The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented.

The Role of Root Exudates in Rhizosphere Interactions with Plants and Other Organisms

Contents
 	Summary	30	
I.	Allocation in perspective	31	
II.	Topics of this review	32	
III.	Methodology	32	
IV.	Environmental effects	33	
V.	Ontogeny	36	
VI.	Differences between species	40	
VII.	Physiology and molecular regulation	41	
VIII.	Ecological aspects	42	
IX.	Perspectives	45	
 	Acknowledgements	45	
 	References	45	
 	Appendices A1–A4	49	


Summary
We quantified the biomass allocation patterns to leaves, stems and roots in vegetative plants, and how this is influenced by the growth environment, plant size, evolutionary history and competition. Dose–response curves of allocation were constructed by means of a meta-analysis from a wide array of experimental data. They show that the fraction of whole-plant mass represented by leaves (LMF) increases most strongly with nutrients and decreases most strongly with light. Correction for size-induced allocation patterns diminishes the LMF-response to light, but makes the effect of temperature on LMF more apparent. There is a clear phylogenetic effect on allocation, as eudicots invest relatively more than monocots in leaves, as do gymnosperms compared with woody angiosperms. Plants grown at high densities show a clear increase in the stem fraction. However, in most comparisons across species groups or environmental factors, the variation in LMF is smaller than the variation in one of the other components of the growth analysis equation: the leaf area : leaf mass ratio (SLA). In competitive situations, the stem mass fraction increases to a smaller extent than the specific stem length (stem length : stem mass). Thus, we conclude that plants generally are less able to adjust allocation than to alter organ morphology.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2011.03952.x

Biomass allocation to leaves, stems and roots: meta-analyses of interspecific variation and environmental control

Arbuscular mycorrhiza (AM), a symbiosis between plants and members of an ancient phylum of fungi, the Glomeromycota, improves the supply of water and nutrients, such as phosphate and nitrogen, to the host plant. In return, up to 20% of plant-fixed carbon is transferred to the fungus. Nutrient transport occurs through symbiotic structures inside plant root cells known as arbuscules. AM development is accompanied by an exchange of signalling molecules between the symbionts. A novel class of plant hormones known as strigolactones are exuded by the plant roots. On the one hand, strigolactones stimulate fungal metabolism and branching. On the other hand, they also trigger seed germination of parasitic plants. Fungi release signalling molecules, in the form of 'Myc factors' that trigger symbiotic root responses. Plant genes required for AM development have been characterized. During evolution, the genetic programme for AM has been recruited for other plant root symbioses: functional adaptation of a plant receptor kinase that is essential for AM symbiosis paved the way for nitrogen-fixing bacteria to form intracellular symbioses with plant cells.

Arbuscular mycorrhiza: the mother of plant root endosymbioses.

A survey of 659 papers mostly published since 1987 was conducted to compile a checklist of mycorrhizal occurrence among 3,617 species (263 families) of land plants. A plant phylogeny was then used to map the mycorrhizal information to examine evolutionary patterns. Several findings from this survey enhance our understanding of the roles of mycorrhizas in the origin and subsequent diversification of land plants. First, 80 and 92% of surveyed land plant species and families are mycorrhizal. Second, arbuscular mycorrhiza (AM) is the predominant and ancestral type of mycorrhiza in land plants. Its occurrence in a vast majority of land plants and early-diverging lineages of liverworts suggests that the origin of AM probably coincided with the origin of land plants. Third, ectomycorrhiza (ECM) and its derived types independently evolved from AM many times through parallel evolution. Coevolution between plant and fungal partners in ECM and its derived types has probably contributed to diversification of both plant hosts and fungal symbionts. Fourth, mycoheterotrophy and loss of the mycorrhizal condition also evolved many times independently in land plants through parallel evolution.

Phylogenetic distribution and evolution of mycorrhizas in land plants

Plants associate with a wide range of microorganisms, with both detrimental and beneficial outcomes. Central to plant survival is the ability to recognize invading microorganisms and either limit their intrusion, in the case of pathogens, or promote the association, in the case of symbionts. To aid in this recognition process, elaborate communication and counter-communication systems have been established that determine the degree of ingress of the microorganism into the host plant. In this Review, I describe the common signalling processes used by plants during mutualistic interactions with microorganisms as diverse as arbuscular mycorrhizal fungi and rhizobial bacteria.

Speak, friend, and enter: signalling systems that promote beneficial symbiotic associations in plants.

