SUMMARYArbuscular mycorrhizal (AM) fungi, which date from the Devonian era, must have dealt ver\' earlj' on with problems of plant defence, a tactic which has enabled them to colonize roots of most extant plant taxa. Conversely, plants forming arbuscular mycorrhiza must exert some sort of control over the fungi during symbiotic interactions since mycelial proliferation within roots is confined to the cortical parenchyma tissue. Plants possess a panoply of defence mechanisms which are triggered by microbial attack. Here we review present-day knowledge of defencerelated root responses to colonization by AM fungi, and assess their possible implications in the symbiosis. Weak, transient, unco-ordinated or extremely localized activation of inducible defence responses occurs during compatible interactions in AM which differs from that in plant-pathogen interactions. When symbiosis-related plant genes are mutated, typical defence responses are associated with resulting root resistance to arbuscular mycorrhizal fungi whilst, contrastingly, plants constitutively expressing defence genes are unaffected in their symbiotic capacities. The mechanisms by which plants can modulate defence responses during symbiotic interactions and the \vay in which AM fungi might contend with these are discussed.

Crystals of thallium dihydrogendiphosphate Tl

New diphosphates AErP 2 O 7 , A = Rb (1), Cs (2), and HEuP 2 O 7 Á3H 2 O (3) have been prepared via soft chemistry route from evaporation of aqueous solution. Their crystal structures have been solved by single crystal diffraction data. The compounds (1) and (2) crystallize in the monoclinic space group P2 1 /c with the following unit cell dimensions: (1) a = 7.7248(4) Å , b = 10.9342(6) Å , c = 8.6716(3) Å , b = 105.434(4)°and Z = 4; (2) a = 7.9192(4) Å , b = 10.8606(4) Å , c = 8.7684(5) Å , b = 104.317(5)°and Z = 4. Compound (3) crystallises in the triclinic space group P -1, with the unit cell parameters: a = 6.4036(5) Å , b = 6.8753(6) Å , c = 9.7964(8) Å , a = 81.706(7)°, b = 80.218(7)°, c = 88.381(7)°. Crystal structure of (1) and (2) consists of 3D open framework built from [ErO 6 ] octahedra sharing corners with [P 2 O 7 ] units, leading to the formation of channels running along c direction in which A ? [A = Rb(1), Cs(2)] ions are located. The compound (3), on the other hand, exhibits a lamellar structure, in which edge-sharing polyhedra of eightfold coordinated europium form chains along [010]. These chains are joined in turn to each others using diphosphates bridges. A network of O-HÁÁÁO hydrogen bonds reinforces the cohesion of the structure of (3). The diphosphate groups in (1), (2) and (3) adopts an eclipsed configuration. IR and Raman spectra of the three new phosphates show usual signals related to the P 2 O 7 diphosphate group and the water molecule. TG and DTA studies were carried out on crystals of (3). The data are correlated to the crystal structures ones.

Rare earth diphosphates have attracted much attention, due mainly to their optical properties [refs, 1,2 As a continuation of these investigations of lanthanides diphosphates, 4 we hereby report on the synthesis and crystal structure of KDyP2O7·2H2O.Aqueous solutions of DyCl3·6H2O (1.5 mmol, 0.565 g) and K4P2O7 (1.5 mmol, 0.495 g) were mixed with a few ml of dilute HCl. The mixture was stirred for 1 day. Then the evaporation of solvent water was allowed for around two weeks at room temperature. Prismatic colorless crystals were deposited, with the chemical composition determined after structure refinements (Fig. 1). They were filtered off and washed with a water-ethanol mixture (20:80).A selected colorless crystal was collected at 293 K, using a CAD-4, Enraf-Nonius 4-circles diffractometer. Data were corrected for Lorentz and polarisation effects. The structure was refined using SHELXL-97 (Sheldrick, 1997).5 Starting atomic coordinates were taken from isotypic KTbP2O7·2H2O. 4 The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were found in successive difference-Fourier maps and refined isotropically.