Characterization and expression analysis of TERMINAL FLOWER1 homologs from cultivated alloteraploid cotton (Gossypium hirsutum) and its diploid progenitors

The seasonal cycle and persistence of a plant is governed by a combination of the determinate or indeterminate status of shoot and root apical meristems. A perennial plant is one in which the apical meristem of at least one of its shoot axes remains indeterminate beyond the first growth season.TERMINAL FLOWER1 (TFL1) genes play important roles in regulating flowering time, the fate of inflorescence meristem and perenniality. To investigate the role of TFL1-like genes in the determination of the apical meristems in an industrially important crop cultivated for its fibers, we isolated and characterized two TFL1 homologs (TFL1a and TFL1b) from tetraploid cultivated cotton (Gossypium hirsutum) and its diploid progenitors (Gossypium arboreum and Gossypium raimondii). All isolated genes maintain the same exon–intron organization. Their phylogenetic analysis at the amino acid level confirmed that the isolated sequences are TFL1-like genes and collocate in the TFL1 clade of the PEBP protein family. Expression analysis revealed that the genes TFL1a and TFL1b have slightly different expression patterns, suggesting different functional roles in the determination of the meristems. Additionally, promoter analysis by computational methods revealed the presence of common binding motifs in TFL1-like promoters. These are the first reported TFL1-like genes isolated from cotton, the most important crop for the textile industry.     

Anaerobic induction and tissue-specific expression of maize Adh1 promoter in transgenic rice plants and their progeny.

Summary In order to analyze expression of the maize alcohol dehydrogenase 1 gene (Adh1), its promoter was fused with the gusA reporter gene and introduced into rice by protoplast transformation. Histochemical analysis of transgenic plants and their progeny showed that the maize Adh1 promoter is constitutively expressed in root caps, anthers, anther filaments, pollen, scutellum, endosperm and shoot and root meristem of the embryo. Induction of expression by the Adh1 promoter was examined using seedlings derived from selfed progeny of the transgenic plants. The results showed that expression of the Adh1 promoter was strongly induced (up to 81-fold) in roots of seedlings after 24 h of anaerobic treatment, concomitant with an increase in the level of gusA mRNA. 2,4-D also induced Adh1 promoter-directed expression of gusA to a similar extent. In contrast, little induction by anaerobic treatment was detected in transformed calli, leaves or roots of primary transformants or shoots of seedlings. A detailed examination of seedling roots during anaerobic treatment revealed that the induction started first at the meristem and after 3 h there was strong induction in the elongation zone which is located 1–2 mm above the meristem; the induction then progressed upward from this region. Our results suggest that transgenic rice plants carring the gusA reporter gene fused with promoters are useful for the study of anaerobic regulation of genes derived from graminaceous species.     

Expression of CENTRORADIALIS (CEN) and CEN-like genes in tobacco reveals a conserved mechanism controlling phase change in diverse species.

Plant species exhibit two primary forms of flowering architecture, namely, indeterminate and determinate. Antirrhinum is an indeterminate species in which shoots grow indefinitely and only generate flowers from their periphery. Tobacco is a determinate species in which shoot meristems terminate by converting to a flower. We show that tobacco is responsive to the CENTRORADIALIS (CEN) gene, which is required for indeterminate growth of the shoot meristem in Antirrhinum. Tobacco plants overexpressing CEN have an extended vegetative phase, delaying the switch to flowering. Therefore, CEN defines a conserved system controlling shoot meristem identity and plant architecture in diverse species. To understand the underlying basis for differences between determinate and indeterminate architectures, we isolated CEN-like genes from tobacco (CET genes). In tobacco, the CET genes most similar to CEN are not expressed in the main shoot meristem; their expression is restricted to vegetative axillary meristems. As vegetative meristems develop into flowering shoots, CET genes are downregulated as floral meristem identity genes are upregulated. Our results suggest a general model for tobacco, Antirrhinum, and Arabidopsis, whereby the complementary expression patterns of CEN-like genes and floral meristem identity genes underlie different plant architectures.     

Temporal root responses in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> L. to chromate reveal structural and regulatory mechanisms involving the SOLITARY ROOT/IAA14 repressor for maintenance of identity meristem genes

The Arabidopsis root system is modified in response to stress generated by high concentrations of nonessential ions such as chromate [Cr(VI)]. In this work, the distribution of auxin and its transporters PIN1 and PIN7, as well as the expression of genes that maintain the identity of the root meristem, were analyzed in Arabidopsis thaliana wild-type (WT) seedlings and in a mutant affected in the SOLITARY ROOT (SLR1/IAA14) locus, which is required for root response to Cr(VI). We show that primary root inhibition, auxin transporter levels, and expression of meristem identity genes were maintained in the slr-1 mutants but not in WT plants in response to Cr(VI) in a time- and concentration-dependent manner. Notably, the outermost single cell layer of the lateral root cap, which normally dies and tends to peel off, remains viable and increases in size following exposure of WT plants, but not slr-1 mutants, to Cr(VI). Our results suggest that (1) the primary root tip senses Cr(VI), (2) the external lateral root cap may play a protective role during Cr(VI) exposure, and (3) Cr(VI) impacts cell division in root meristems via auxin redistribution and SLR1/IAA14 function, influencing the expression of root meristem genes.     

