Specialized 16SrX phytoplasmas induce diverse morphological and physiological changes in their respective fruit crops.

The host-pathogen combinations-Malus domestica (apple)/`Candidatus Phytoplasma mali´, Prunus persica (peach)/`Ca. P. prunorum´ and Pyrus communis (pear)/`Ca. P. pyri´ show different courses of diseases although the phytoplasma strains belong to the same 16SrX group. While infected apple trees can survive for decades, peach and pear trees die within weeks to few years. To this date, neither morphological nor physiological differences caused by phytoplasmas have been studied in these host plants. In this study, phytoplasma-induced morphological changes of the vascular system as well as physiological changes of the phloem sap and leaf phytohormones were analysed and compared with non-infected plants. Unlike peach and pear, infected apple trees showed substantial reductions in leaf and vascular area, affecting phloem mass flow. In contrast, in infected pear mass flow and physicochemical characteristics of phloem sap increased. Additionally, an increased callose deposition was detected in pear and peach leaves but not in apple trees in response to phytoplasma infection. The phytohormone levels in pear were not affected by an infection, while in apple and peach trees concentrations of defence- and stress-related phytohormones were increased. Compared with peach and pear trees, data from apple suggest that the long-lasting morphological adaptations in the vascular system, which likely cause reduced sap flow, triggers the ability of apple trees to survive phytoplasma infection. Some phytohormone-mediated defences might support the tolerance.

Molecular Characterization of Indigenous Rhizobia from Kenyan Soils Nodulating with Common Beans.

Kenya is the seventh most prominent producer of common beans globally and the second leading producer in East Africa. However, the annual national productivity is low due to insufficient quantities of vital nutrients and nitrogen in the soils. Rhizobia are symbiotic bacteria that fix nitrogen through their interaction with leguminous plants. Nevertheless, inoculating beans with commercial rhizobia inoculants results in sparse nodulation and low nitrogen supply to the host plants because these strains are poorly adapted to the local soils. Several studies describe native rhizobia with much better symbiotic capabilities than commercial strains, but only a few have conducted field studies. This study aimed to test the competence of new rhizobia strains that we isolated from Western Kenya soils and for which the symbiotic efficiency was successfully determined in greenhouse experiments. Furthermore, we present and analyze the whole-genome sequence for a promising candidate for agricultural application, which has high nitrogen fixation features and promotes common bean yields in field studies. Plants inoculated with the rhizobial isolate S3 or with a consortium of local isolates (COMB), including S3, produced a significantly higher number of seeds and seed dry weight when compared to uninoculated control plants at two study sites. The performance of plants inoculated with commercial isolate CIAT899 was not significantly different from uninoculated plants (p > 0.05), indicating tight competition from native rhizobia for nodule occupancy. Pangenome analysis and the overall genome-related indices showed that S3 is a member of R. phaseoli. However, synteny analysis revealed significant differences in the gene order, orientation, and copy numbers between S3 and the reference R. phaseoli. Isolate S3 is phylogenomically similar to R. phaseoli. However, it has undergone significant genome rearrangements (global mutagenesis) to adapt to harsh conditions in Kenyan soils. Its high nitrogen fixation ability shows optimal adaptation to Kenyan soils, and the strain can potentially replace nitrogenous fertilizer application. We recommend that extensive fieldwork in other parts of the country over a period of five years be performed on S3 to check on how the yield changes with varying whether conditions.

Guardians of the phloem - forisomes and beyond.

The phloem is a highly specialized vascular tissue that forms a fundamentally important transport and signaling pathway in plants. It is therefore a system worth protecting. The main function of the phloem is to transport the products of photosynthesis throughout the whole plant, but it also transports soluble signaling molecules and propagates electrophysiological signals. The phloem is constantly threatened by mechanical injuries, phloem-sucking pests and parasites, and the spread of pathogens, which has led to the evolution of efficient defense mechanisms. One such mechanism involves structural phloem proteins, which are thought to facilitate sieve element occlusion following injury and to defend the plant against pathogens. In leguminous plants, specialized structural phloem proteins known as forisomes form unique mechanoproteins via sophisticated molecular interaction and assembly mechanisms, thus enabling reversible sieve element occlusion. By understanding the structure and function of forisomes and other structural phloem proteins, we can develop a toolbox for biotechnological applications in material science and medicine. Furthermore, understanding the involvement of structural phloem proteins in plant defense mechanisms will allow phloem engineering as a new strategy for the development of crop varieties that are resistant to pests, pathogens and parasites.

