Possible involvement of cytokinin in nitrate-mediated root growth in maize Qiuying Tian, Fanjun Chen, Fusuo Zhang & Guohua Mi 1 Department of Plant Nutrition, College of Resources and Environmental Science, China Agricultural Uni- versity, 100094, Beijing, China. 1 Corresponding author* Received 8 March 2005. Accepted in revised form 2 May 2005 Key words: cytokinin, maize (Zea mays L.), nitrate, root growth Abstract Response of root system architecture to nutrient availability in soils is an essential way for plants to adapt to soil environments. Nitrate can affect root development either as a result of changes in the external concentration, or through changes in the internal nutrient status of the plant. Nevertheless, less is known about the physiological mechanisms. In the present study, two maize (Zea mays L.) inbred lines (478 and Wu312) were used to study a possible role of cytokinin in nitrate-mediated root growth in nutrient solu- tions. Root elongation of 478 was more sensitive to high nitrate supply than that of Wu312. Medium high nitrate (5 mM) inhibited root elongation in 478, while, root elongation in Wu312 was only inhibited at high NO 3 ) supply (20 mM). Under high nitrate supply, the root elongation zone in 478 became swollen and the site of lateral root elongation was close towards the root tip. Both of the phenomena are typical of root growth induced by exogenous cytokinin treatments. Correspondingly, zeatin and zeatin nucleotide (Z + ZR) concentrations were increased at higher nitrate supply in 478, whereas they were constant in Wu312. Furthermore, exogenous cytokinin 6-benzylaminopurine (6-BA) completely reversed the stimula- tory effect of low nitrate on root elongation. Therefore, it is supposed that the inhibitory effect of high concentration of nitrate on root elongation is, at least in part, mediated by increased cytokinin level in roots. High nitrate supply may have negative influences on root apex activity by affecting cytokinin metabolism so that root apical dominance is weakened and, therefore, root elongation is suppressed and lateral roots grow closer to the root apex. Nitrate suppressed lateral root elongation in Wu312 at con- centration higher than 5 mM. In 478, however, this phenomenon was not significant even at 20 mM nitrate. Although exogenous 6-BA (20 nM) could suppress lateral root elongation as well, the inhibitory effect of high NO 3 ) concentration of nitrate on lateral root growth cannot be explained by changes in endogenous cytokinin alone. Abbreviations: 6-BA – 6-benzylaminopurine; ELISA – enzyme-linked immunosorbent assay; LR – lateral root; NR – nitrate reductase; Z – zeatin; ZR – zeatin nucleotide Introduction Nutrient supply can affect root development either as a result of changes in the external con- centration, or through changes in the internal nutrient status of the plant (Forde and Lorenzo, 2001). There is an increase in the number of LRs initiated in the nutrient-rich zone and an increase in the growth rate of both new and existing LRs, leading to a greatly increased root density within the nutrient-rich zone. This response has been shown for NO 3 ) , NH 4 + , and phosphate, but K + does not elicit the same effect (Drew, 1975; Drew and Saker, 1975). One of the nutrients that have been studied in great detail is nitrate, which has both positive and * FAX No: +86-10-62731016. E-mail: miguohua@cau.edu.cn Plant and Soil (2005) 277:185–196 Ó Springer 2005 DOI 10.1007/s11104-005-6837-5