The Arabidopsis Dual-Affinity Nitrate Transporter Gene AtNRT1.1 (CHL1) Is Activated and Functions in Nascent Organ Development during Vegetative and Reproductive Growth

The AtNRT1.1 (CHL1) transporter provides a primary mechanism for nitrate uptake 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 cortex. A time-course study showed that CHL1 is activated in the primary root tip early in seedling development and at the earliest stages of lateral root formation. Strong CHL1 expression also was found in shoots, concentrated 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. chl1 mutants showed a reduction in the growth of nascent roots, stems, leaves, and flower buds. The growth of nascent primary roots was inhibited in the mutants even in the absence of added nitrate, whereas elongation of lateral root primordia was inhibited specifically at low nitrate and acidic pH. Interestingly, chl1 mutants also displayed a late-flowering phenotype. These results indicate that CHL1 is activated and functions in the growth of nascent organs in both shoots and roots during vegetative and reproductive growth.