Embryo-patterning genes and reinforcement cues determine cell fate in theArabidopsis thalianaroot

The majority of plant organs arise from groups of continuously dividing cells, the meristems. Little is known about mechanisms of cell specification in meristems. Within theArabidopsisroot meristem, the fate of every cell can be predicted accurately, and the origin of these cells during the formation of the embryonic root primordium is known. Laser ablations reveal that, despite the regularity in cell lineage, position remains important to reinforce cell specification. Genetic analysis has revealed that many genes involved in the specification of the main cell types in the root act early, during embryogenesis, and an important question is whether the same or other genes are involved in the reinforcement of specification. Sub-specification of cell types, as exemplified by epidermal root hair cell specification, involves two pathways, one of which may act to reinforce earlier patterning events mediated by the other.     

Different promoter regions control level and tissue specificity of expression of Agrobacterium rhizogenes rolB gene in plants

Expression of the rolB gene of A. rhizogenes T-DNA triggers root differentiation in transformed plant cells. In order to study the regulation of this morphogenetic gene, the GUS reporter gene was placed under the control of several deleted fragments of the rolB 5 non-coding region: carrot disc transformations and the analysis of transgenic tobacco plants containing these constructions identified the presence of distinct regulatory domains in the rolB promoter. Two regions (located from positions –623 to –471 and from –471 to –341, from the translation start codon) control the level but not the tissue specificity of rolB expression: progressive deletions of the rolB promoter starting from position –1185 to –341, although at different levels, maintained the same pattern of GUS expression — maximal in root meristems and less pronounced in the vascular tissue of aerial organs. Further deletion of 35 bp, from –341 to –306, drastically affected tissue specificity: GUS activity was still clearly detectable in the vascular tissue of the aerial organs while expression in the root meristem was totally suppressed. Analysis of transgenic embryos and seedlings confirmed that distinct promoter domains are responsible for meristematic (root) and differentiated (vascular) expression of rolB. Finally, we present data concerning the effects of plant hormones on the expression of rolB-GUS constructions.     

Transformation of maize by 2,4-dihydroxy-7-methoxy-2H-1,4-benzo- xazin-3(4H)-one resistant Agrobacterium strains

One of the important factors responsible for recalcitrance of maize tissue towards Agrobacterium-mediated transformation is the presence of 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA), an inhibitory metabolite found in maize cells. DIMBOA-resistant strains of Agrobacterium tumefaciens were used to transfer genes coding for GUS (-glucuronidase) and NPTII (neomycine phosphotransferase II) in maize shoot apical meristems derived from 20 day-old seedlings and immature embryos. GUS expression was higher (21–34%) in the apical meristem and was dependent on the type of infecting strain and explant-age. The PCR analysis of selected tissues confirmed the presence of GUS gene in the transformed cells.     

Transgenic cotton (<Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.) seedlings expressing a tobacco glutathione <Emphasis Type="Italic">S</Emphasis>-transferase fail to provide improved stress tolerance

Transgenic cotton (Gossypium hirsutum L.) lines expressing the tobacco glutathione S-transferase (GST) Nt107 were evaluated for tolerance to chilling, salinity, and herbicides, antioxidant enzyme activity, antioxidant compound levels, and lipid peroxidation. Although transgenic seedlings exhibited ten-fold and five-fold higher GST activity under normal and salt-stress conditions, respectively, germinating seedlings did not show improved tolerance to salinity, chilling conditions, or herbicides. Glutathione peroxidase (GPX) activity in transgenic seedlings was 30% to 60% higher under normal conditions, but was not different than GPX activity in wild-type seedlings under salt-stress conditions. Glutathione reductase, superoxide dismutase, ascorbate peroxidase, and monodehydroascorbate reductase activities were not increased in transgenic seedlings under salt-stress conditions, while dehydroascorbate reductase activity was decreased in transgenic seedlings under salt-stress conditions. Transgenic seedlings had 50% more oxidized glutathione when exposed to salt stress. Ascorbate levels were not increased in transgenic seedlings under salt-stress conditions. Malondialdehyde content in transgenic seedlings was nearly double that of wild-type seedlings under normal conditions and did not increase under salt-stress conditions. These results show that expression of Nt107 in cotton does not provide adequate protection against oxidative stress and suggests that the endogenous antioxidant system in cotton may be disrupted by the expression of the tobacco GST.     

Mapping meristem respiration of Prunus persica (L.) Batsch seedlings: potential respiration of the meristems, O2 diffusional constraints and combined effects on root growth

Root system architecture partially results from meristem activities, which themselves depend on endogenous and environmental factors, such as O2 depletion. In this study, meristem respiration and growth was measured in the root systems of three Prunus persica (L.) Batsch seedlings. The spatial distribution of meristem respiration within the root system was described, and the relationship between the respiration rates and meristem radii was analysed, using a model of radial O2 diffusion and consumption within the root. Histological observations were also used to help interpret the results. Respiration rates were linearly correlated to the root growth rates (rho 2 = 0.9). Respiration reached values greater than 3.5 x 10(-13) mol O2 s-1 for active meristems. The taproot meristem consumed more O2 than the rest of the entire root system meristems. Similarly, the first order lateral meristems used more O2 than the second order ones. A near hyperbolic relationship between respiration rates and meristem radii was observed. This can be explained by a model of radial O2 diffusion and consumption within the root. Therefore, only one maximum potential respiration rate and one O2 diffusion coefficient was estimated for all the meristems.