Evaluation Of Tests to Determine Resistance of Zantedeschia spp. (Araceae) to Soft Rot caused by Erwinia carotovora subsp. carotovora

Bacterial soft rot caused by Erwinia carotovora subsp. carotovora is a major disease in Zantedeschia spp., particularly in cultivars from the section Aestivae. The disease can be partly controlled by cultural measures, but by combining cultural methods with resistant plant material a promising strategy for control of soft rot can be developed. No tests are available for resistance testing in breeding Zantedeschia spp. Therefore, three tests developed for use in potato breeding were adapted for use on eight cultivars of Zantedeschia spp. Variation was found in all three tests. Resistant control cultivar Zantedeschia aethiopica Crowborough scored most resistant in all three tests. Within the section Aestivae, degrees of susceptibility were identified that were in agreement with each other and with field observations, indicating reliability of two of the methods in which tubers were used. The correlation coefficient of these two tests was high. A new non-destructive test method was developed for use on seedlings which involved immersion of leaf disks in a bacterial suspension. The percentage of decayed leaf area was a measure of resistance and results were in general agreement with the other tests. These methods will be useful for breeding for soft rot resistance and performing genetic analyses.     

The role of siderophores in potato tuber yield increase by Pseudomonas putida in a short rotation of potato

The effect of treatment of potato seed tubers withPseudomonas putida isolate WCS358 on tuber yield was studied in different crop rotations at the Experimental Farm De Schreef, near Lelystad. With untreated, tuber yield in a 1:3 (short) rotation compared to yield in a 1:6 (long) rotation of potato was decreased by 11% at 86 days (seed tuber harvest) and by 14% at 130 days (ware potato harvest) after seeding. Seed tuber treatment with the wild-type isolate WCS358 increased tuber yield with 13% in a short rotation of potato 86 days after seeding, whereas a siderophore-negative Tn5 transposon mutant of this isolate had no effect on tuber yield. Seed tuber treatment with the wild-type isolate or the siderophore-negative mutant in a long rotation of potato had no effect on tuber yield. At 130 days after seeding no effect of any of the seed tuber treatments was found in both short and long rotations of potato.Root colonization by siderophore-producing Tn5 transposon mutants of WCS358 was decreased at the end of the growing season. No difference in root colonization between siderophore-producing and siderophore-negative Tn5 transposon mutants was found at 130 days after seeding.Siderophore production seems to be a prerequisite in potato tuber yield increase by WCS358 under field conditions. This is the first time that the involvement of siderophores in growth stimulation has been demonstrated in the field.Samenvatting De invloed van een behandeling van aardappelpootgoed metPseudomonas putida isolaat WCS358 op de knolopbrengst werd onderzocht in verschillende gewasrotaties on een proefveld van proefboerderij De Schreef, Flevopolder. In de controlebehandelingen werd in een nauwe aardappelrotatie (1:3) een reductie van 11% in opbrengst van pootaardappelen (86 dagen na het poten) geconstateerd ten opzichte van een ruime aardappelrotatie (1:6); 130 dagen na het poten werd een vermindering met 14% gevonden in de opbrengst van consumptieaardappelen.Pootgoedbehandeling met het siderofoorproducerende isolaat WCS358 verhoogde de opbrengst van pootaardappelen in de 1:3-rotatie met 13%. Een Tn5-transposonmutant van dit isolaat die het vermogen sideroforen te produceren had verloren, had geen effect op de opbrengst. In de 1:6-rotatie had behandeling van pootgoed met WCS358 geen effect op de opbrengst van pootaardappelen.Zowel in de nauwe (1:3) als in de ruimte (1:6) rotatie werd (130 dagen na het poten), geen effect van behandeling van pootgoed met WCS358 op de opbrengst van consumptieaardappelen gevonden.Wortelkolonisatie door siderofoorproducerende Tn5-transposonmutanten van WCS358 nam aan het eind van het seizoen af. Er werd, 130 dagen na het poten, geen verschil in wortelkolonisatie geconstateerd tussen siderofoorproducerende en siderofoornegatieve Tn5-transposonmutanten.Siderofoorproduktie blijkt een voorwaarde te zijn voor verhoging van de knolopbrengst door WCS358 onder veldomstandigheden. De verhoging van de knolopbrengst treedt alleen op in de nauwe aardappelrotatie. Dit is de eerste keer dat de betrokkenheid van sideroforen bij groeistimulatie onder veldomstandigheden is aangetoond.     

