Phosphorus is a major nutrient acquired by roots via high-affinity inorganic phosphate (Pi) transporters. In this paper, we describe the tissue-specific regulation of tomato (Lycopersicon esculentum L.) Pi-transporter genes by Pi. The encoded peptides of the LePT1 and LePT2 genes belong to a family of 12 membrane-spanning domain proteins and show a high degree of sequence identity to known high-affinity Pi transporters. Both genes are highly expressed in roots, although there is some expression of LePT1 in leaves. Their expression is markedly induced by Pi starvation but not by starvation of nitrogen, potassium, or iron. The transcripts are primarily localized in root epidermis under Pi starvation. Accumulation of LePT1 message was also observed in palisade parenchyma cells of Pi-starved leaves. Our data suggest that the epidermally localized Pi transporters may play a significant role in acquiring the nutrient under natural conditions. Divided root-system studies support the hypothesis that signal(s) for the Pi-starvation response may arise internally because of the changes in cellular concentration of phosphorus.

For a better understanding of the molecular and biochemical processes involved in orthophosphate (Pi) uptake at the root/soil interface, we cloned a Pi-transporter c DNA (LePT1) from a root air-specific cDNA library of tomato (Lycopersicon esculentum Mill.). The corresponding protein belongs to the growing family of ion transporters with twelve putative transmembrane domains. It is highly homologous to recently isolated Pi transporters from higher plants, yeast and fungi. When expressed in a Pi-uptake-deficient yeast mutant, the L. esculentum phosphate transporter 1 (LePT1) protein exhibits an apparent Km of 31 MicroM. The transporter is still active at submicromolar Pi concentrations and mediates highest Pi uptake at pH 5. The activity of LePT1 is dependent on the electrochemical membrane potential mediated by the yeast P-type H + - ATPase. Transcript levels of LePT1 in tomato seedlings are detectable in all vegetative organs under Pi-sufficient conditions, with highest concentrations in root hairs. In situ hybridization studies demonstrate cell-specific expression of LePT1 in the tomato root. The LePT1 mRNA is detectable in peripheral cell layers such as rhizodermal and root cap cells. Under Pi-deprivation condition, mRNA levels are also detectable in young stelar tissue. This work presents molecular and biochemical evidence for distinct root cells playing an important role in Pi acquisition at the root/soil interface.

Here, orthologous genes of six phosphate transporter (PiT) genes, which are members of the Pht1 and Pht2 families in tomato and potato, have been cloned from the solanaceous species pepper, eggplant and tobacco. Overall, expressions of these genes in pepper, eggplant and tobacco showed similar patterns to those in tomato and potato: P-starvation enhancement in both leaves and roots for Pht1;1, P-depletion induction exclusively in roots for Pht1;2, mycorrhizal enhancement for Pht1;3, and mycorrhizal induction for both Pht1;4 and Pht1;5. In the roots of nonmycorrhizal eggplant, SmPht1;3, SmPht1;4 and SmPht1;5 were also expressed under extreme P starvation. Mycorrhizal symbiosis under high-P supply conditions reduced plant growth, with concurrent enhancement of Pht1;2 expression in the roots of pepper as well as eggplant. In addition, the mycorrhizal symbiosis down-regulated the expression of Pht2;1 genes greatly in the leaves of pepper and tobacco. The discrepancies between the evolutionary distances of the PiT genes and their expression patterns among the five species suggest greater complexity in function of PiT in plants than previously expected.

The results obtained from this study provide new insights into the evolutionary expansion, functional divergence and genetic redundancy of the Pht1 genes in response to Pi deficiency and mycorrhizal symbiosis in tomato.