Replication in the phloem is not necessary for efficient vascular transport of tobacco mosaic tobamovirus.
Plant viruses move systemically from one leaf to another via phloem. However, the viral functions needed for systemic movement are not fully elucidated. An experimental system was designed to study the effects of low temperature on the vascular transport of the tobacco mosaic tobamovirus (TMV). Vascular transport of TMV from lower inoculated leaves to upper non-inoculated leaves via a stem segment kept at low temperature (4 degrees C) was not affected. On the other hand, several experiments were performed on tobacco leaves to demonstrate that virus replication did not occur at the same temperature. The data suggest that replication of TMV in the phloem of wild-type tobacco plants is not necessary for the vascular transport of TMV, and that the virus moves with photoassimilates as suggested previously. (+info)
Transformation of the collateral vascular bundles into amphivasal vascular bundles in an Arabidopsis mutant.
Arabidopsis inflorescence stems develop a vascular pattern similar to that found in most dicots. The arrangement of vascular tissues within the bundle is collateral, and vascular bundles in the stele are arranged in a ring. Although auxin has been shown to be an inducer of vascular differentiation, little is known about the molecular mechanisms controlling vascular pattern formation. By screening ethyl methanesufonate-mutagenized populations of Arabidopsis, we have isolated an avb1 (amphivasal vascular bundle) mutant with a novel vascular pattern. Unlike the collateral vascular bundles seen in the wild-type stems, the vascular bundles in the avb1 stems were similar to amphivasal bundles, i.e. the xylem completely surrounded the phloem. Furthermore, branching vascular bundles in the avb1 stems abnormally penetrated into the pith, which resulted in a disruption in the ring-like arrangement of vascular bundles in the stele. The avb1 mutation did not affect leaf venation pattern and root vascular organization. Auxin polar transport assay indicated that the avb1 mutation did not disrupt the auxin polar transport activity in inflorescence stems. The avb1 mutation also exhibited pleiotropic phenotypes, including curled stems and extra cauline branches. Genetic analysis indicated that the avb1 mutation was monogenic and partially dominant. The avb1 locus was mapped to a region between markers mi69 and ASB2, which is covered by a yeast artificial chromosome clone, CIC9E2, on chromosome 5. Isolation of the avb1 mutant provides a novel means to study the evolutionary mechanisms controlling the arrangement of vascular tissues within the bundle, as well as the mechanisms controlling the arrangement of vascular bundles in the stele. (+info)
The irregular xylem3 locus of Arabidopsis encodes a cellulose synthase required for secondary cell wall synthesis.
The irregular xylem3 (irx3) mutant of Arabidopsis has a severe deficiency in secondary cell wall cellulose deposition that leads to collapsed xylem cells. The irx3 mutation has been mapped to the top arm of chromosome V near the marker nga106. Expressed sequence tag clone 75G11, which exhibits sequence similarity to cellulose synthase, was found to be tightly linked to irx3, and genomic clones containing the gene corresponding to clone 75G11 complemented the irx3 mutation. Thus, the IRX3 gene encodes a cellulose synthase component that is specifically required for the synthesis of cellulose in the secondary cell wall. The irx3 mutant allele contains a stop codon that truncates the gene product by 168 amino acids, suggesting that this allele is null. Furthermore, in contrast to radial swelling1 (rsw1) plants, irx3 plants show no increase in the accumulation of beta-1,4-linked glucose in the noncrystalline cell wall fraction. IRX3 and RSW1 fall into a distinct subgroup (Csa) of Arabidopsis genes showing homology to bacterial cellulose synthases. (+info)
CUT1, an Arabidopsis gene required for cuticular wax biosynthesis and pollen fertility, encodes a very-long-chain fatty acid condensing enzyme.
Land plants secrete a layer of wax onto their aerial surfaces that is essential for survival in a terrestrial environment. This wax is composed of long-chain, aliphatic hydrocarbons derived from very-long-chain fatty acids (VLCFAs). Using the Arabidopsis expressed sequence tag database, we have identified a gene, designated CUT1, that encodes a VLCFA condensing enzyme required for cuticular wax production. Sense suppression of CUT1 in transgenic Arabidopsis plants results in waxless (eceriferum) stems and siliques as well as conditional male sterility. Scanning electron microscopy revealed that this was a severe waxless phenotype, because stems of CUT1-suppressed plants were completely devoid of wax crystals. Furthermore, chemical analyses of waxless plants demonstrated that the stem wax load was reduced to 6 to 7% of wild-type levels. This value is lower than that reported for any of the known eceriferum mutants. The severe waxless phenotype resulted from the downregulation of both the decarbonylation and acyl reduction wax biosynthetic pathways. This result indicates that CUT1 is involved in the production of VLCFA precursors used for the synthesis of all stem wax components in Arabidopsis. In CUT1-suppressed plants, the C24 chain-length wax components predominate, suggesting that CUT1 is required for elongation of C24 VLCFAs. The unique wax composition of CUT1-suppressed plants together with the fact that the location of CUT1 on the genetic map did not coincide with any of the known ECERIFERUM loci suggest that we have identified a novel gene involved in wax biosynthesis. CUT1 is currently the only known gene with a clearly established function in wax production. (+info)
Feedback regulation of GA5 expression and metabolic engineering of gibberellin levels in Arabidopsis.