Evaluation of the transition matrix for comminuting pea seeds in an impact mill using a linear neural network

Legumes fix atmospheric nitrogen by symbiosis with soil bacteria in root nodules. Legume yields are limited by the low capacity of N2 fixation. Hypernodulating mutants have been selected decades ago to try to increase nodule number. However, literature data show that N fixation of hypernodulating mutants was not increased compared to parental lines. Here, we study the functional basis of limited N fixation associated to hypernodulation. We grew two wild type genotypes and nine hypernodulating mutants of pea in hydroponics in three greenhouse experiments. We measured the following traits related to N nutrition during the vegetative period: nodule number, plant N uptake, nodule-specific activity, and plant and nodule concentrations. Genetic and environmental variations induced nodule gradients. These gradients were used to set quantitative relationships between N nutrition traits and nodule number. We compared the relationships obtained for hypernodulating and for wild types. N nutrition traits were analysed together with C nutrition traits, through correlation networks. Our results show that higher nodule number of hypernodulating mutants is correlated with lower levels of nodule activity, from −25 to −60 %, by comparison to the wild type. Higher nodule number of hypernodulating mutants is also correlated with lower total N uptake by symbiotic fixation, from −0 to −60 %, by comparison to the wild type. Findings demonstrate that N nutrition is not a factor limiting growth in hypernodulating mutants, as shown by N nutrition index higher than 1, indicating N nutrition in excess. The correlations suggest that limited N2 fixation in hypernodulating mutants arises from restricted shoot growth, which limits the plant capacity to accumulate N. Furthermore, symbiotic efficiency decreased with increasing nodule number, down to a minimal value for hypernodulating mutants. Thus, to overcome the trade-off between N benefits from N2 fixation and carbon nodulation costs, the hypernodulation trait should be associated with high shoot growth capacity in breeding programs.

/pdf/pea-nodule-gradients-explain-n-nutrition-and-limited-4x3f2gv0ba.pdf

Pea nodule gradients explain N nutrition and limited symbiotic fixation in hypernodulating mutants

Trabajo presentado en la 6th International Food Legume Research Conference y la 7th International Conference of Legume Genetics and Genomics (IFLRC VI & ICLGG VII), celebrada en Saskatoon (Canada) del 7 al 11 de julio de 2014.

LEGumes for the Agriculture of TOmorrow (LEGATO project)

Identification of molecular markers tightly linked to low tannin and vicine-convicine content in faba beans.

Nous proposons un diagnostic des forces de recherche investies sur les legumineuses proteagineuses importantes pour l’Europe (pois, feveroles lupins, pois chiches) par une analyse des publications scientifiques au niveau mondial et francais sur la periode 2000-2009, en regard des investissements sur le soja et sur les especes modeles. Le corpus mondial identifie sur cette periode 23640, 18946, et 1335 references respectivement sur le soja, les proteagineux et les especes legumineuses modeles. La taille du corpus mondial « proteagineux + legumes secs + Arabidopsis thaliana » a augmente de 33% entre les periodes 1990-1999 et 2000-2009, avec amplification des contributions sur Arabidopsis thaliana et une stabilite dans la repartition entre especes de legumineuses a graines qui est correlee aux volumes produits. Sur la periode 2000-2009 la France contribue a 28% des publications mondiales sur les especes legumineuses modeles (Medicago truncatula, Lotus japonicus). Environ 11% des publications sur proteagineux font appel a une espece legumineuse modele en France a comparer a 1% dans le monde. Les publications sur les especes legumineuses modeles sont en majorite issues des disciplines de la genetique et de la physiologie vegetale. Le pois est l’espece de legumineuse a graine la plus etudiee en France (genetique, physiologie, protection des plantes agronomie-ecophysiologie sont les disciplines preponderantes), alors que les travaux sur le soja sont dominants au niveau mondial (les memes disciplines sont mobilisees que pour les proteagineux). Une premiere analyse par disciplines et themes permet d’identifier des signaux forts et faibles dans les publications francaises. Nous elargirons prochainement au niveau mondial notre diagnostic par thematique, en le combinant a une etude des blocs geographiques mondiaux impliques, et une analyse du reseau des collaborations internationales etablies par la recherche francaise, incluant une description des journaux impliques.

https://hal.archives-ouvertes.fr/hal-01173194/document

Analyse bibliométrique des publications scientifiques françaises et mondiales sur les protéagineux au cours de la période 2000-2009 : comparaison au soja et aux espèces modèles

Gérard Duc

Papers

Evaluation of the transition matrix for comminuting pea seeds in an impact mill using a linear neural network

Pea nodule gradients explain N nutrition and limited symbiotic fixation in hypernodulating mutants

LEGumes for the Agriculture of TOmorrow (LEGATO project)

Identification of molecular markers tightly linked to low tannin and vicine-convicine content in faba beans.

Analyse bibliométrique des publications scientifiques françaises et mondiales sur les protéagineux au cours de la période 2000-2009 : comparaison au soja et aux espèces modèles