Presymptomatic visualization of plant-virus interactions by thermography.

Salicylic acid (SA), produced by plants as a signal in defense against pathogens, induces metabolic heating mediated by alternative respiration in flowers of thermogenic plants, and, when exogenously applied, increases leaf temperature in nonthermogenic plants. We have postulated that the latter phenomenon would be detectable when SA is synthesized locally in plant leaves. Here, resistance to tobacco mosaic virus (TMV) was monitored thermographically before any disease symptoms became visible on tobacco leaves. Spots of elevated temperature that were confined to the place of infection increased in intensity from 8 h before the onset of visible cell death, and remained detectable as a halo around the ongoing necrosis. Salicylic acid accumulates during the prenecrotic phase in TMV-infected tobacco and is known to induce stomatal closure in certain species. We show that the time course of SA accumulation correlates with the evolution of both localized thermal effect and stomatal closure. Since the contribution of leaf respiration is marginal, we concluded that the thermal effect results predominantly from localized, SA-induced stomatal closure. The presymptomatic temperature increase could be of general significance in incompatible plant-pathogen interactions.     

Hydroxamate-Stimulated O(2) Uptake in Roots of Pisum sativum and Zea mays, Mediated by a Peroxidase : Its Consequences for Respiration Measurements

Low concentrations of salicylhydroxamic acid (<5 millimolar) stimulate O₂ uptake in intact roots of Pisum sativum. We demonstrate that the hydroxamate-stimulated O₂ uptake does not reside in the mitochondria. We also show that the hydroxamate-stimulated O₂ uptake is due to the activation of a peroxidase catalyzing reduction of O₂. This peroxidase, which can use both NADH and NADPH as a substrate, is stimulated by low concentrations of monophenols, e.g. salicylhydroxamic acid and 2-methoxyphenol. It is inhibited by high (20 millimolar) concentrations of salicylhydroxamic acid, cyanide, and scavengers of the superoxide free radical ion, e.g. ascorbate, gentisic acid, and catechol. In the presence of gentisic acid, O₂ uptake by intact pea roots was no longer stimulated by low concentrations of salicylhydroxamic acid. The consequence of the present finding for in vivo respiration measurements is that the use of low concentrations of salicylhydroxamic acid and uncoupler is reliable only in the presence of a suitable superoxide free radical scavenger which prevents activation of the peroxidase. It also confirms that high concentrations of salicylhydroxamic acid (20-25 millimolar) can be safely used in short-term experiments to assess the activity of the alternative path in intact roots.     

The regulation of glycolysis and electron transport in roots

The respiration of roots and isolated root mitochondria was investigated in Phaseolus vulgaris L., Spinacea oleracea L.; Triticum aestivum L., and Zea mays L. Although the respiration of both intact roots and isolated mitochondria displayed resistance to cyanide and sensitivity to SHAM, the percentage resistance and inhibition in roots was not the same as that in the mitochondria, with the exception of wheat. Adding FCCP to roots stimulated oxygen uptake and equalized the effects of SHAM and cyanide on roots and mitochondria. In spinach and maize roots, FCCP stimulated both the cytochrome and alternative pathways, while in bean roots, only the alternative pathway was stimulated. FCCP had little effect on wheat root respiration rates. Potential in vivo rates of oxygen uptake were estimated by expressing rates obtained with isolated mitochondria on a fumarase activity basis, and fumarase activity on a root weight basis. In wheat roots the potential rate was approximately equal to the measured in vivo rate; in the other species the potential rates were substantially greater than measured rates, but approximately equal to uncoupled in vivo rates. Key glycolytic intermediates in roots were measured, and it was found that the phosphofructokinase and pyruvate kinase reactions were displaced far from equilibrium, the degree of displacement being approximately equal in roots with little, and roots with substantial, alternative path engagement. Thus, although glycolysis is controlled, the regulation of this pathway appears to be quite flexible. The results are discussed in terms of possible regulatory mechanisms.     

Cyanide-resistant respiration in roots and leaves. Measurements with intact tissues and isolated mitochondria

A comparison was made between the oxygen uptake of roots and leaves and of mitochondria isolated from the same tissues. Ten species were included in this study: three legumes, one C3-monocotyledon, one C4-monocotyledon, the rest non-leguminous C3-dicotyledons. Root and leaf respiration in all species examined displayed substantial resistance to KCN (0.1–1.0 mM) and the cyanide-resistant respiration was completely inhibited by salicylhydroxamic acid (SHAM; 10–20 mM). SHAM alone inhibited oxygen uptake to varying degrees, depending on the species. Mitochondria were isolated from roots and leaves of many of the species examined and also displayed cyanide-resistant oxygen uptake, which was sensitive to both SHAM and tetraethylthiuram disulfide (disulfiram). Concentrations of SHAM greater than 2 mM caused inhibition of the cytochrome path as well as of the alternative path in isolated mitochondria. Respiration rates of intact roots and leaves in the presence of varying concentrations of SHAM alone were plotted against those obtained in the presence of both SHAM and KCN. This plot showed that in vivo the cytochrome pathway was not affected by 10 or 20 mM SHAM in the external solution. We conclude that the activity of the alternative pathway in intact roots and leaves can be reliably estimated by comparing SHAM-sensitivity and cyanide-resistance of respiration.     

