title: Ammonium inhibits primary root growth by reducing the length of meristem and elongation zone and decreasing elemental expansion rate in the root apex in Arabidopsis thaliana.
abstract: The inhibitory effect of ammonium on primary root growth has been well documented; however the underlying physiological and molecular mechanisms are still controversial. To avoid ammonium toxicity to shoot growth, we used a vertical two-layer split plate system, in which the upper layer contained nitrate and the lower layer contained ammonium. In this way, nitrogen status was maintained and only the apical part of the root system was exposed to ammonium. Using a kinematic approach, we show here that 1 mM ammonium reduces primary root growth, decreasing both elemental expansion and cell production. Ammonium inhibits the length of elongation zone and the maximum elemental expansion rate. Ammonium also decreases the apparent length of the meristem as well as the number of dividing cells without affecting cell division rate. Moreover, ammonium reduces the number of root cap cells but appears to affect neither the status of root stem cell niche nor the distal auxin maximum at the quiescent center. Ammonium also inhibits root gravitropism and concomitantly down-regulates the expression of two pivotal auxin transporters, AUX1 and PIN2. Insofar as ammonium inhibits root growth rate in AUX1 and PIN2 loss-of-function mutants almost as strongly as in wild type, we conclude that ammonium inhibits root growth and gravitropism by largely distinct pathways.
authors.affiliation: Key Lab of Plant-Soil Interaction, MOE, Center for Resources, Environment, and Food Security, College of Resources and Environmental Sciences, China Agricultural University, Beijing, China.
authors.forename: Ying
authors.initials: Y
authors.lastname: Liu
authors.forename: Ningwei
authors.initials: N
authors.lastname: Lai
authors.forename: Kun
authors.initials: K
authors.lastname: Gao
authors.forename: Fanjun
authors.initials: F
authors.lastname: Chen
authors.forename: Lixing
authors.initials: L
authors.lastname: Yuan
authors.forename: Guohua
authors.initials: G
authors.lastname: Mi
mesh_terms: D017360:Arabidopsis / Q000033:anatomy & histology / Q000166:cytology / Q000187:drug effects* / Q000254:growth & development*; D029681:Arabidopsis Proteins / Q000235:genetics; D001696:Biomechanical Phenomena; D002452:Cell Count; D002455:Cell Division / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation / Q000187:drug effects; D005308:Fertilizers; D018522:Gravitropism / Q000187:drug effects; D026901:Membrane Transport Proteins / Q000235:genetics; D018519:Meristem / Q000033:anatomy & histology / Q000166:cytology / Q000187:drug effects* / Q000254:growth & development*; D009154:Mutation; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology*
chemical_list: D029681:Arabidopsis Proteins; D005308:Fertilizers; D026901:Membrane Transport Proteins; D000644:Quaternary Ammonium Compounds; C100794:AUX1 protein, Arabidopsis; C117013:AT5G57090 protein, Arabidopsis