Distribution, Characterization and the Commercialization of Elite Rhizobia Strains in Africa.

Grain legumes play a significant role in smallholder farming systems in Africa because of their contribution to nutrition and income security and their role in fixing nitrogen. Biological Nitrogen Fixation (BNF) serves a critical role in improving soil fertility for legumes. Although much research has been conducted on rhizobia in nitrogen fixation and their contribution to soil fertility, much less is known about the distribution and diversity of the bacteria strains in different areas of the world and which of the strains achieve optimal benefits for the host plants under specific soil and environmental conditions. This paper reviews the distribution, characterization, and commercialization of elite rhizobia strains in Africa.

The Physiological and Biochemical Effects on Napier Grass Plants Following Napier Grass Stunt Phytoplasma Infection.

Napier grass stunt (NGS) phytoplasma, a phloem-limited bacterium, infects Napier grass leading to severe yield losses in East Africa. The infected plants are strongly inhibited in growth and biomass production. In this study, phytoplasma-induced morphological changes of the vascular system and physiological changes were analyzed and compared with uninfected plants. The study showed that the phytoplasmas are more abundant in source leaves and range from 103 bacteria/µg total DNA in infected roots to 106 in mature Napier grass leaves. Using microscopical, biochemical, and physiological tools, we demonstrated that the ultrastructure of the phloem and sieve elements is severely altered in the infected plants, which results in the reduction of both the mass flow and the translocation of photoassimilates in the infected leaves. The reduced transport rate inhibits the photochemistry of photosystem II in the infected plants, which is accompanied by loss of chloroplastic pigments in response to the phytoplasma infection stress eventually resulting in yellowing of diseased plants. The phytoplasma infection stress also causes imbalances in the levels of defense-related antioxidants, glutathione, ascorbic acid, reactive oxygen species (ROS), and-in particular-hydrogen peroxide. This study shows that the infection of NGS phytoplasma in the phloem of Napier grass has an impact on the primary metabolism and activates a ROS-dependent defense response.

Studying the Function of Phytoplasma Effector Proteins Using a Chemical-Inducible Expression System in Transgenic Plants.

Phytoplasmas are bacterial pathogens that live mainly in the phloem of their plant hosts. They dramatically manipulate plant development by secreting effector proteins that target developmental proteins of their hosts. Traditionally, the effects of individual effector proteins have been studied by ectopic overexpression using strong, ubiquitously active promoters in transgenic model plants. However, the impact of phytoplasma infection on the host plants depends on the intensity and timing of infection with respect to the developmental stage of the host. To facilitate investigations addressing the timing of effector protein activity, we have established chemical-inducible expression systems for the three most well-characterized phytoplasma effector proteins, SECRETED ASTER YELLOWS WITCHES' BROOM PROTEIN 11 (SAP11), SAP54 and TENGU in transgenic Arabidopsis thaliana. We induced gene expression either continuously, or at germination stage, seedling stage, or flowering stage. mRNA expression was determined by quantitative reverse transcription PCR, protein accumulation by confocal laser scanning microscopy of GFP fusion proteins. Our data reveal tight regulation of effector gene expression and strong upregulation after induction. Phenotypic analyses showed differences in disease phenotypes depending on the timing of induction. Comparative phenotype analysis revealed so far unreported similarities in disease phenotypes, with all three effector proteins interfering with flower development and shoot branching, indicating a surprising functional redundancy of SAP54, SAP11 and TENGU. However, subtle but mechanistically important differences were also observed, especially affecting the branching pattern of the plants.

Species-Specific and Distance-Dependent Dispersive Behaviour of Forisomes in Different Legume Species.

