Arabidopsis plants responding to phosphorus (P) deficiency increase lateral root formation and reduce primary root elongation. In addition the number and length of root hairs increases in response to P deficiency. Here we studied the patterns of radical oxygen species (ROS) in the roots of Arabidopsis seedlings cultured on media supplemented with high or low P concentration. We found that P availability affected ROS distribution in the apical part of roots. If plants were grown on high P medium, ROS were located in the root elongation zone and quiescent centre. At low P ROS were absent in the elongation zone, however, their synthesis was detected in the primary root meristem. The proximal part of roots was characterized by ROS production in the lateral root primordia and in elongation zones of young lateral roots irrespective of P concentration in the medium. On the other hand, plants grown at high or low P differed in the pattern of ROS distribution in older lateral roots. At high P, the elongation zone was the primary site of ROS production. At low P, ROS were not detected in the elongation zone. However, they were present in the proximal part of the lateral root meristem. These results suggest that P deficiency affects ROS distribution in distal parts of Arabidopsis roots. Under P-sufficiency ROS maximum was observed in the elongation zone, under low P, ROS were not synthesized in this segment of the root, however, they were detected in the apical root meristem.Keywords Hydrogen peroxide Á Lateral roots Á Phosphate availability Á Superoxide Á Root growth Á Root system architecture Abbreviations DCF 2 0 ,7 0 -Dichlorofluorescein DCFH 2 0 ,7 0 -Dichlorodihydrofluorescein NBT Nitroblue tetrazolium QC Quiescent centre ROS Radical oxygen species

The aim of this study was to test the effect of auxin treatment on selected parameters of the redox metabolism in roots. We found that auxin application results in a reduction in the H 2 O 2 level in roots. The hormone stimulated CuZn-superoxide dismutase, but simultaneously increased the activities of catalase, cell wall bound ferulic acid peroxidase, and soluble peroxidase izoenzymes. The analysis of the expression of genes coding for the cytosolic izoform of CuZn-superoxide dismutase, catalase, and cell wall associated peroxidase (TPX 1) involved in cell wall stiffening and lignification revealed the stimulatory effect of exogenous auxin on the expression of the aforementioned genes. The enzyme activity and gene expression in the roots of control and auxin-treated plants were studied in daily intervals, during a 3-day-long growth cycle. The stimulatory effect of auxin on the enzymatic activity was transient with the highest stimulation observed on the second day of treatment. On the third day, the activities of the enzymes decreased. The maximal enzyme activities were preceded by a rise in gene expression. The increase in the level of CuZn-superoxide dismutase and catalase transcripts were detected after 1 day of auxin treatment. Then the expression of the aforementioned genes decreased. The period of auxin-dependent stimulation of the TPX 1 gene expression encompassed the first and the second day of treatment. Auxin stimulated CuZnsuperoxide dismutase and catalase activities only in the distal zone of the root while peroxidase activity was increased by auxin in the distal as well as in the proximal parts of the organ.

Addendum to: Tyburski J, Dunajska K, Tretyn A. Reactive oxygen species localization in roots of Arabidopsis thaliana seedlings under phosphate deficiency. Plant Growth Regul 2009; 59:27-36; DOI: 10.1007/s10725-009-9385-9. T he developmental response of the Arabidopsis root system to low phosphorus (P) availability involves the reduction in primary root elongation accompanied by the formation of numerous lateral roots. We studied the roles of selected redox metabolites, namely, radical oxygen species (ROS) and ascorbic acid (ASC) in the regulation of root system architecture by different P availability. Rapidly growing roots of plants grown on P-sufficient medium synthesize ROS in root elongation zone and quiescent centre. We have demonstrated that the arrest of root elongation at low P medium coincides with the disappearance of ROS from the elongation zone. P-starvation resulted in a decrease in ascorbic acid level in roots. This correlated with a decrease in cell division activity. On the other hand, feeding P-deficient plants with ASC, stimulated mitotic activity in the primary root meristem and partly reversed the inhibition of root growth imposed by low P conditions. In this paper, we discuss the idea of the involvement of redox agents in the regulation of root system architecture under low P availability. Phosphorus (P) Availability Defines the Patterns of Root Architecture in Arabidopsis Affecting Cell Divisions and ElongationThe seedlings of Arabidopsis thaliana develop clearly distinguishable patterns of root system architecture in response to variable P availability. Cultures at low P concentration (1 μM) result in a reduction in primary root growth, increased lateral root formation and enhanced root hair development. On the other hand, at high P concentration (1 mM), the root system is composed of a long primary root with few lateral roots and short root hairs. 1-3Responses of the root system to P deficiency are dependent on changes in cell proliferation. Arrest of primary root growth at low P availability is due to the inhibition of cell division and the onset of cell differentiation within the primary root meristem. Mitotic activity is relocated to the sites of lateral root formation, which results in an increased lateral root density. In a manner similar to the primary root tip, cell differentiation in older lateral roots occurs within the apical root meristem, which is followed by an arrest in lateral root elongation. 4,5 Besides the reduction in cell division rate, low P treatments inhibit cell growth in root elongation zone. 5,6 Promotion of lateral root development and the arrest of cell divisions in the apices of roots of P-starved plants results from changes in auxin transport and/or sensitivity.3,4 However, the processes affected by P-deficiency, namely, cell division and elongation are to a large extent, regulated by redox factors like radical oxygen species (ROS) or ascorbate. 7Redox agents are involved in an auxindependent patterning in the root apical meristem. Cells of the quiescent ...