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The skin is the largest organ of the human body and builds a barrier to protect us from the harmful environment and also from unregulated loss of water. Keratinocytes form the skin barrier by undergoing a highly complex differentiation process that involves changing their morphology and structural integrity, a process referred to as cornification. Alterations in the epidermal cornification process affect the formation of the skin barrier. Typically, this results in a disturbed barrier, which allows the entry of substances into the skin that are immunologically reactive. This contributes to and promotes inflammatory processes in the skin but also affects other organs. In many common skin diseases, including atopic dermatitis and psoriasis, a defect in the formation of the skin barrier is observed. In these diseases the cytokine composition within the skin is different compared to normal human skin. This is the result of resident skin cells that produce cytokines, but also because additional immune cells are recruited. Many of the cytokines found in defective skin are able to influence various processes of differentiation and cornification. Here we summarize the current knowledge on cytokines and their functions in healthy skin and their contributions to inflammatory skin diseases.

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Herbaceous plants grown with free access to nutrients exhibit inherent differences in maximum relative growth rate (RGR) and rate of nutrient uptake. Measured rates of root respiration are higher in fast-growing species than in slow-growing ones. Fast-growing herbaceous species, however, exhibit lower rates of respiration than would be expected from their high rates of growth and nitrate uptake. We investigated why the difference in root O 2 uptake between fast-and slow-growing species is relatively small. Inhibition of respiration by the build-up of CO 2 in closed cuvettes, diurnal variation in respiration rates or an increasing ratio of respiratory CO 2 release to O 2 uptake (RQ) with increasing RGR failed to explain the relatively low root respiration rates in fast-growing grasses. Furthermore, differences in alternative pathway activity can at most only partly explain why the difference in root respiration between fast-and slow-growing grasses is relatively small. Although specific respiratory costs for maintenance of biomass are slightly higher in the fast-growing Dactylis glomerata L. than those in the slowgrowing Festuca ovina L., they account for 50% of total root respiration in both species. The specific respiratory costs for ion uptake in the fast-growing grass are onethird of those in the slow-growing grass [0·41 versus 1·22 mol O 2 mol (NO 3 -) -1 ]. We conclude that this is the major cause of the relatively low rates of root respiration in fast-growing grasses.Key-words: Dactylis glomerata L.; Festuca ovina L.; alternative pathway; diurnal variation; elevated CO 2 ; ion uptake; relative growth rate; root respiration; specific respiratory costs. INTRODUCTIONWhen herbaceous plants are grown with free access to nutrients, they exhibit inherent differences in relative growth rate (RGR) and rate of nutrient uptake Poorter et al. 1991;Garnier 1992; Van der Werf, Welschen & Lambers 1992). For example, fast-growing species exhibit RGR values that are 3-fold higher than those of slow-growing species . Similarly, the rate of net nitrate uptake (NNUR) is 4-to 6-fold higher in fast-growing species than in slow-growing ones (Poorter et al. 1991). Rates of root respiration (r t , O 2 uptake per unit root mass and time) are expected to be higher also, since more respiratory energy is needed for growth and ion uptake.Although the measured rates of root respiration are indeed approximately 1·7-fold higher in fast-growing species than in slow-growing ones, they are not as high as predicted from their high rates of growth and ion uptake. To predict rates of root respiration, assumptions have to be made about the specific respiratory costs for energy-requiring processes. Poorter et al. (1991) calculated the expected rates of root respiration in fast-growing and slow-growing herbs using the differences in RGR and NNUR and assuming the same specific respiratory costs for ion uptake and for growth and maintenance of biomass in fast-and slowgrowing species. To do this they used the specific respiratory costs determ...

In an earlier study, treatment of radish seed with the bacterium Pseudomonas fluorescens WCS374 suppressed fusarium wilt of radish (Fusarium oxysporum f. sp. raphani) in a commercial greenhouse [Leeman et al., 1991b[Leeman et al., , 1995a. In this greenhouse, the areas with fusarium wilt were localized or expanded very slowly, possibly due to disease suppressiveness of the soil. To study this phenomenon, fungi were isolated from radish roots collected from the greenhouse soil. Roots grown from seed treated with WCS374 were more abundantly colonized by fungi than were roots from nonbacterized plants. Among these were several species known for their antagonistic potential. Three of these fungi, Acremonium rutilum, Fusarium oxysporum and Verticillium lecanii, were evaluated further and found to suppress fusarium wilt of radish in a pot bioassay. In an induced resistance bioassay on rockwool, F. oxysporum and V. lecanii suppressed the disease by the apparent induction of systemic disease resistance. In pot bioassays with the Pseudomonas spp. strains, the pseudobactin-minus mutant 358PSB-did not suppress fusarium wilt, whereas its wild type strain (WCS358) suppressed disease presumably by siderophore-mediated competition for iron. The wild type strains of WCS374 and WCS417, as well as their pseudobactin-minus mutants 374PSB-and 417PSB-suppressed fusarium wilt. The latter is best explained by the fact that these strains are able to induce systemic resistance in radish, which operates as an additional mode of action. Co-inoculation in pot bioassays, ofA. rutilum, E oxysporum or V. lecanii with the Pseudomonas spp. WCS358, WCS374 or WCS417, or their pseudobactin-minus mutants, significantly suppressed disease (except forA. rutilum/417PSB-and all combinations with 358PSB-), compared with the control treatment, if the microorganisms were applied in inoculum densities which were ineffective in suppressing disease as separate inocula. If one or both of the microorganism(s) of each combination were applied as separate inocula in a density which suppressed disease, no additional suppression of disease was observed by the combination. The advantage of the co-inoculation is that combined populations significantly suppressed disease even when their individual population density was too low to do so. This may provide more consistent biological control. The co-inoculation effect obtained in the pot bioassays suggests that 9 //" . co-operation of P. fluorescens WCS374 and indigenous antagomsts~eould have been mvolved in the suppression of fusarium wilt of radish in the commercial greenhouse trials.Abbreviations: CFU = colony forming units; KB ~ King's B; PGPR = plant growth-promoting rhizobacteria; CQ = colonization quotient.