Forisomes are giant fusiform protein complexes composed of sieve element occlusion (SEO) protein monomers, exclusively found in sieve elements (SEs) of legumes. Forisomes block the phloem mass flow by a Ca2+-induced conformational change (swelling and rounding). We studied the forisome reactivity in four different legume species-Medicago sativa, Pisum sativum, Trifolium pratense and Vicia faba. Depending on the species, we found direct relationships between SE diameter, forisome surface area and distance from the leaf tip, all indicative of a developmentally tuned regulation of SE diameter and forisome size. Heat-induced forisome dispersion occurred later with increasing distance from the stimulus site. T. pratense and V. faba dispersion occurred faster for forisomes with a smaller surface area. Near the stimulus site, electro potential waves (EPWs)-overlapping action (APs), and variation potentials (VPs)-were linked with high full-dispersion rates of forisomes. Distance-associated reduction of forisome reactivity was assigned to the disintegration of EPWs into APs, VPs and system potentials (SPs). Overall, APs and SPs alone were unable to induce forisome dispersion and only VPs above a critical threshold were capable of inducing forisome reactions.

The Beneficial Root-Colonizing Fungus Mortierella hyalina Promotes the Aerial Growth of Arabidopsis and Activates Calcium-Dependent Responses That Restrict Alternaria brassicae-Induced Disease Development in Roots.

The endophytic fungus Mortierella hyalina colonizes the roots of Arabidopsis thaliana and stimulates growth and biomass production of the aerial parts but not of roots. An exudate fraction from the fungus induces rapid and transient cytoplasmic Ca2+elevation in the roots. The Ca2+ response does not require the well-characterized (co)receptors BAK1, CERK1, and FLS2 for pathogen-associated molecular patterns, and the Ca2+ channels GLR-2.4, GLR-2.5, and GLR-3.3 or the vacuolar TWO PORE CHANNEL1, which might be involved in cytoplasmic Ca2+ elevation. We isolated an ethyl-methane-sulfonate-induced Arabidopsis mutant that is impaired in this Ca2+ response. The roots of the mutant are impaired in M. hyalina-mediated suppression of immune responses after Alternaria brassicae infection, i.e., jasmonate accumulation, generation of reactive oxygen species, as well as the activation of jasmonate-related defense genes. Furthermore, they are more colonized by M. hyalina than wild-type roots. We propose that the mutant gene product is involved in a Ca2+-dependent signaling pathway activated by M. hyalina to suppress immune responses in Arabidopsis roots.

A Poly(A) Ribonuclease Controls the Cellotriose-Based Interaction between Piriformospora indica and Its Host Arabidopsis.

Piriformospora indica, an endophytic root-colonizing fungus, efficiently promotes plant growth and induces resistance to abiotic stress and biotic diseases. P. indica fungal cell wall extract induces cytoplasmic calcium elevation in host plant roots. Here, we show that cellotriose (CT) is an elicitor-active cell wall moiety released by P. indica into the medium. CT induces a mild defense-like response, including the production of reactive oxygen species, changes in membrane potential, and the expression of genes involved in growth regulation and root development. CT-based cytoplasmic calcium elevation in Arabidopsis (Arabidopsis thaliana) roots does not require the BAK1 coreceptor or the putative Ca2+ channels TPC1, GLR3.3, GLR2.4, and GLR2.5 and operates synergistically with the elicitor chitin. We identified an ethyl methanesulfonate-induced mutant (cytoplasmiccalcium elevation mutant) impaired in the response to CT and various other cellooligomers (n = 2-7), but not to chitooligomers (n = 4-8), in roots. The mutant contains a single nucleotide exchange in the gene encoding a poly(A) ribonuclease (AtPARN; At1g55870) that degrades the poly(A) tails of specific mRNAs. The wild-type PARN cDNA, expressed under the control of a 35S promoter, complements the mutant phenotype. Our identification of cellotriose as a novel chemical mediator casts light on the complex P. indica-plant mutualistic relationship.

Herbivore-Triggered Electrophysiological Reactions: Candidates for Systemic Signals in Higher Plants and the Challenge of Their Identification.

