The apoplast-habitat of endophytic dinitrogen-fixing bacteria and their significance for the nitrogen nutrition of nonleguminous plants

Effects of fertilizer reduction coupled with straw returning on soil fertility, wheat root endophytic bacteria, and the occurrence of wheat crown rot

Excessive fertilization is associated with nutrient loss, soil compaction, and weak plant resistance. Straw returning can increase soil fertility with a consequent reduction in fertilizer, but the effects of fertilizer reduction coupled with straw returning on crop endophytic microbes and crop disease are poorly understood. Therefore, using metagenomic sequencing methods we investigated the responses of soil fertility, diversity, the function of root endophytic bacteria, and the occurrence of wheat crown rot due to the application of fertilizer (no, moderate and excessive fertilizer) coupled with or without straw returning after 7 years of treatments. The results showed that, after excessive fertilization, the wheat crown rot became severe, registering a disease index of 23. Compared with excessive fertilization, moderate fertilization coupled with straw returning significantly reduced the incidence of wheat crown rot, the disease index was reduced by 38.50%, and the richness and diversity of endophytic bacteria were increased by 61.20 and 11.93%, respectively, but the soil fertility was not significantly affected. In addition, moderate fertilization coupled with straw returning changed the community structure of endophytic bacteria and increased the relative abundance of carbohydrate metabolism and nitrogen fixation-related genes by 4.72 and 9.32%, respectively. Our results indicated that fertilizer reduction coupled with straw returning reduced the occurrence of wheat crown rot, increased the diversity of endophytic bacteria, and changed the community structure and function of endophytic bacteria, which will provide a better understanding of the interaction of fertilization coupled with straw returning, endophytic bacteria and wheat crown rot.

Azospirillum-plant relationships: physiological, molecular, agricultural, and environmental advances (1997-2003)

This review presents a critical and comprehensive documentation and analysis of the developments in agricultural, environmental, molecular, and physiological studies related to Azospirillum cells, and to Azospirillum interactions with plants, based solely on information published between 1997 and 2003. It was designed as an update of previous reviews (Bashan and Levanony 1990; Bashan and Holguin 1997a), with a similar scope of interest. Apart from an update and critical analysis of the current knowledge, this review focuses on the central issues of Azospirillum research today, such as, (i) physiological and molecular studies as a general model for rhizosphere bacteria; (ii) co-inoculation with other microorganisms; (iii) hormonal studies and re-consideration of the nitrogen contribution by the bacteria under specific environmental conditions; (iv) proposed Azospirillum as a non-specific plant-growth-promoting bacterium; (v) re-introduction of the "Additive Hypothesis," which suggests involvement of multiple mechanisms employed by the bacteria to affect plant growth; (vi) comment on the less researched areas, such as inoculant and pesticide research; and (vii) proposes possible avenues for the exploitation of this bacterium in environmental areas other than agriculture.Key words: Azospirillum, plant–bacteria interaction, plant-growth-promoting bacteria, PGPB, PGPR, rhizosphere bacteria.

Location and quantification of phosphorus and other elements in fully hydrated, soil-grown arbuscular mycorrhizas: a cryo-analytical scanning electron microscopy study

•  Concentrations of phosphorus (P), potassium (K), magnesium (Mg) and calcium (Ca) were determined in situ in fully hydrated arbuscular mycorrhizas by cryo-analytical scanning electron microscopy. The field- and glasshouse-grown plants (subterranean and white clovers, field pea and leek) were colonized by indigenous mycorrhizal fungi. •  The [P] in intraradical hyphae was generally 60-170 mM, although up to 600 mM was recorded, and formed strong linear relationships with [K], up to 350 mM, and [Mg], up to 175 mM. Little Ca was detected. The turgid branches of young arbuscules contained 30-50 mM P, up to 100 mM K and little Mg. Collapsing arbuscule branches and clumped arbuscules had greatly elevated Ca (30-250 mM), but otherwise differed little from young arbuscule branches in elemental concentration. •  The [P] was low or undetectable in 86% of uncolonized cortical cell vacuoles, but was generally elevated in vacuoles surrounding an arbuscule and in the liquid surrounding hyphae in intercellular spaces. •  Our results suggest that both young arbuscules and intercellular hyphae are sites for P-transfer, that Mg²⁺ and K⁺ are probably balancing cations for P anions in hyphae, and that host cells may limit arbuscule lifespan through deposition of material rich in Ca.

