PREFERENTIAL EXPRESSION OF AN AMMONIUM TRANSPORTER AND OF 2 PUTATIVE NITRATE TRANSPORTERS IN ROOT HAIRS OF TOMATO

Root hairs as specialized epidermal cells represent part of the outerm ost interface between a plant and its soil environment, They make up t o 70% of the root surface and, therefore, are likely to contribute sig nificantly to nutrient uptake, To study uptake systems for mineral nit rogen, three genes homologous to Arabidopsis nitrate and ammonium tran sporters (AtNrt1 and AtAmt1) were isolated from a root hair-specific t omato cDNA library. Accumulation of LeNrt1-1, LeNrt1-2, and LeAmt1 tra nscripts was root-specific, with no detectable transcripts in stems or leaves. Expression was root cell type-specific and regulated by nitro gen availability. LeNrt1-2 mRNA accumulation was restricted to root ha irs that had been exposed to nitrate. In contrast, LeNrt1-1 transcript s were detected in root hairs as well as other root tissues under all nitrogen treatments applied, Analogous to LeNrt1-1, the gene LeAmt1 wa s expressed under all nitrogen conditions tested, and root hair-specif ic mRNA accumulation was highest following exposure to ammonium. Expre ssion of LeAMT1 in an ammonium uptake-deficient yeast strain restored growth on low ammonium medium, confirming its involvement in ammonium transport. Root hair specificity and characteristics of substrate regu lation suggest an important role of the three genes in uptake of miner al nitrogen.