In stressed plants, electrophysiological reactions (elRs) are presumed to contribute to long-distance intercellular communication between distant plant parts. Because of the focus on abiotic stress-induced elRs in recent decades, biotic stress-triggered elRs have been widely ignored. It is likely that the challenge to identify the particular elR types (action potential [AP], variation potential, and system potential [SP]) was responsible for this course of action. Thus, this survey focused on insect larva feeding (Spodoptera littoralis and Manduca sexta) that triggers distant APs, variation potentials, and SPs in monocotyledonous and dicotyledonous plant species (Hordeum vulgare, Vicia faba, and Nicotiana tabacum). APs were detected only after feeding on the stem/culm, whereas SPs were observed systemically following damage to both stem/culm and leaves. This was attributed to the unequal vascular innervation of the plant and a selective electrophysiological connectivity of the plant tissue. However, striking variations in voltage patterns were detected for each elR type. Further analyses (also in Brassica napus and Cucurbita maxima) employing complementary electrophysiological approaches in response to different stimuli revealed various reasons for these voltage pattern variations: an intrinsic plasticity of elRs, a plant-specific signature of elRs, a specific influence of the applied (a)biotic trigger, the impact of the technical approach, and/or the experimental setup. As a consequence, voltage pattern variations, which are not irregular but rather common, need to be included in electrophysiological signaling analysis. Due to their widespread occurrence, systemic propagation, and respective triggers, elRs should be considered as candidates for long-distance communication in higher plants.

Filamentous sieve element proteins are able to limit phloem mass flow, but not phytoplasma spread.

In Fabaceae, dispersion of forisomes-highly ordered aggregates of sieve element proteins-in response to phytoplasma infection was proposed to limit phloem mass flow and, hence, prevent pathogen spread. In this study, the involvement of filamentous sieve element proteins in the containment of phytoplasmas was investigated in non-Fabaceae plants. Healthy and infected Arabidopsis plants lacking one or two genes related to sieve element filament formation-AtSEOR1 (At3g01680), AtSEOR2 (At3g01670), and AtPP2-A1 (At4g19840)-were analysed. TEM images revealed that phytoplasma infection induces phloem protein filament formation in both the wild-type and mutant lines. This result suggests that, in contrast to previous hypotheses, sieve element filaments can be produced independently of AtSEOR1 and AtSEOR2 genes. Filament presence was accompanied by a compensatory overexpression of sieve element protein genes in infected mutant lines in comparison with wild-type lines. No correlation was found between phloem mass flow limitation and phytoplasma titre, which suggests that sieve element proteins are involved in defence mechanisms other than mechanical limitation of the pathogen.

Aphid salivary proteases are capable of degrading sieve-tube proteins.

Sieve tubes serve as transport conduits for photo-assimilates and other resources in angiosperms and are profitable targets for piercing-sucking insects such as aphids. Sieve-tube sap also contains significant amounts of proteins with diverse functions, for example in signalling, metabolism, and defence. The identification of salivary proteases in Acyrthosiphon pisum led to the hypothesis that aphids might be able to digest these proteins and by doing so suppress plant defence and access additional nitrogen sources. Here, the scarce knowledge of proteases in aphid saliva is briefly reviewed. In order to provide a better platform for discussion, we conducted a few tests on in vitro protease activity and degradation of sieve-tube sap proteins of Cucurbita maxima by watery saliva. Inhibition of protein degradation by EDTA indicates the presence of different types of proteases (e.g. metalloproteses) in saliva of A. pisum. Proteases in the watery saliva from Macrosiphum euphorbiae and A. pisum were able to degrade the most abundant phloem protein, which is phloem protein 1. Our results provide support for the breakdown of sieve-element proteins by aphid saliva in order to suppress/neutralize the defence responses of the plant and to make proteins of sieve-tube sap accessible as a nitrogen source, as is discussed in detail. Finally, we discuss whether glycosylation of sieve-element proteins and the presence of protease inhibitors may confer partial protection against the proteolytic activity of aphid saliva.

