Root architecture differences have been linked to the survival of plants on phosphate (P)-deficient soils, as well as to the improved yields of P-efficient crop cultivars. To understand how these differences arise, we have studied the root architectures of P-deficient Arabidopsis (Arabidopsis thaliana Columbia-0) plants. A striking aspect of the root architecture of these plants is that their primary root elongation is inhibited when grown on P-deficient medium. Here, we present evidence suggesting that this inhibition is a result of iron (Fe) toxicity. When the Fe concentration in P-deficient medium is reduced, we observe elongation of the primary root without an increase in P availability or a corresponding change in the expression of P deficiencyregulated genes. Recovery of the primary root elongation is associated with larger plant weights, improved ability to take up P from the medium, and increased tissue P content. This suggests that manipulating Fe availability to a plant could be a valuable strategy for improving a plant's ability to tolerate P deficiency.

Increasing emissions of heavy metals such as cadmium, mercury, and arsenic into the environment pose an acute problem for all organisms. Considerations of the biochemical basis of heavy metal detoxification in animals have focused exclusively on two classes of peptides, the thiol tripeptide, glutathione (GSH, ␥-Glu-CysGly), and a diverse family of cysteine-rich low molecular weight proteins, the metallothioneins. Plants and some fungi, however, not only deploy GSH and metallothioneins for metal detoxification but also synthesize another class of heavy metal binding peptides termed phytochelatins (PCs) from GSH. Here we show that PC-mediated heavy metal detoxification is not restricted to plants and some fungi but extends to animals by demonstrating that the ce-pcs-1 gene of the nematode worm Caenorhabditis elegans encodes a functional PC synthase whose activity is critical for heavy metal tolerance in the intact organism.Plants and some fungi post-translationally synthesize novel peptides termed phytochelatins (PCs) 1 when exposed to heavy metals. Fabricated from the ubiquitous thiol tripeptide GSH and related thiols in a novel transpeptidation reaction catalyzed by PC synthases (␥-glutamylcysteinyltransferase; EC 2.3.2.15), PCs have the general structure (␥-Glu-Cys) n -Xaa, contain 2-11 ␥-Glu-Cys repeats, chelate heavy metals at high affinity, and facilitate the vacuolar sequestration of heavy metals, most notably Cd 2ϩ (1-3). Although it is more than a decade since the first report of the partial purification of a heavy metal-, primarily Cd 2ϩ -activated PC synthase from plant extracts (1), it is only recently that the small family of genes encoding these enzymes has been identified in plants and the fission yeast Schizosaccharomyces pombe (4 -6). As exemplified by the clone from Arabidopsis thaliana (AtPCS1), these genes encode 45-55-kDa proteins that are sufficient for heavy metalactivated PC synthesis from GSH both in vivo and in vitro (6, 7).An unexpected outcome of the cloning of AtPCS1 and its equivalents from other plants and S. pombe was the identification of a single-copy gene homolog (accession number Z66513) in the nematode worm Caenorhabditis elegans (4 -6). Designated ce-pcs-1, this gene encodes a hypothetical 40.8-kDa protein (CePCS1) bearing 32% identity (45% similarity) to AtPCS1 in an overlap of 367 amino acid residues (6). Disclosure of a PCS1 homolog in the genome of C. elegans was surprising in that it raised for the first time the possibility that not only GSH and metallothioneins (8) but also PCs might participate in metal homeostasis in at least some animals.In the report that follows we demonstrate unequivocally that ce-pcs-1 encodes a bona fide PC synthase whose activity is necessary for the detoxification of heavy metals in the intact organism. Discovery of the PC synthase-dependent pathway in the model organism C. elegans establishes a firm basis for determining the ubiquity of this pathway in other animals and for elucidation of the identity and organization of the cellular machiner...

Tissue functions and mechanical coupling of cells must be integrated throughout development. A striking example of this coupling is the interactions of body wall muscle and hypodermal cells in Caenorhabditis elegans. These tissues are intimately associated in development and their interactions generate structures that provide a continuous mechanical link to transmit muscle forces across the hypodermis to the cuticle. Previously, we established that mup-4 is essential in embryonic epithelial (hypodermal) morphogenesis and maintenance of muscle position. Here, we report that mup-4 encodes a novel transmembrane protein that is required for attachments between the apical epithelial surface and the cuticular matrix. Its extracellular domain includes epidermal growth factor-like repeats, a von Willebrand factor A domain, and two sea urchin enterokinase modules. Its intracellular domain is homologous to filaggrin, an intermediate filament (IF)-associated protein that regulates IF compaction and that has not previously been reported as part of a junctional complex. MUP-4 colocalizes with epithelial hemidesmosomes overlying body wall muscles, beginning at the time of embryonic cuticle maturation, as well as with other sites of mechanical coupling. These findings support that MUP-4 is a junctional protein that functions in IF tethering, cell–matrix adherence, and mechanical coupling of tissues.

Caenorhabditis elegans strains mutant for the unc-27 gene show abnormal locomotion and muscle structure. Experiments revealed that unc-27 is one of four C. elegans troponin I genes and that three mutant alleles truncate the protein: recessive and presumed null allele e155 terminates after nine codons; semidominant su142sd eliminates the inhibitory and C-terminal regions; and semidominant su195sd abbreviates the extreme C-terminus. Assays of in vivo muscular performance at high and low loads indicated that su142sd is most deleterious, with e155 least and su195sd intermediate. Microscopy revealed in mutant muscle a prevalent disorder of dense body positioning and a less well defined sarcomeric structure, with small islands of thin filaments interspersed within the overlap region of A bands and even within the H zone. The mutants' rigid paralysis and sarcomeric disarray are consistent with unregulated contraction of the sarcomeres, in which small portions of each myofibril shorten irregularly and independently of one another, thereby distorting the disposition of filaments. The exacerbated deficits of su142sd worms are compatible with involvement in vivo of the N-terminal portion of troponin I in enhancing force production, and the severe impairment associated with su195sd highlights importance of the extreme C-terminus in the protein's inhibitory function.

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