The plant hormone auxin regulates various developmental processes including root formation, vascular development, and gravitropism. Mutations within the AUX1 gene confer an auxin-resistant root growth phenotype and abolish root gravitropic curvature. Polypeptide sequence similarity to amino acid permeases suggests that AUX1 mediates the transport of an amino acid-like signaling molecule. Indole-3-acetic acid, the major form of auxin in higher plants, is structurally similar to tryptophan and is a likely substrate for the AUX1 gene product. The cloned AUX1 gene can restore the auxin-responsiveness of transgenic aux1 roots. Spatially, AUX1 is expressed in root apical tissues that regulate root gravitropic curvature.

Plants employ a specialized transport system composed of separate influx and efflux carriers to mobilize the plant hormone auxin between its site(s) of synthesis and action. Mutations within the permease‐like AUX1 protein significantly reduce the rate of carrier‐mediated auxin uptake within Arabidopsis roots, conferring an agravitropic phenotype. We are able to bypass the defect within auxin uptake and restore the gravitropic root phenotype of aux1 by growing mutant seedlings in the presence of the membrane‐permeable synthetic auxin, 1‐naphthaleneacetic acid. We illustrate that AUX1 expression overlaps that previously described for the auxin efflux carrier, AtPIN2, using transgenic lines expressing an AUX1 promoter:: uidA (GUS) gene. Finally, we demonstrate that AUX1 regulates gravitropic curvature by acting in unison with the auxin efflux carrier to co‐ordinate the localized redistribution of auxin within the Arabidopsis root apex. Our results provide the first example of a developmental role for the auxin influx carrier within higher plants and supply new insight into the molecular basis of gravitropic signalling.

Our aim was to generate and prove the concept of “smart” plants to monitor plant phosphorus (P) status in Arabidopsis. Smart plants can be genetically engineered by transformation with a construct containing the promoter of a gene up-regulated specifically by P starvation in an accessible tissue upstream of a marker gene such as β-glucuronidase (GUS). First, using microarrays, we identified genes whose expression changed more than 2.5-fold in shoots of plants growing hydroponically when P, but not N or K, was withheld from the nutrient solution. The transient changes in gene expression occurring immediately (4 h) after P withdrawal were highly variable, and many nonspecific, shock-induced genes were up-regulated during this period. However, two common putative cis-regulatory elements (a PHO-like element and a TATA box-like element) were present significantly more often in the promoters of genes whose expression increased 4 h after the withdrawal of P compared with their general occurrence in the promoters of all genes represented on the microarray. Surprisingly, the expression of only four genes differed between shoots of P-starved and -replete plants 28 h after P was withdrawn. This lull in differential gene expression preceded the differential expression of a new group of 61 genes 100 h after withdrawing P. A literature survey indicated that the expression of many of these “late” genes responded specifically to P starvation. Shoots had reduced P after 100 h, but growth was unaffected. The expression of SQD1, a gene involved in the synthesis of sulfolipids, responded specifically to P starvation and was increased 100 h after withdrawing P. Leaves of Arabidopsis bearing a SQD1::GUS construct showed increased GUS activity after P withdrawal, which was detectable before P starvation limited growth. Hence, smart plants can monitor plant P status. Transferring this technology to crops would allow precision management of P fertilization, thereby maintaining yields while reducing costs, conserving natural resources, and preventing pollution.

We have investigated the subcellular localization, the domain topology, and the amino acid residues that are critical for the function of the presumptive Arabidopsis thaliana auxin influx carrier AUX1. Biochemical fractionation experiments and confocal studies using an N-terminal yellow fluorescent protein (YFP) fusion observed that AUX1 colocalized with plasma membrane (PM) markers. Because of its PM localization, we were able to take advantage of the steep pH gradient that exists across the plant cell PM to investigate AUX1 topology using YFP as a pH-sensitive probe. The YFP-coding sequence was inserted in selected AUX1 hydrophilic loops to orient surface domains on either apoplastic or cytoplasmic faces of the PM based on the absence or presence of YFP fluorescence, respectively. We were able to demonstrate in conjunction with helix prediction programs that AUX1 represents a polytopic membrane protein composed of 11 transmembrane spanning domains. In parallel, a large aux1 allelic series containing null, partial-loss-of-function, and conditional mutations was characterized to identify the functionally important domains and amino acid residues within the AUX1 polypeptide. Whereas almost all partial-loss-of-function and null alleles cluster in the core permease region, the sole conditional allele aux1-7 modifies the function of the external C-terminal domain.

NASC operates an Affymetrix 'GeneChip' (microarray) service for the Arabidopsis thaliana community. All data produced by the service are publicly available through our microarray data base 'NASCArrays' published at http://affymetrix. arabidopsis.info. The data are accessible through text searching and a series of data mining tools. All data are annotated with sample preparation details, and the original Affymetrix data are available for download. The database aims to be MIAME supportive and provide a coordinated resource for re searchers interested in the transcriptome of Arabidopsis. Using this database, data produced will be shared with other databases worldwide.