Direct and individual analysis of stress-related phytohormone dispersion in the vascular system of Cucurbita maxima after flagellin 22 treatment.

• The stress-related phytohormones, salicylic acid (SA) and abscisic acid (ABA), and the three jasmonates, jasmonic acid (JA), cis-12-oxo-phytodienoic acid (cis-OPDA), and (+)-7-iso-jasmonoyl-L-isoleucine (JA-Ile), were investigated in phloem and xylem exudates of Cucurbita maxima. • Phloem and xylem exudates were separately collected and analysed via liquid chromatography-mass spectrometry. • We show direct evidence for all three jasmonates, ABA, and SA in both phloem and xylem exudates of C. maxima. JA and JA-Ile concentrations are higher in xylem (JA: c(xylem) ≈ 199.5 nM, c(phloem) ≈ 43.9 nM; JA-Ile: c(xylem) ≈ 7.9 nM, c(phloem) ≈ 1.6 nM), whereas ABA and SA concentrations are higher in phloem exudates (ABA: c(xylem) ≈ 37.1 nM, c(phloem) ≈ 142.6 nM; SA: c(xylem) ≈ 61.6 nM, c(phloem) ≈ 1319 nM). During bacteria-derived flagellin 22 (flg22)-triggered remote root-to-shoot signalling, phytohormone concentration changed rapidly both in phloem and xylem. • The unequal distribution of phytohormones suggests that phloem and xylem have distinct roles in defence responses. Our data shed light on systemic phytohormone signalling and help explain how plants cope with environmental challenges by lateral exchange between phloem and xylem. Our analysis is a starting point for further investigations of how phytohormones contribute to phloem- and xylem-based defence signalling.

Spread the news: systemic dissemination and local impact of Ca²âº signals along the phloem pathway.

We explored the idea of whether electropotential waves (EPWs) primarily act as vehicles for systemic spread of Ca(2+) signals. EPW-associated Ca(2+) influx may trigger generation and amplification of countless long-distance signals along the phloem pathway given the fact that gating of Ca(2+)-permeable channels is a universal response to biotic and abiotic challenges. Despite fundamental differences, both action and variation potentials are associated with a sudden Ca(2+) influx. Both EPWs probably disperse in the lateral direction, which could be of essential functional significance. A vast set of Ca(2+)-permeable channels, some of which have been localized, is required for Ca(2+)-modulated events in sieve elements. There, Ca(2+)-permeable channels are clustered and create so-called Ca(2+) hotspots, which play a pivotal role in sieve element occlusion. Occlusion mechanisms play a central part in the interaction between plants and phytopathogens (e.g. aphids or phytoplasmas) and in transient re-organization of the vascular symplasm. It is argued that Ca(2+)-triggered systemic signalling occurs in partly overlapping waves. The forefront of EPWs may be accompanied by a burst of free Ca(2+) ions and Ca(2+)-binding proteins in the sieve tube sap, with a far-reaching impact on target cells. Lateral dispersion of EPWs may induce diverse Ca(2+) influx and handling patterns (Ca(2+) signatures) in various cell types lining the sieve tubes. As a result, a variety of cascades may trigger the fabrication of signals such as phytohormones, proteins, or RNA species released into the sap stream after product-related lag times. Moreover, transient reorganization of the vascular symplasm could modify cascades in disjunct vascular cells.

Interaction of xylem and phloem during exudation and wound occlusion in Cucurbita maxima.

Collection of cucurbit exudates from cut petioles has been a powerful tool for gaining knowledge on phloem sap composition without full notion of the complex exudation mechanism. Only few publications explicitly mentioned that exudates were collected from the basal side of the cut, which exudes more copiously than the apical side. This is surprising since only exudation from the apical side is supposedly driven by phloem pressure gradients. Composition of carbohydrates and pH values at both wounding sides are equal, whereas protein concentration is higher at the basal side. Apparently, exudation is far more complex than just the delivery of phloem sap. Xylem involvement is indicated by lower protein concentrations after elimination of root pressure. Moreover, dye was sucked into xylem vessels owing to relaxation of negative pressure after cutting. The lateral water efflux from the vessels increases turgor of surrounding cells including sieve elements. Simultaneously, detached parietal proteins (PP1/PP2) induce occlusion of sieve plates and cover wound surface. If root pressure is strong enough, pure xylem sap can be collected after removal of the occlusion plug at the wound surface. The present findings provide a mechanism of sap exudation in Cucurbita maxima, in which the contribution of xylem water is integrated.

