Systemic nitrogen demand signaling

Shivalingappa BangiPALB- 5052

Crop Physiology UAS, GKVK Bengaluru

Why Nitrogen is important ??????

It is an essential macronutrient that impacts plant growth and development.

There is an abundant supply of nitrogen in the earth’s atmosphere— N2 gas comprises nearly 79% of air.

It is an important component of chlorophyll, amino acids, nucleic acids and secondary metabolites.

The nitrogen regulatory mechanisms are mostly characterized at the physiological level but still remain largely unknown at the molecular level

Nitrogen cycle

Nitrogen Uptake

Organic form Inorganic form

Nitrate, Ammonium

and Urea

Amino acid, Proteins

and peptides

Nitrate is one of the most abundant N sources in the soil.

Under various environmental conditions, plants need to acquire nitrate efficiently from the soil, distribute it between source and sink organs, and adjust nitrate homeostasis at the cellular level.

To do so, plants use a combination of transporters and channels with diverse ranges of affinity and specificity.

Nitrate transporter and channels in higher plants

NRT1 (Nitrate transporter-1) Dual-affinity (transport and signalling)

NRT2 (Nitrate transporter-2)

CLC family (Chloride channel)

SLAH3 (Slow anion channel- associated 1 homolog 3)- uptake, allocation and sensing of nitrate

CLC a

CLC b

O’Brien et al., 2016Nitrate signaling pathway in Arabidopsis roots

PLC- phspholipase CTGA1- TGACG MOTIF- BINDING FACTORABF3- AUXIN SIGNALING F-BOX 3NAC4- NAC DOMAIN CONTAINING PROTEIN

Wang et al., 2012

How Nitrate movement in root system ????

NAXT1 – Nitrate excretion transporter 1

Wang et al., 2012Regulation of root-to-shoot nitrate transport

Co- cortexPe- pericycleXy - xylemPh – phloem En - endodermis

Wang et al., 2012

Directions of nitrate movement mediated by transporters and channels CIPK3 - Calcineurin-B like (CBL)-CBL-interacting protein kinase

CPK21- calcium-dependant protein kinase 21 NAXT1 – Nitrate excretion transporter 1

NRT2.1NRT2.2NRT2.4NRT1.1NRT1.2

HATS

LATS

Uptake

Efflux

NAXT1

Xylem

SLAH3

NRT1.1

Vacuole

CLCa,b

NO3 -

H+

Vacuole

NRT2.7

NO3 - H+ (?)

Embryo cell

Mesophyll cell

Guard cell

seed

NRT1.6

NRT1.7Remobilize

NRT 1.8

NRT 1.9

NRT 1.5

Role of Nitrate transporters in whole plant system ??

Wang et al., 2012Nitrate remobilization from older to younger leaves

Wang et al., 2012

Nitrate transport to embryos F - Funiculus

Components of the influx and efflux of nitrate and ammonium in roots.

Hachiya et al., 2016NSCC- non-selective cation channelsAMT- Ammonium transporters

Conclusion Further study is required to understand the function of other transporters. For example, there are at 80 genes in the rice (Oryza sativa) NRT1 family but to date only two of them have been functionally characterized.

In Arabidopsis also less than one third of NRT1 and NRT2 genes have been characterized. Now a big question to plant scientists is that why higher plants need so many nitrate transporters? and how those genes were function and how these coordinate the external stimuli with internal growth, it requires more study.

REFERENCE

HACHIYA, T., AND SAKAKIBARA, H., 2016, Interactions between nitrate and ammonium in their uptake, allocation, assimilation, and signaling in plants. Journal of Experimental Botany, erw449.

O'BRIEN, J. A., VEGA, A., BOUGUYON, E., KROUK, G., GOJON, A., CORUZZI, G., AND GUTIÉRREZ, R. A., 2016, Nitrate transport, sensing, and responses in plants. Molecular Plant, 9(6): 837-856.

WANG, Y. Y., HSU, P. K., AND TSAY, Y. F., 2012, Uptake, allocation and signaling of nitrate. Trends in plant science, 17(8): 458-467.