Gluconacetobacter diazotrophicus, a sugar cane endosymbiont, produces a bacteriocin against Xanthomonas albilineans, a sugar cane pathogen

Gluconacetobacter diazotrophicus in liquid culture secretes proteins into the medium. Both medium containing Gluconacetobacter protein and a solution of this protein after acetone precipitation appeared to inhibit the growth of Xanthomonas albilineans in solid culture. This apparent inhibition of bacterial growth has, in fact, been revealed to be lysis of bacterial cells, as demonstrated by transmission electron microscopy. Fractionation of the Gluconacetobacter protein mixture in size-exclusion chromatography reveals a main fraction with lysozyme-like activity which produces lysis of both living bacteria and isolated cell walls.

Endophytic Colonization of Plants by the Biocontrol Agent Rhizobium etli G12 in Relation to Meloidogyne incognita Infection

ABSTRACT The external and internal colonization of potato and Arabidopsis roots by the biocontrol strain Rhizobium etli G12 containing a plasmidborne trp promoter green fluorescent protein transcriptional fusion, pGT-trp, was studied in the presence and absence of the root-knot nematode Meloidogyne incognita. Plant colonization behavior and biocontrol potential of the marked strain G12(pGT-trp) was not altered compared with the parental strain. Plasmid pGT-trp was stable for more than 80 generations without selection and conferred sufficient fluorescence to detect single bacterial cells in planta. Although bacteria were found over the entire rhizoplane, they preferentially colonized root tips, the emerging lateral roots, and galled tissue caused by Meloidogyne infestation. Internal colonization of potato roots was mainly observed in epidermal cells, especially root hairs. G12(pGT-trp) colonization was also observed in inner Arabidopsis root tissues in areas of vascularization. In the presence of M. incognita, G12(pGT-trp) colonized the interior of nematode galls in high numbers. In some cases, bacterial colonization even extended from the galled tissue into adjacent root tissue. The internally colonized sites in roots were often discontinuous. Fluorescence microscopy of gfp-tagged rhizobacteria was a sensitive and a rapid technique to study external and internal colonization of plant roots by bacteria interacting with nematodes.

The apoplast and its significance for plant mineral nutrition

It has only recently become apparent that the apoplast plays a major role in a diverse range of processes, including intercellular signalling, plant-microbe interactions and both water and nutrient transport. Broadly defined, the apoplast constitutes all compartments beyond the plasmalemma - the interfibrillar and intermicellar space of the cell walls, and the xylem, including its gas- and water-filled intercellular space - extending to the rhizoplane and cuticle of the outer plant surface. The physico-chemical properties of cell walls influence plant mineral nutrition, as nutrients do not simply pass through the apoplast to the plasmalemma but can also be adsorbed or fixed to cell-wall components. Here, current progress in understanding the significance of the apoplast in plant mineral nutrition is reviewed. The contribution of the root apoplast to short-distance transport and nutrient uptakes is examined particularly in relation to Na⁺ toxicity and Al³⁺ tolerance. The review extends to long-distance transport and the role of the apoplast as a habitat for microorganisms. In the leaf, the apoplast might have benefits over the vacuole as a site for short-term nutrient storage and solute exchange with the atmosphere. Contents Summary 167 I. Introduction 168 II. The properties of the apoplast and its implication for solute movement 168 1. The middle lamella 168 2. The primary wall 168 3. The secondary cell wall 169 III. The root apoplast - nutrient uptake and short-distance transport 170 IV. The apoplast as a compartment for long distance transport 174 V. The apoplast - habitat for microorganisms 175 VI. The apoplast of leaves - a compartment of storage and of reactions 177 1. Transport routes in the leaf apoplast 177 2. Methods of studying apoplastic solutes 177 3. Solute relations in the leaf apoplast 178 4. Concentration gradients in the leaf apoplast 179 5. Ion relations in the leaf apoplast and symptoms of deficiency and toxicity 179 6. Ion relations in the leaf apoplast - influence of nutrient supply 180 7. The leaf apoplast - compartment for transient ion storage 180 8. Ion fluxes between apoplast and symplast 181 9. Apoplastic ion balance 181 10. Leaf apoplast - interaction with the atmosphere 183 VII. Conclusions 183 Acknowledgements 183 References 183.