The Arabidopsis dual-affinity nitrate transporter gene AtNRT1.1. (CHL1) isactivated and functions in nascent organ development during vegetative andreproductive growth

The AtNRT1.1 (CHL1) transporter provides a primary mechanism for nitrate up take in Arabidopsis and is expected to localize to the epidermis and cortex of the mature root, where the bulk of nitrate uptake occurs. Using fusions to GFP/GUS marker genes, we found CHL1 expression concentrated in the tips of primary and lateral roots, with very low signals in the epidermis and c ortex. A time-course study showed that CHL1 is activated in the primary roo t tip early in seedling development and at the earliest stages of lateral r oot formation. Strong CHL1 expression also was found in shoots, concentrate d in young leaves and developing flower buds but not in the shoot meristem. These expression patterns were confirmed by immunolocalization and led us to examine CHL1 function specifically in the growth of developing organs. c hl1 mutants showed a reduction in the growth of nascent roots, stems, leave s, and flower buds. The growth of nascent primary roots was inhibited in th e mutants even in the absence of added nitrate, whereas elongation of later al root primordia was inhibited specifically at low nitrate and acidic pH. Interestingly, chl1 mutants also displayed a late-flowering phenotype. Thes e results indicate that CHL1 is activated and functions in the growth of na scent organs in both shoots and roots during vegetative and reproductive gr owth.