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.

Root Nutrient Foraging

“…For instance, PR elongation is strongly inhibited by systemic calcium deficiency, whereas LR elongation is even slightly induced (Gruber et al, 2013). Likewise, NH 4 + concentrations that stimulate LR formation concomitantly suppress meristem size of the parental root (Lima et al, 2010;Liu et al, 2013b). As suggested by the specific effect of mild and prolonged nitrogen deficiency in TAR2-regulated or CLE-CLV1-regulated LR elongation (Araya et al, 2014;Ma et al, 2014), a dose-dependent or pronounced morphological response in LRs may rely on not only the size of a signal but also, the cell type-specific expression pattern of receptors translating these systemic signals into developmental changes.…”

Role of Arbuscular Mycorrhizal Fungi in the Nitrogen Uptake of Plants: Current Knowledge and Research Gaps

“…Valentine et al [96] also reported, that NH4 + fed AM roots had lower numbers of vesicles and arbuscules than NO3 − supplied roots. An excess of NH4 + as sole N source is often considered to be toxic for plants, and inhibits root growth [97].…”

Comparison of Chromatin Epigenetic Modification Patterns among Root Meristem, Elongation and Maturation Zones in Maize (<b><i>Zea mays</i></b> L.)

“…The regulation of auxin distribution in roots controls both cell division and elongation [Blilou et al, 2005]. Research indicates that ammonium influences root growth by reducing both the meristem and elongation zones and decreasing the elemental expansion rate in the root apex [Liu Y et al, 2013].…”