Low-temperature-induced changes in trehalose, mannitol and arabitol associated with enhanced tolerance to freezing in ectomycorrhizal basidiomycetes (Hebeloma spp.).

Ectomycorrhizal fungi have been shown to survive sub-zero temperatures in axenic culture and in the field. However, the physiological basis for resistance to freezing is poorly understood. In order to survive freezing, mycelia must synthesise compounds that protect the cells from frost damage, and certain fungal-specific soluble carbohydrates have been implicated in this role. Tissue concentrations of arabitol, mannitol and trehalose were measured in axenic cultures of eight Hebeloma strains of arctic and temperate origin grown at 22, 12, 6 and 2 degrees C. In a separate experiment, mycelia were frozen to -5 degrees C after pre-conditioning at either 2 degrees C or 22 degrees C. For some, especially temperate strains, there was a clear increase in specific soluble carbohydrates at lower growth temperatures. Trehalose and mannitol were present in all strains and the highest concentrations (close to 2.5% and 0.5% dry wt.) were recorded only after a cold period. Arabitol was found in four strains only when grown at low temperature. Cold pre-conditioning enhanced recovery of mycelia following freezing. In four out of eight strains, this was paralleled by increases in mannitol and trehalose concentration at low temperature that presumably contribute towards cryoprotection. The results are discussed in an ecological context with regard to mycelial overwintering in soil.

Ectomycorrhizal symbiosis can enhance plant nutrition through improved access to discrete organic nutrient patches of high resource quality.

It is known that roots can respond to patches of fertility; however, root proliferation is often too slow to exploit resources fully, and organic nutrient patches may be broken down and leached, immobilized or chemically fixed before they are invaded by the root system. The ability of fungal hyphae to exploit resource patches is far greater than that of roots due to their innate physiological and morphological plasticity, which allows comprehensive exploration and rapid colonization of resource patches in soils. The fungal symbionts of ectomycorrhizal plants excrete significant quantities of enzymes such as chitinases, phosphatases and proteases. These might allow the organic residue to be tapped directly for nutrients such as N and P. Pot experiments conducted with nutrient-stressed ectomycorrhizal and control willow plants showed that when high quality organic nutrient patches were added, they were colonized rapidly by the ectomycorrhizal mycelium. These established willows (0.5 m tall) were colonized by Hebeloma syrjense P. Karst. for 1 year prior to nutrient patch addition. Within days after patch addition, colour changes in the leaves of the mycorrhizal plants (reflecting improved nutrition) were apparent, and after I month the concentration of N and P in the foliage of mycorrhizal plants was significantly greater than that in non-mycorrhizal plants subject to the same nutrient addition. It seems likely that the mycorrhizal plants were able to compete effectively with the wider soil microbiota and tap directly into the high quality organic resource patch via their extra-radical mycelium. We hypothesize that ectomycorrhizal plants may reclaim some of the N and P invested in seed production by direct recycling from failed seeds in the soil. The rapid exploitation of similar discrete, transient, high-quality nutrient patches may have led to underestimations when determining the nutritional benefits of ectomycorrhizal colonization.

Adaptation to long-term cold-girdling in genotypes of common bean (Phaseolus vulgaris L.).

The effect of long-term cold-girdling on phloem transport and resource allocation in whole plants of common bean is described. Wide differences were found between genotypes, with some maintaining translocation when cold-girdled. This provides evidence to support passive phloem transport. The possibilities that cold-girdling may physically block transport and/or disrupt root-shoot signalling are discussed.

The DL gene system in common bean: a possible mechanism for control of root-shoot partitioning.

Crosses between certain genotypes of common bean result in dwarfing of F1 plants and lethal dwarfing in a proportion of the F2 population. This is under the control of the semi-dominant alleles, DL1 and DL2 at two complementary loci which are expressed in the root and shoot respectively. The various DL genotypes can be simulated by grafting. The graft combination DL1 DL1 dl2 dl2 /dl1 dl1 DL2 DL2 was found to have a significantly higher root dry matter fraction than either parent. Lethally dwarfed plants (DL1 DL1 DL2 DL2 ) and the analogous lethal graft combination (dl1 dl1 DL2 DL2 /DL1 DL1 dl2 dl2 ) exhibit failure of root growth and have very low root fractions. Hybrids or graft combinations with failed roots ceased growth and accumulated large amounts of starch throughout their hypocotyls. In sterile culture, both lethal dwarfs and lethal graft combinations were able to grow roots if sucrose was added to the growth medium. This indicates that a failure of sucrose translocation to the roots is probably responsible for failed root growth. Data from screening the DL genotypes of 49 cultivars could be fully explained using the DL system hypothesis, and grafting proved to be efficient for identifying DL genotype. The DL system might be of fundamental importance in root-shoot partitioning. Current evidence favours the hypothesis that failure of root growth is the outcome of excessively high sink strength of shoots compared to roots, which might arise from signalling incompatibilities between the genotypes.