The gibberellin (GA) 20-oxidase encoded by the GA5 gene of Arabidopsis directs GA biosynthesis to active GAs, whereas that encoded by the P16 gene of pumpkin endosperm leads to biosynthesis of inactive GAs. Negative feedback regulation of GA5 expression was demonstrated in stems of Arabidopsis by bioactive GAs but not by inactive GA. In transgenic Arabidopsis plants overexpressing P16, there was a severe reduction in the amounts of C20-GA intermediates, accumulation of large amounts of inactive GA25 and GA17, a reduction in GA4 content, and a small increase in GA1. However, due to feedback regulation, expression of GA5 and GA4, the gene coding for the subsequent 3beta-hydroxylase, was greatly increased to compensate for the effects of the P16 transgene. Consequently, stem height was only slightly reduced in the transgenic plants. (+info)
Responses of plant vascular systems to auxin transport inhibition.
To assess the role of auxin flows in plant vascular patterning, the development of vascular systems under conditions of inhibited auxin transport was analyzed. In Arabidopsis, nearly identical responses evoked by three auxin transport inhibitor substances revealed an enormous plasticity of the vascular pattern and suggest an involvement of auxin flows in determining the sites of vascular differentiation and in promoting vascular tissue continuity. Organs formed under conditions of reduced auxin transport contained increased numbers of vascular strands and cells within those strands were improperly aligned. In leaves, vascular tissues became progressively confined towards the leaf margin as the concentration of auxin transport inhibitor was increased, suggesting that the leaf vascular system depends on inductive signals from the margin of the leaf. Staged application of auxin transport inhibitor demonstrated that primary, secondary and tertiary veins became unresponsive to further modulations of auxin transport at successive stages of early leaf development. Correlation of these stages to anatomical features in early leaf primordia indicated that the pattern of primary and secondary strands becomes fixed at the onset of lamina expansion. Similar alterations in the leaf vascular responses of alyssum, snapdragon and tobacco plants suggest common functions of auxin flows in vascular patterning in dicots, while two types of vascular pattern alterations in Arabidopsis auxin transport mutants suggest that at least two distinct primary defects can result in impaired auxin flow. We discuss these observations with regard to the relative contributions of auxin transport, auxin sensitivity and the cellular organisation of the developing organ on the vascular pattern. (+info)
Two cytosolic cyclophilin genes of Arabidopsis thaliana differently regulated in temporal- and organ-specific expression.
We have previously isolated two closely related genes (ATCYP1 and ATCYP2) each encoding a cytosolic cyclophilin of Arabidopsis thaliana. Here we tested expression patterns of these two genes by Northern analysis and by histochemical analysis with transgenic plants carrying the promoter: beta-glucuronidase (GUS) fusion. The results showed that ATCYP1 is predominantly transcribed in vascular tissue and flowers, but ATCYP2 is at higher levels in younger leaves. The different expression patterns seemed to be conferred by the quite different promoter structures carrying various cis elements. Our finding suggests that the two cyclophilins have different roles in Arabidopsis thaliana cells. (+info)
Transgenic tobacco plants expressing the Drosophila Polycomb (Pc) chromodomain show developmental alterations: possible role of Pc chromodomain proteins in chromatin-mediated gene regulation in plants.
The chromodomain of the Drosophila Polycomb (Pc) protein has been introduced into tobacco nuclei to determine its location in the nucleus and its effect on plant development. Pc is a repressor of homeotic Drosophila genes that shares a well-conserved, although not identical, chromodomain with a structural heterochromatin component, Heterochromatin Protein 1. The chromodomains might therefore play a common role in chromatin repression. An analysis of transgenic plants expressing the Pc chromodomain, which was linked to the green fluorescent protein, suggested that the Pc chromodomain has distinct target regions in the plant genome. Transgenic plants expressing the Pc chromodomain had phenotypic abnormalities in their leaves and flowers, indicating a disruption in development. In axillary shoot buds of plants displaying altered leaf phenotypes, enhanced expression of a homeodomain gene, which is downregulated in wild-type leaves, was found. In Drosophila, Pc has been shown to possess distinct chromosome binding activity and to be involved in the regulation of development-specific genes. Our results support the assumptions that the heterologous chromodomain affects related functions in Drosophila and in plants, and that chromatin modification mechanisms are involved in the regulation of certain plant genes, in a manner similar to chromatin-mediated gene regulation in Drosophila. (+info)