The evolution of land flora transformed the terrestrial environment. Land plants evolved from an ancestral charophycean alga from which they inherited developmental, biochemical, and cell biological attributes. Additional biochemical and physiological adaptations to land, and a life cycle with an alternation between multicellular haploid and diploid generations that facilitated efficient dispersal of desiccation tolerant spores, evolved in the ancestral land plant. We analyzed the genome of the liverwort Marchantia polymorpha, a member of a basal land plant lineage. Relative to charophycean algae, land plant genomes are characterized by genes encoding novel biochemical pathways, new phytohormone signaling pathways (notably auxin), expanded repertoires of signaling pathways, and increased diversity in some transcription factor families. Compared with other sequenced land plants, M. polymorpha exhibits low genetic redundancy in most regulatory pathways, with this portion of its genome resembling that predicted for the ancestral land plant. PAPERCLIP.

Uptake and translocation of cationic nutrients play essential roles in physiological processes including plant growth, nutrition, signal transduction, and development. Approximately 5% of the Arabidopsis genome appears to encode membrane transport proteins. These proteins are classified in 46 unique families containing approximately 880 members. In addition, several hundred putative transporters have not yet been assigned to families. In this paper, we have analyzed the phylogenetic relationships of over 150 cation transport proteins. This analysis has focused on cation transporter gene families for which initial characterizations have been achieved for individual members, including potassium transporters and channels, sodium transporters, calcium antiporters, cyclic nucleotide-gated channels, cation diffusion facilitator proteins, natural resistance-associated macrophage proteins (NRAMP), and Zn-regulated transporter Fe-regulated transporterlike proteins. Phylogenetic trees of each family define the evolutionary relationships of the members to each other. These families contain numerous members, indicating diverse functions in vivo. Closely related isoforms and separate subfamilies exist within many of these gene families, indicating possible redundancies and specialized functions. To facilitate their further study, the PlantsT database (http://plantst.sdsc.edu) has been created that includes alignments of the analyzed cation transporters and their chromosomal locations.

Metal cation homeostasis is essential for plant nutrition and resistance to toxic heavy metals. Many plant metal transporters remain to be identified at the molecular level. In the present study, we have isolated AtNramp cDNAs from Arabidopsis and show that these genes complement the phenotype of a metal uptake deficient yeast strain, Metal cations are essential for plant nutrition (1). Metals such as iron (Fe), manganese (Mn), and copper (Cu) are necessary cofactors for many enzymatic reactions. Some metals such as zinc (Zn) play important structural roles in proteins. Furthermore, metal cations recently have been shown to be involved in signaling in animals (2) and plants (3). However, plants also need to control against excessive accumulation of essential cations and toxic heavy metals, such as cadmium (Cd 2ϩ ), lead, mercury, and arsenic.Metal transporters are essential to maintain intracellular metal homeostasis (4). In plants, metal cation transporters play important roles in several steps of metal nutrition. These transport proteins mediate metal uptake in root cells and metal transfer between cells and organs. Metal transporters are involved in metal detoxification by mediating the transport of metal cations or metal chelates from the cytosol to the vacuolar compartment (5-8). A better knowledge of the mechanisms of metal transport in plants is needed for understanding and manipulating plant nutrition and plant resistance to toxic heavy metals.Uptake of metal cations has been investigated in various species. These studies point to multiple pathways for the lowand high-affinity and constitutive and inducible inf lux of metal cations, such as Zn and Fe (9 -11). It has been suggested that toxic metals such as cadmium enter plant cells by transporters for essential cations, such as . For example, in pea, Fe deficiency, which is known to stimulate high-affinity Fe uptake also stimulates Cd 2ϩ uptake (12). Recently, screening of plant cDNA libraries for genes able to restore the growth defects of yeast metal transport mutants has led to the identification of plant genes encoding transporters that allow the uptake of Cu (16), Fe, Mn, and Zn (17-19) and various cations (14) in yeast. The ion transport function of these cloned transporters has not yet been directly analyzed in roots.Genes encoding members of the Nramp family of integral membrane proteins were identified through very diverse genetic screens (20). Nramp1 was cloned in mouse as a locus involved in intracellular bacterial pathogen sensitivity (21). Nramp homologous sequences have now been identified in bacteria, fungi, plants, and animals. It was discovered recently that some Nramp proteins function as metal transporters. SMF1, a yeast Nramp homologue, was shown to encode a manganese transporter (22). Then, DCT1͞Nramp2, a mammalian homologue of the Nramp genes, was isolated in a functional screen for Fe transport systems and shown to encode a broad specificity metal transporter (23). DCT1͞Nramp2 was also shown to be a genetic determinant for anemia,...

Understanding the functional connections between genes, proteins, metabolites and mineral ions is one of biology's greatest challenges in the postgenomic era. We describe here the use of mineral nutrient and trace element profiling as a tool to determine the biological significance of connections between a plant's genome and its elemental profile. Using inductively coupled plasma spectroscopy, we quantified 18 elements, including essential macro- and micronutrients and various nonessential elements, in shoots of 6,000 mutagenized M2 Arabidopsis thaliana plants. We isolated 51 mutants with altered elemental profiles. One mutant contains a deletion in FRD3, a gene known to control iron-deficiency responses in A. thaliana. Based on the frequency of elemental profile mutations, we estimate 2-4% of the A. thaliana genome is involved in regulating the plant's nutrient and trace element content. These results demonstrate the utility of elemental profiling as a useful functional genomics tool.

Abscisic acid (ABA) regulates vital physiological responses, and a number of events in the ABA signaling cascade remain t o be identified. To allow quantitative analysis of genetic signaling mutants, patch-clamp experiments were developed and performed with the previously inaccessible Arabidopsis guard cells from the wild type and ABA-insensitive (abi) mutants. Slow anion channels have been proposed t o play a rate-limiting role in ABA-induced stomatal closing. We now directly demonstrate that ABA strongly activates slow anion channels in wild-type guard cells. Furthermore, ABA-induced anion channel activation and stomatal closing were suppressed by protein phosphatase inhibitors. In abil-7 and abi2-7 mutant guard cells, ABA activation of slow anion channels and ABA-induced stomatal closing were abolished. These impairments in ABA signaling were partially rescued by kinase inhibitors in abil but not in abi2 guard cells. These data provide cell biological evidence that the abi2 locus disrupts early ABA signaling, that abil and abi2 affect ABA signaling at different steps in the cascade, and that protein kinases act as negative regulators of ABA signaling in Arabidopsis. New models for ABA signaling pathways and roles for abil, abi2, and protein kinases and phosphatases are discussed.