Inhibition of very long acyl chain sphingolipid synthesis modifies membrane dynamics during plant cytokinesis.

Plant cytokinesis requires intense membrane trafficking and remodeling to form a specific membrane structure, the cell plate that will ultimately separate the daughter cells. The nature and the role of lipids involved in the formation of the cell plate remain unclear. Plant membranes are particularly rich in sphingolipids such as glucosyl-ceramides with long (16 carbons) or very long (24 carbons) acyl chains. We reveal here that inhibition of the synthesis of sphingolipids with very long acyl chains induces defective cell plates with persistent vesicular structures and large gaps. Golgi-derived vesicles carrying material toward the cell plate display longer vesicle-vesicle contact time and their cargos accumulate at the cell plate, suggesting membrane fusion and/or recycling defects. In vitro fusion experiments between artificial vesicles show that glycosphingolipids with very long acyl chains stimulate lipid bilayer fusion. Therefore we propose that the very long acyl chains of sphingolipids are essential structural determinants for vesicle dynamics and membrane fusion during cytokinesis.

OCTOPUS, a polarly localised membrane-associated protein, regulates phloem differentiation entry in Arabidopsis thaliana.

Vascular development is embedded into the developmental context of plant organ differentiation and can be divided into the consecutive phases of vascular patterning and differentiation of specific vascular cell types (phloem and xylem). To date, only very few genetic determinants of phloem development are known. Here, we identify OCTOPUS (OPS) as a potentiator of phloem differentiation. OPS is a polarly localised membrane-associated protein that is initially expressed in provascular cells, and upon vascular cell type specification becomes restricted to the phloem cell lineage. OPS mutants display a reduction of cotyledon vascular pattern complexity and discontinuous phloem differentiation, whereas OPS overexpressers show accelerated progress of cotyledon vascular patterning and phloem differentiation. We propose that OPS participates in vascular differentiation by interpreting longitudinal signals that lead to the transformation of vascular initials into differentiating protophloem cells.

Natural variation of nitrate uptake and nitrogen use efficiency in Arabidopsis thaliana cultivated with limiting and ample nitrogen supply.

Eighteen accessions of Arabidopsis thaliana were grown with low (N-) and high (N+) nitrogen supply. N uptake was monitored by feeding plants with 15N-enriched nutritive solution over 24 h. Biomass [fresh matter (FM) and dry matter (DM)], N concentration (N%), and 15N content were monitored and computed to determine the nitrogen use efficiency (NUE) and nitrogen uptake efficiency (NupE). NUE has been estimated as the ratio between biomass and N concentration (DM/N%) and NupE as the concentration of 15N in plants [microg (g(-1) DM)]. Accession traits were analysed to detect common and individual genotype features. The genetic variation in NUE at high N input was mainly explained by variation in N uptake. Even though plants managed N uptake and N metabolism differently under N+ and N-, NUE was similar in these two conditions, showing that NUE was exclusively genetically determined. Hierarchical classification revealed that the physiological classes arising were similar under N- and N+. Both wasteful and efficient genotypes were detected. Three extreme genotypes, Col-0, Bur-0, and Tsu-0, were noted. Bur-0 and Tsu-0 exhibited high NUE and large biomass. Col-0 showed the reverse: low NUE and low biomass. Bur-0 appeared poorly tolerant of a high N supply. The present data will facilitate the choice of Arabidopsis accessions as parents of recombinant inbred line populations suitable for the mapping of quantitiative trait loci related to NUE, NupE, and N storage capacity.

High-resolution whole-mount imaging of three-dimensional tissue organization and gene expression enables the study of Phloem development and structure in Arabidopsis.

Currently, examination of the cellular structure of plant organs and the gene expression therein largely relies on the production of tissue sections. Here, we present a staining technique that can be used to image entire plant organs using confocal laser scanning microscopy. This technique produces high-resolution images that allow three-dimensional reconstruction of the cellular organization of plant organs. Importantly, three-dimensional domains of gene expression can be analyzed with single-cell precision. We used this technique for a detailed examination of phloem cells in the wild type and mutants. We were also able to recognize phloem sieve elements and their differentiation state in any tissue type and visualize the structure of sieve plates. We show that in the altered phloem development mutant, a hybrid cell type with phloem and xylem characteristics develops from initially normally differentiated protophloem cells. The simplicity of sieve element data collection allows for the statistical analysis of structural parameters of sieve plates, essential for the calculation of phloem conductivity. Taken together, this technique significantly improves the speed and accuracy of the investigation of plant growth and development.