Combining Fluorescent Pseudomonas spp. Strains to Enhance Suppression of Fusarium Wilt of Radish

Fusarium wilt diseases, caused by the fungus Fusarium oxysporum, lead to significant yield losses of crops. One strategy to control fusarium wilt is the use of antagonistic, root-colonizing Pseudomonas spp. It has been demonstrated that different strains of these bacteria suppress disease by different mechanisms. Therefore, application of a mixture of these biocontrol strains, and thus of several suppressive mechanisms, may represent a viable control strategy. A prerequisite for biocontrol by combinations of biocontrol agents can be the compatibility of the co-inoculated micro-organisms. Hence, compatibility between several Pseudomonas spp. strains, that have the ability to suppress fusarium wilt of radish, was tested in vitro on KB agar plates. Growth of P. fluorescens strain RS111 was strongly inhibited by Pseudomonas spp. strains RE8, RS13, RS56 and RS158, whereas a mutant of strain RS111 (RS111-a) was insensitive to inhibition by these strains. Strains RS111 and RS111-a only slightly inhibited some other strains. Suppression of fusarium wilt of radish in a potting soil bioassay by the incompatible combination of RE8 and RS111 was comparable to the effects of the single strains. However, disease suppression by the compatible combination of RE8 and RS111-a was significantly better as compared to the single strains. In contrast, the incompatible combination of RS56 with RS111 resulted in enhanced disease suppression as compared to the single strains. Increased disease suppression by combinations of RS13 or RS158 with RS111 or RS111-a was not observed. This indicates that specific interactions between biocontrol strains influence disease suppression by combinations of these strains.     

Ecosystem services in grassland associated with biotic and abiotic soil parameters

Biotic soil parameters have so far seldom played a role in practical soil assessment and management of grasslands. However, the ongoing reduction of external inputs in agriculture would imply an increasing reliance on ecosystem self-regulating processes. Since soil biota play an important role in these processes and in the provision of ecosystem services, biological soil parameters should be an integral part of soil assessment. The general objective of the current study is to investigate to what extent biotic soil parameters provide additional value in soil quality assessment of grassland on sandy soils. We measured abiotic and biotic soil parameters together with process parameters underlying ecosystem services in 20 permanent production grasslands. Cross-validated stepwise regression was used to identify abiotic and biotic soil parameters that explained the soil ecosystem services soil structure maintenance, water regulation, supply of nutrients, and grassland production, respectively.Process parameters underlying the ecosystem service soil structure maintenance such as bulk density and the percentage of sub-angular blocky elements were mainly influenced by SOM and its qualities. The correlations between penetration resistance at 0-10 cm and the percentage of soil crumbs with earthworms suggested a relationship to earthworm activity. Parameters underlying the service of water regulation showed no clear relationship to biotic soil parameters. Water infiltration rate in the field was explained by the penetration resistance at 10-20 cm. Process parameters underlying the service of nutrients’ supply such as the potentially mineralizable C and N were mainly determined by soil total N. The potential C and N mineralization were more related to biotic soil parameters, whereby each parameter was the other’s antithesis. The grassland production without N fertilization viz. the nitrogen supply capacity of the soil measured as N yield, was mainly explained by soil organic matter (SOM) and soil moisture, and to a lesser extent by soil total N. One gram of SOM per kg of dry soil corresponded to 3.21 kg N yield ha⁻¹, on top of a constant of 15.4 kg N ha⁻¹. The currently applied calculations in the Dutch grassland fertilization recommendation, underestimated in 85% of the production grasslands, the measured nitrogen supply capacity of the soil by on average 42 kg N ha⁻¹ (31%). This legitimizes additional research to improve the currently applied recommendations for sandy soils. The response of N yield to N fertilization ranged from 35 to 102%. This wide range emphasizes the importance of a better recommendation base to target N fertilizer. The response of N yield to N fertilization was predicted by the total number of enchytraeids, the underlying mechanism of which needs further investigation on different soil types. This knowledge can be important for the optimal use of fertilizer and its consequences for environmental quality.     

Rhizobacteria-mediated Induced Systemic Resistance: Triggering, Signalling and Expression

Selected strains of rhizosphere bacteria reduce disease by activating a resistance mechanism in the plant named rhizobacteria-mediated induced systemic resistance (ISR). Rhizobacteria-mediated ISR resembles pathogen-induced systemic acquired resistance (SAR) in that both types of induced resistance render uninfected plant parts more resistant towards a broad spectrum of plant pathogens. Some rhizobacteria trigger the salicylic acid (SA)-dependent SAR pathway by producing SA at the root surface. In other cases, rhizobacteria trigger a different signalling pathway that does not require SA. The existence of a SA-independent ISR pathway has been demonstrated in Arabidopsis thaliana. In contrast to pathogen-induced SAR, ISR induced by Pseudomonas fluorescens WCS417r is independent of SA accumulation and pathogenesis-related (PR) gene activation but, instead, requires responsiveness to the plant hormones jasmonic acid (JA) and ethylene. Mutant analyses showed that ISR follows a novel signalling pathway in which components from the JA and ethylene response are successively engaged to trigger a defensive state that, like SAR, is controlled by the regulatory factor NPR1. Interestingly, simultaneous activation of both the JA/ethylene-dependent ISR pathway and the SA-dependent SAR pathway results in an enhanced level of protection. Thus combining both types of induced resistance provides an attractive tool for the improvement of disease control. This review focuses on the current status of our research on triggering, signalling, and expression of rhizobacteria-mediated ISR in Arabidopsis.     

