Gravitropism in plants: Hydraulics and wall growth properties of responding cells

Gravitropism is the asymmetrical alteration of plant growth in response to a change in the gravity vector, with the typical result that stems grow up and roots grow down. The gravity response is important for plants because it enables them to grow their aerial parts in a mechanically stable (upright) position and to develop their roots and leaves to make efficient use of soil nutrients and sunlight. The elucidation of gravitropic responses will tell much about how gravity exerts its morphogenetic effects on plants and how plants regulate their growth at the cellular and molecular levels

Measuring Plant Cell Wall Extension (Creep) Induced by Acidic pH and by Alpha-Expansin

Growing plant cell walls characteristically exhibit a property known as 'acid growth', by which we mean they are more extensible at low pH (< 5) 1. The plant hormone auxin rapidly stimulates cell elongation in young stems and similar tissues at least in part by an acid-growth mechanism 2, 3. Auxin activates a H+ pump in the plasma membrane, causing acidification of the cell wall solution. Wall acidification activates expansins, which are endogenous cell wall-loosening proteins 4, causing the cell wall to yield to the wall tensions created by cell turgor pressure. As a result, the cell begins to enlarge rapidly. This 'acid growth' phenomenon is readily measured in isolated (nonliving) cell wall specimens. The ability of cell walls to undergo acid-induced extension is not simply the result of the structural arrangement of the cell wall polysaccharides (e.g. pectins), but depends on the activity of expansins 5. Expansins do not have any known enzymatic activity and the only way to assay for expansin activity is to measure their induction of cell wall extension. This video report details the sources and preparation techniques for obtaining suitable wall materials for expansin assays and goes on to show acid-induced extension and expansin-induced extension of wall samples prepared from growing cucumber hypocotyls

Cloning, characterisation and expression analysis of cDNA clones encoding cell wall-modifying enzymes isolated from ripe apples

Fruit softening is accompanied by modifications of the cell wall pectic and hemicellulosic fractions, as the result of the combined action of several cell wall-modifying enzymes. The objective of this work was to clone specific cDNAs that encode isoforms of cell wall-modifying enzymes, which are expressed during the final stages of apple softening, and to establish a temporal sequence of their accumulation. A cDNA library enriched with mRNA isolated from over-ripe fruit was constructed and screened. A pectin methylesterase (MdPME1), a pectate lyase (MdPL1), an -l-arabinofuranosidase (MdAF1) an endo-1,4- -glucanase (MdEG1), two xyloglucan endotransglucosylase/hydrolases (Md-XTH1 and Md-XTH2), and an alpha-expansin (MdEXPA3) specific cDNAs were identified by homology-based cloning, and their mRNA accumulation was examined during fruit growth and ripening. The expression of an apple -galactosidase ( -Gal; pABG1) and a polygalacturonase (PG; pGDPG-1) mRNA previously reported was also investigated using the same biological material. Transcripts of all enzymes, except MdPME1, could be unambiguously detected by semi-quantitative RT-PCR in fruit during ripening. However, transcripts of MdEG1 were more abundant at fruit set and MdPL1 exhibited higher expression before commercial maturity. The strongest RT-PCR signals in over-ripe fruit were observed for PG, -Gal and Md-XTH1 clones. Two XTHs were detected in over-ripe fruit. While Md-XTH1 acts constitutively during fruit development, Md-XTH2 showed a ripening-related pattern. The Md-XTH2-encoded protein was heterologously expressed in Saccharomyces cerevisiae and showed both transglycosylase and hydrolase activities. Expression analyses conducted in flowers, peduncles, young and expanded leaves, and petioles of senescent leaves revealed that none of the cloned cDNAs is fruit specifi

Managing Water under Uncertainty and Risk: The United Nations World Water Development Report 4

This report introduces new aspects of water issues: 1) it reintroduces the 12 challenge area reports that provided the foundation for the first two World Water Development Reports (WWDR); 2) 4 new reports on water quality, groundwater, gender, and desertification, land degradation and drought; 3) in recognition that the global challenges of water can vary considerably across countries and regions, a series of 5 regional reports have been included; 4) a deeper analysis of the main external forces of freshwater resources and possibilities for their future evolution; 5) managing water under uncertainty and risk