Differential distribution of the plant hormone auxin within tissues mediates a variety of developmental processes. Cellular auxin levels are determined by metabolic processes including synthesis, degradation, and (de)conjugation, as well as by auxin transport across the plasma membrane. Whereas transport of free auxins such as naturally occurring indole-3-acetic acid (IAA) is well characterized, little is known about the transport of auxin precursors and metabolites. Here, we identify a mutation in the ABCG37 gene of Arabidopsis that causes the polar auxin transport inhibitor sensitive1 (pis1) phenotype manifested by hypersensitivity to auxinic compounds. ABCG37 encodes the pleiotropic drug resistance transporter that transports a range of synthetic auxinic compounds as well as the endogenous auxin precursor indole-3-butyric acid (IBA), but not free IAA. ABCG37 and its homolog ABCG36 act redundantly at outermost root plasma membranes and, unlike established IAA transporters from the PIN and ABCB families, transport IBA out of the cells. Our findings explore possible novel modes of regulating auxin homeostasis and plant development by means of directional transport of the auxin precursor IBA and presumably also other auxin metabolites.PDR9 | PDR8 | IBA transport | auxin synthesis P lants have evolved outstanding capacities to adapt their metabolism and development to respond to their environment. Changes in the availability and distribution of endogenous signaling molecules-plant hormones-play important roles in these responses (1). The phytohormone auxin, perceived by TIR1/AFB receptor proteins and interpreted by downstream nuclear signaling pathway, is an important signal that mediates transcriptional developmental reprogramming (reviewed in refs. 2 and 3). The differential distribution of auxin within tissues is essential for many adaptive responses including embryo and leaf patterning, root and stem elongation, lateral root initiation, and leaf expansion (4). Differential distribution of the major active auxin, IAA, depends on its intercellular transport and metabolic processes that involve biosynthesis by several pathways and release from storage forms including amide-or ester-linked conjugates with amino acids, peptides, and sugars (reviewed in ref. 5). The role of another endogenously occurring auxinic compound IBA is still unclear. It has been proposed that IBA acts independently of IAA (6), but a number of recent genetic findings suggest that IBA functions as an important precursor to IAA during conversion resembling peroxisomal fatty acid β-oxidation (5, 7). Besides metabolism, a crucial process controlling cellular auxin levels is the directional, intercellular auxin transport that depends on specialized influx and efflux carriers (reviewed in ref. 8). IAA transporters include amino acid permeases-like AUXIN RESISTANT1 (AUX1) mediating auxin influx (9-11), the PIN-FORMED (PIN) efflux carriers (12)(13)(14), and the MULTIDRUG RESISTANCE/P-GLYCOPROTEIN (PGP) class of ATP-Binding Cassette (ABC) auxin trans...

SUMMARYMultistep phosphorelay (MSP) signaling mediates responses to a variety of important stimuli in plants. In Arabidopsis MSP, the signal is transferred from sensor histidine kinase (HK) via histidine phosphotransfer proteins (AHP1-AHP5) to nuclear response regulators. In contrast to ancestral two-component signaling in bacteria, protein interactions in plant MSP are supposed to be rather nonspecific. Here, we show that the C-terminal receiver domain of HK CKI1 (CKI1 RD ) is responsible for the recognition of CKI1 downstream signaling partners, and specifically interacts with AHP2, AHP3 and AHP5 with different affinities. We studied the effects of Mg 2+ , the co-factor necessary for signal transduction via MSP, and phosphorylation-mimicking BeF 3 ) on CKI1 RD in solution, and determined the crystal structure of free CKI1 RD and CKI1 RD in a complex with Mg 2+ . We found that the structure of CKI1 RD shares similarities with the only known structure of plant HK, ETR1 RD , with the main differences being in loop L3. Magnesium binding induces the rearrangement of some residues around the active site of CKI1 RD , as was determined by both X-ray crystallography and NMR spectroscopy. Collectively, these results provide initial insights into the nature of molecular mechanisms determining the specificity of MSP signaling and MSP catalysis in plants.

A semi-nested polymerase chain reaction (snPCR) for detecting proviral DNA of ovine lentivirus (OvLV) in peripheral blood mononuclear cells was developed. Primers for snPCR were situated within the gag gene of the Maedi-Visna virus (MVV) genome. A comparison between the snPCR and serological tests (agar gel immunodiffusion test, immunoblot) were performed using 98 ovine blood samples. Thirty (30.6%) of the 98 sheep examined had antibodies specific for the MVV. PCR showed 21 of them to be positive and nine seropositive animals to be PCR negative. Six of the 68 serologically negative sheep were found to be PCR positive, probably due to delayed seroconversion. The PCR amplification products of these six sheep were sequenced and subjected to phylogenetic analysis. The resulting phylogenetic tree of partial gag gene sequences confirmed that the ovine lentivirus genotype in the Czech Republic is more closely related to the prototype MVV isolates than to the caprine arthritis encephalitis viruses.