Growth rate, plant development and water relations of the ABA-deficient tomato mutant sitiens

Given the close relationship between a plant's growth rate and its pattern of biomass allocation and the effects of abscisic acid (ABA) on biomass allocation, we studied the influence of ABA on biomass allocation and growth rate of wildtype tomato ( Lycopersicon esculentum Mill. cv. Moneymaker) plants and their strongly ABA-deficient mutant sitiens. The relative growth rate of sitiens was 22% lower than that of the wildtype, as the result of a decreased specific leaf area. The net assimilation rate and the leaf weight ratio were not affected. The mutant showed a much higher transpiration rate and lower hydraulic conductance of the roots. These two factors resulted in sitiens having a significantly lower leaf water potential and turgor. resulting in reduced leaf expansion and, consequently, a lower specific leaf area relative to the wildtype. Addition of ABA to the sitiens roots resulted in phenotypic reversion to the wildtype. We conclude that the influence of ABA-deficiency on biomass allocation and relative growth rate is the result of altered water relations in the plants, rather than of a direct effect on sink strength of different plant organs.     

Root and leaf attributes accounting for the performance of fast- and slow-growing grasses at different nutrient supply

Despite their difference in potential growth rate, the slow-growing Brachypodium pinnatum and the fast-growing Dactylis glomerata co-occur in many nutrient-poor calcareous grasslands. They are known to respond differently to increasing levels of N and P. An experiment was designed to measure which characteristics are affected by nutrient supply and contribute to the ecological performance of these species. Nutrient acquisition and root and shoot traits of these grasses were studied in a garden experiment with nine nutrient treatments in a factorial design of 3 N and 3 P levels each. D. glomerata was superior to B. pinnatum in nutrient acquisition and growth in all treatments. B. pinnatum was especially poor in P acquisition. Both species responded to increasing N supply and to a lesser extent to increasing P supply by decreasing their root length and increasing their leaf area per total plant weight. D. glomerata showed a higher plasticity. In most treatments, the root length ratio (RLR) and the leaf area ratio (LAR) were higher for D. glomerata. A factorization of these parameters into components expressing biomass allocation, form (root fineness or leaf thickness) and density (dry matter content) shows that the low density of the biomass of D. glomerata was the main cause for the higher RLR and LAR. The biomass allocation to the roots showed a considerable plasticity but did not differ between the species. B. pinnatum had the highest leaf weight ratio. Root fineness was highly plastic in D. glomerata, the difference with B. pinnatum being mainly due to the thick roots of D. glomerata at high nutrient supply. The leaf area/leaf fresh weight ratio did not show any plasticity and was slightly higher for B. pinnatum. It is concluded, that the low density of the biomass of D. glomerata is the pivotal trait responsible for its faster growth at all nutrient levels. It enables simultaneously a good nutrient acquisition capacity by the roots as well as a superior carbon acquisition by the leaves. The high biomass density of B. pinnatum will then result in a lower nutrient requirement due to a slower turnover, which in the long term is advantageous under nutrient-poor conditions.     

The effect of nitrate-nitrogen supply on bacteria and bacterial-feeding fauna in the rhizosphere of different grass species

Summary Microbial growth in the rhizosphere is affected by the release of organic material from roots, so differences in carbon budgets between plants may affect their rhizosphere biology. This was tested by sampling populations of bacteria and bacteriophagous fauna from the rhizosphere of Lolium perenne, Festuca arundinacea, Poa annua, and Poa pratensis, under conditions of high and low nitrate availability. Concentrations of soluble phenolics and lignin varied considerably between the species but were not related to differences in rhizosphere biology. L. perenne and F. arundinacea supported fewer bacteria than the Poa species. There was no significant rhizosphere effect on the groups of protozoa. The major indicators of rhizosphere productivity were the bacterial-feeding nematodes (mainly Acrobeloides spp.), and there was a large positive effect of added nitrate. Nematode biomass was significantly lower in the rhizosphere of the slow-growing P. pratensis compared with the fast-growing P. annua, indicating that the differential allocation of carbon has affects on rhizosphere biology. A large rhizosphere effect on enchytraeid worms was also observed, and their potential importance in the rhizosphere is discussed.