The Effects of Founding Events and Agricultural Practices on the Genetic Structure of Three Metapopulations of Globodera pallida

To assess the genetic constitution of the Globodera pallida populations in the Netherlands and the effects of agricultural practices, three geographically separated metapopulations, in total consisting of 226 local populations, were analyzed by two-dimensional gel electrophoresis (2-DGE) of total proteins. This technique allows the accurate assessment of allele frequencies in homogenates of mixtures of individuals. Based on the estimated average heterozygosity, the average proportion of polymorphic loci and the average number of alleles per locus, the genetic diversity among 226 local G. pallida populations was small. The small genetic basis of G. pallida in the Netherlands will facilitate the identification of Solanum genotypes with a broad and durable resistance to G. pallida. Instead of clusters of local populations with unique alleles, a continuous range of allele frequencies was observed. Analysis of the three metapopulations by the Shannon-Weaver index and Nei's G(ST) revealed that the metapopulation from a region with sandy-loam soils was clearly distinguishable from the remaining two; the local populations within this metapopulation were more similar and the genetic diversity within the individual local populations was significantly higher than the local populations from the two remaining regions. These regions are characterized by wider crop rotation schemes and a very limited use of nematicides. The less intensive cultivation of potatoes in these regions with sandy-clay soils resulted in relatively little variation within and more variation between local nematode populations. To our knowledge, the effects of agricultural practices on the genetic constitution of potato cyst nematode populations have not been pinpointed before.     

Earthworm species composition affects the soil bacterial community and net nitrogen mineralization

Knowledge of the effects of species diversity within taxonomic groups on nutrient cycling is important for understanding the role of soil biota in sustainable agriculture. We hypothesized that earthworm species specifically affect nitrogen mineralization, characteristically for their ecological group classifications, and that earthworm species interactions would affect mineralization through competition and facilitation effects. A mesocosm experiment was conducted to investigate the effect of three earthworm species, representative of different ecological groups (epigeic: Lumbricus rubellus; endogeic: Aporrectodea caliginosa tuberculata; and anecic: Lumbricus terrestris), and their interactions on the bacterial community, and on nitrogen mineralization from ¹⁵N-labelled crop residue and from soil organic matter.Our results indicate that L. rubellus and L. terrestris enhanced mineralization of the applied crop residue whereas A. caliginosa had no effect. On the other hand, L. rubellus and A. caliginosa enhanced mineralization of the soil organic matter, whereas L. terrestris had no effect. The interactions between different earthworm species affected the bacterial community and the net mineralization of soil organic matter. The two-species interactions between L. rubellus and A. caliginosa, and L. rubellus and L. terrestris, resulted in reduced mineral N concentrations derived from soil organic matter, probably through increased immobilization in the bacterial biomass. In contrast, the interaction between A. caliginosa and L. terrestris resulted in increased bacterial growth rate and reduced total soil C. When all three species were combined, the interaction between A. caliginosa and L. terrestris was dominant. We conclude that the effects of earthworms on nitrogen mineralization depend on the ecological traits of the earthworm species present, and can be modified by species interactions. Knowledge of these effects can be made useful in the prevention of nutrient losses and increased soil fertility in agricultural systems, that typically have a low earthworm diversity.     

Reversible optical transcription of supramolecular chirality into molecular chirality

In nature, key molecular processes such as communication, replication, and enzyme catalysis all rely on a delicate balance between molecular and supramolecular chirality. Here we report the design, synthesis, and operation of a reversible, photoresponsive, self-assembling molecular system in which molecular and supramolecular chirality communicate. It shows exceptional stereoselectivity upon aggregation of the molecules during gel formation with the solvent. This chirality is locked by photochemical switching, a process that is subsequently used to induce an inverted chiral supramolecular assembly as revealed by circular dichroism spectroscopy. The optical switching between different chiral aggregated states and the interplay of molecular and supramolecular chirality offer attractive new prospects for the development of molecular memory systems and smart functional materials.