No abstract

Nodulated legumes require more P than legumes growing on mineral nitrogen, but little is known about the basis for the higher P requirement. Experiments were conducted to determine how Rhizobium tropici responds to P limitation and to understand how P is partitioned between the symbionts under conditions of adequate or limiting P. Free-living R. tropici responds to P stress by increasing P transport capacity and inducing both an acid and an alkaline phosphatase. This P-stress response occurs when the medium P concentration decreases below 1 p~. Both P-stressinducible phosphatases are found in bacteroids taken from plants growing with adequate P, suggesting that P levels in the symbiosome space is low enough to induce the expression of these enzymes. Bacteroid alkaline phosphatase-specific activity was highest during vegetative growth of the bean plant, but decreased approximately 75% during the host reproductive stages. In hydroponic experiments 32P-tracer studies showed that in vivo rates of P accumulation were significantly higher in bacteroids from P-limited plants compared with those from plants that had been supplied with adequate P. In contrast, label accumulation in leaves was greatest in plants grown with adequate P.On a worldwide basis, most cultivated soils have insufficient P for maximum crop yields. Legumes are particularly affected because they are typically cultured symbiotically, and it has been shown that legumes dependent on symbiotic nitrogen fixation have higher P requirements than legumes grown with a nitrogen fertilizer. Soybean (Cassman et al., 1981~;Israel, 1987), clover (Powell, 1977), common bean Bliss, 1987, 1989), pigeon pea (Itoh, 1987), and cowpea (Cassman, et al., 1981~) a11 respond positively to P fertilization. Growth and symbiotic parameters increased by P fertilization include whole-plant N concentration, plant dry matter, nodule number, nodule mass, nitrogenase activity, and specific nodule nitrogenase activity.The reasons for this P response are poorly understood. In soybean symbiotic N, fixation places a P tax on the host plant (Israel, 1987). Field studies with peanuts have found that P concentrations in the stems and seeds of nodulating ' This work was initiated while T.R.M. was a National Science Foundation postdoctoral fellow in plant biology (BIR-9203796) lines are lower than those in nonnodulating isolines (Sahrawat, et al., 1988), suggesting that P distribution within the plant is different when the legume plant grows symbiotically. The higher P requirement of symbiotically grown legumes is apparently not due to different abilities of roots in the two N environments to absorb P (Israel, 1987), which implies that an optimum symbiotic interaction between the host plant and rhizobia depends on an efficient allocation and use of available P.Most P metabolism studies with rhizobia have used freeliving cells. P requirements for growth (Keyser and Munns, 1979;Cassman et al., 1981aCassman et al., , 1981b, P transport rates, and P storage (Cassman et al., 1981b;Beck and Mun...

A broad survey of most of the major geyser basins within Yellowstone National Park (Wyoming, USA) was conducted to identify the flowering plants which tolerate high rhizosphere temperatures (> or = 40 degrees C) in geothermally heated environments. Under such conditions, five species of monocots and four species of dicots were repeatedly found. The predominant flowering plants in hot soils (>40 degrees C at 2-5 cm depth) were grasses, primarily Dichanthelium lanuginosum. Long-term (weeks to months) rhizosphere temperatures of individual D. lanuginosum above 40 degrees C were recorded at several different locations, both in the summer and winter. The potential role of heat shock proteins (HSPs) in the apparent adaptation of these plants to chronically high rhizosphere temperatures was examined. Antibodies to cytoplasmic class I small heat shock proteins (sHSPs) and to HSP101 were used in Western immunoblot analyses of protein extracts from plants collected from geothermally heated soils. Relatively high levels of proteins reacting with anti-sHSP antibodies were consistently detected in root extracts from plants experiencing rhizosphere temperatures above 40 degrees C, though these proteins were usually not highly expressed in leaf extracts from the same plants. Proteins reacting with antibodies to HSP101 were also present both in leaf and root extracts from plants collected from geothermal soils, but their levels of expression were not as closely related to the degree of heat exposure as those of sHSPs.

BackgroundDispersal from Candida albicans biofilms that colonize catheters is implicated as a primary factor in the link between contaminated catheters and life threatening blood stream infections (BSI). Appropriate in vitro C. albicans biofilm models are needed to probe factors that induce detachment events.ResultsUsing a flow through system to culture C. albicans biofilms we characterized a detachment process which culminates in dissociation of an entire early stage biofilm from a silicone elastomer surface. We analyzed the transcriptome response at time points that bracketed an abrupt transition in which a strong adhesive association with the surface is weakened in the initial stages of the process, and also compared batch and biofilm cultures at relevant time points. K means analysis of the time course array data revealed categories of genes with similar patterns of expression that were associated with adhesion, biofilm formation and glycoprotein biosynthesis. Compared to batch cultures the biofilm showed a pattern of expression of metabolic genes that was similar to the C. albicans response to hypoxia. However, the loss of strong adhesion was not obviously influenced by either the availability of oxygen in the medium or at the silicone elastomer surface. The detachment phenotype of mutant strains in which selected genes were either deleted or overexpressed was characterized. The microarray data indicated that changes associated with the detachment process were complex and, consistent with this assessment, we were unable to demonstrate that transcriptional regulation of any single gene was essential for loss of the strong adhesive association.ConclusionThe massive dispersal of the early stage biofilm from a biomaterial surface that we observed is not orchestrated at the level of transcriptional regulation in an obvious manner, or is only regulated at this level by a small subpopulation of cells that mediate adhesion to the surface.