Foraging for space and avoidance of physical obstructions by plant roots: a comparative study of grasses from contrasting habitats

Physical obstructions that reduce space for root growth can profoundly affect plant performance. The aim of this study was to investigate the ability of roots to avoid obstructions and forage for usable space, and to reveal the mechanism involved. Eight grass species from four genera were examined. Each genus included species characteristic of habitats with high and low nutrient availability. The ability to limit root mass and to adjust morphology within substrate containing obstructions in the form of gravel was investigated. A treatment with activated carbon, which adsorbs organic compounds, was used to examine the possible involvement of root exudates in responses to obstructions. Only species characteristic of nutrient-poor habitats restricted placement of root mass in substrate containing obstructions, and this response disappeared in the presence of activated carbon. Root morphological responses to obstructions differed from those shown in response to nutrient-poor conditions or compacted soil. These results suggest that the ability to avoid obstructions is dependent on the sensitivity of roots to their own exudates accumulating in the vicinity of obstructions. This is similar to other behavioural responses in which cues or signals are used to adjust growth before stressful conditions are encountered.     

Crystal structure of cockroach allergen Bla g 2, an unusual zinc binding aspartic protease with a novel mode of self-inhibition

The crystal structure of Bla g 2 was solved in order to investigate the structural basis for the allergenic properties of this unusual protein. This is the first structure of an aspartic protease in which conserved glycine residues, in two canonical DTG triads, are substituted by different amino acid residues. Another unprecedented feature revealed by the structure is the single phenylalanine residue insertion on the tip of the flap, with the side-chain occupying the S1 binding pocket. This and other important amino acid substitutions in the active site region of Bla g 2 modify the interactions in the vicinity of the catalytic aspartate residues, increasing the distance between them to approximately 4A and establishing unique direct contacts between the flap and the catalytic residues. We attribute the absence of substantial catalytic activity in Bla g 2 to these unusual features of the active site. Five disulfide bridges and a Zn-binding site confer stability to the protein, which may contribute to sensitization at lower levels of exposure than other allergens.     

Denaturation of a membrane transport protein by urea: The erythrocyte anion exchanger

Summary Chloride equilibrium exchange was measured in the presence of intracellular and extracellular urea, several different alkylureas and thiourea. Urea half-inhibited Cl exchange at about 2.5m, but the other, less polar analogs had significantly higher potencies; e.g., butylurea half-inhibited at about 60mm. Onset and reversal of inhibition occurred within less than 2 sec. The inhibition exhibited no obvious sigmoidal dependence on urea concentration, and at low concentrations dimethylurea was a noncompetitive inhibitor of Cl exchange. However, at higher concentrations the Dixon plots were curved upward and a Hill analysis of the dimethylurea data yielded a Hill coefficient of at least 1.5. When present on only one side of the membrane, the slowly penetrating thiourea inhibited Cl exchange with a higher potency from the outside of the cell. Cl/Br exchange was inhibited less under conditions of self-inhibition of anion exchange than in the absence of self-inhibition. These data indicate that the ureas inactivate the anion transporter by a reversible denaturation process, and that the function of the anion transport mechanism may be more sensitive to small perturbations of protein structure than are spectroscopically derived structural parameters.     

Kinetics of the effect of amiloride on the permeability of the apical membrane of rabbit descending colon to sodium

Summary The effects of the addition of graded concentrations of amiloride, (A) _m , to the mucosal bathing solution on the permeability of the apical membrane of rabbit descending colon to Na (P _Na ^m ) were determined when the Na activity in the mucosal bathing solution, (Na) _m , was 18, 32 or 100mm.P _Na ^m was obtained from current-voltage relations determined on tissues bathed with a high-K serosal solution before and after the addition of a maximally inhibitory concentration of amiloride to the mucosal solution as described by Turnheim et al. (Turnheim, K., Thompson, S.M., Schultz. S.G. 1983.J. Membrane Biol. 76:299–309).The results indicate that: (1) As demonstrated previously (Turnheim et al., 1983),P _Na ^m decreases with increasing (Na) _m . (2)P _Na ^m also decreases hyperbolically with increasing (A) _m . Kinetic analyses of the effect of amiloride onP _Na ^m are consistent with the conclusions that: (i) the stoichiometry between the interaction of amiloride with apical membrane receptors that results in a decrease inP _Na ^m is one-for-one; (ii) there is no evidence for cooperativity between amiloride and these binding sites; (iii) the value of (A) _m needed to halveP _Na ^m at a fixed (Na) _m is 0.6–1.0 m; and, (iv) this value is independent of (Na) _m over the fivefold range studied.These findings are consistent with the notion that the sites with which amiloride interacts to bring about closure of the channels through which Na crosses the apical membrane arekinetically distinct from the sites with which (Na) _m interacts to bring about closure (i.e., self-inhibition). In short, the effects of (Na) _m and (A) _m onP _Na ^m in this tissue appear to be independent and additive.