Christine Picard

Novel approaches in plant breeding for rhizosphere-related traits.

Selection of modern varieties has typically been performed in standardized, high fertility systems with a primary focus on yield. This could have contributed to the loss of plant genes associated with efficient nutrient acquisition strategies and adaptation to soil-related biotic and abiotic stresses if such adaptive strategies incurred a cost to the plant that compromised yield. Furthermore, beneficial interactions between plants and associated soil organisms may have been made obsolete by the provision of nutrients in high quantity and in readily plant available forms. A review of evidence from studies comparing older traditional varieties to modern high yielding varieties indeed showed that this has been the case. Given the necessity to use scarce and increasingly costly fertilizer inputs more efficiently while also raising productivity on poorer soils, it will be crucial to reintroduce desirable rhizosphere-related traits into elite cultivars. Traits that offer possibilities for improving nutrient acquisition capacity, plant–microbe interactions and tolerance to abiotic and biotic soil stresses in modern varieties were reviewed. Despite the considerable effort devoted to the identification of suitable donors and of genetic factors associated with these beneficial traits, progress in developing improved varieties has been slow and has so far largely been confined to modifications of traditional breeding procedures. Modern molecular tools have only very recently started to play a rather small role. The few successful cases reviewed in this paper have shown that novel breeding approaches using molecular tools do work in principle. When successful, they involved close collaboration between breeders and scientists conducting basic research, and confirmation of phenotypes in field tests as a ‘reality check’. We concluded that for novel molecular approaches to make a significant contribution to breeding for rhizosphere related traits it will be essential to narrow the gap between basic sciences and applied breeding through more interdisciplinary research that addresses rather than avoids the complexity of plant–soil interactions.

Genotypic and phenotypic diversity in populations of plant-probiotic Pseudomonas spp. colonizing roots

Several soil microorganisms colonizing roots are known to naturally promote the health of plants by controlling a range of plant pathogens, including bacteria, fungi, and nematodes. The use of theses antagonistic microorganisms, recently named plant-probiotics, to control plant-pathogenic fungi is receiving increasing attention, as they may represent a sustainable alternative to chemical pesticides. Many years of research on plant-probiotic microorganisms (PPM) have indicated that fluorescent pseudomonads producing antimicrobial compounds are largely involved in the suppression of the most widespread soilborne pathogens. Phenotype and genotype analysis of plant-probiotic fluorescent pseudomonads (PFP) have shown considerable genetic variation among these types of strains. Such variability plays an important role in the rhizosphere competence and the biocontrol ability of PFP strains. Understanding the mechanisms by which genotypic and phenotypic diversity occurs in natural populations of PFP could be exploited to choose those agricultural practices which best exploit the indigenous PFP populations, or to isolate new plant-probiotic strains for using them as inoculants. A number of different methods have been used to study diversity within PFP populations. Because different resolutions of the existing microbial diversity can be revealed depending on the approach used, this review first describes the most important methods used for the assessment of fluorescent Pseudomonas diversity. Then, we focus on recent data relating how differences in genotypic and phenotypic diversity within PFP communities can be attributed to geographic location, climate, soil type, soil management regime, and interactions with other soil microorganisms and host plants. It becomes evident that plant-related parameters exert the strongest influence on the genotypic and phenotypic variations in PFP populations.

Frequency and biodiversity of 2,4-diacetylphloroglucinol-producing rhizobacteria are differentially affected by the genotype of two maize inbred lines and their hybrid

Rhizobacteria (2808) were isolated on Pseudomonas-selective S1 medium from two maize inbred lines and from their hybrid at three plant growth stages. Positive phl D hybridization was found for 364 of them. The PhlD+ isolates were significantly more numerous in the rhizosphere of the hyrid than in those of parental lines. Furthermore, the frequency of PhlD+ was significantly higher for the hybrid at the flowering stage. An amplified rDNA restriction analysis showed that the hybrid genotype also increases the genetic diversity of PhlD+ populations when compared with its inbred parent lines, and this could be an effect of heterosis. Influence of the hybrid on the frequency and diversity of the bacterial PhlD+ population varied along the plant growth stage.

Genetic diversity of phl D gene from 2,4-diacetylphloroglucinol-producing Pseudomonas spp. strains from the maize rhizosphere

In biocontrol Pseudomonads, phlD is an essential gene involved in the biosynthesis of 2,4-diacetylphloroglucinol (DAPG). HaeIII restriction of amplified phlD gene, previously proposed as the most discriminant analysis, showed no polymorphism among 144 Pseudomonas strains isolated from maize roots. However, these strains fell into three statistically significant DAPG production level groups. phlD sequences of 13 strains belonging to the three DAPG groups revealed a KspI restriction site only in good DAPG-producing strains. This result was confirmed on the 144 strains, 82 of which were identified as good-DAPG producers by both biochemical and amplified phlD KspI restriction analysis. They are candidates as potential biocontrol agents.

Soil antimony pollution and plant growth stage affect the biodiversity of auxin-producing bacteria isolated from the rhizosphere of Achillea ageratum L

Abstract A total of 4512 rhizobacteria were isolated at three stages of plant growth from Achillea ageratum colonizing a polluted site with an antimony concentration gradient. For 222 of these isolates auxin production (aux(+)) was verified in vitro. The percentage of aux(+) isolates increased with soil antimony concentration, as well as with plant growth stage. An amplified rDNA restriction analysis clustered the aux(+) isolates into 51 clusters, one of which was numerically predominant and present throughout plant development and at all antimony concentrations. The aux(+) population was genetically very diverse, and this diversity was related to both antimony concentration and plant growth stage.