the number of cells in root elongation zone [4, 5]. Little is known about the mechanisms underlying the alteration in growth and development of Arabidopsis roots after plants are exposed to limiting P supply. It has been
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