Pea Aphid (Acyrthosiphon pisum) Host Races Reduce Heat-Induced Forisome Dispersion in Vicia faba and Trifolium pratense.

Although phloem-feeding insects such as aphids can cause significant damage to plants, relatively little is known about early plant defenses against these insects. As a first line of defense, legumes can stop the phloem mass flow through a conformational change in phloem proteins known as forisomes in response to Ca2+ influx. However, specialized phloem-feeding insects might be able to suppress the conformational change of forisomes and thereby prevent sieve element occlusion. To investigate this possibility, we triggered forisome dispersion through application of a local heat stimulus to the leaf tips of pea (Pisum sativum), clover (Trifolium pratense) and broad bean (Vicia faba) plants infested with different pea aphid (Acyrthosiphon pisum) host races and monitored forisome responses. Pea aphids were able to suppress forisome dispersion, but this depended on the infesting aphid host race, the plant species, and the age of the plant. Differences in the ability of aphids to suppress forisome dispersion may be explained by differences in the composition and quantity of the aphid saliva injected into the plant. Various mechanisms of how pea aphids might suppress forisome dispersion are discussed.

The Cell Membrane of a Novel Rhizobium phaseoli Strain Is the Crucial Target for Aluminium Toxicity and Tolerance.

Soils with low pH and high aluminium (Al) contamination restrict common bean production, mainly due to adverse effects on rhizobia. We isolated a novel rhizobium strain, B3, from Kenyan soil which is more tolerant to Al stress than the widely used commercial strain CIAT899. B3 was resistant to 50 µM Al and recovered from 100 µM Al stress, while CIAT899 did not. Calcein labeling showed that less Al binds to the B3 membranes and less ATP and mScarlet-1 protein, a cytoplasmic marker, leaked out of B3 than CIAT899 cells in Al-containing media. Expression profiles showed that the primary targets of Al are genes involved in membrane biogenesis, metal ions binding and transport, carbohydrate, and amino acid metabolism and transport. The identified differentially expressed genes suggested that the intracellular γ-aminobutyric acid (GABA), glutathione (GSH), and amino acid levels, as well as the amount of the extracellular exopolysaccharide (EPS), might change during Al stress. Altered EPS levels could also influence biofilm formation. Therefore, these parameters were investigated in more detail. The GABA levels, extracellular EPS production, and biofilm formation increased, while GSH and amino acid level decreased. In conclusion, our comparative analysis identified genes that respond to Al stress in R. phaseoli. It appears that a large portion of the identified genes code for proteins stabilizing the plasma membrane. These genes might be helpful for future studies investigating the molecular basis of Al tolerance and the characterization of candidate rhizobial isolates that perform better in Al-contaminated soils than commercial strains.

CORK1, A LRR-Malectin Receptor Kinase, Is Required for Cellooligomer-Induced Responses in Arabidopsis thaliana.

Cell wall integrity (CWI) maintenance is central for plant cells. Mechanical and chemical distortions, pH changes, and breakdown products of cell wall polysaccharides activate plasma membrane-localized receptors and induce appropriate downstream responses. Microbial interactions alter or destroy the structure of the plant cell wall, connecting CWI maintenance to immune responses. Cellulose is the major polysaccharide in the primary and secondary cell wall. Its breakdown generates short-chain cellooligomers that induce Ca2+-dependent CWI responses. We show that these responses require the malectin domain-containing CELLOOLIGOMER-RECEPTOR KINASE 1 (CORK1) in Arabidopsis and are preferentially activated by cellotriose (CT). CORK1 is required for cellooligomer-induced cytoplasmic Ca2+ elevation, reactive oxygen species (ROS) production, mitogen-associated protein kinase (MAPK) activation, cellulose synthase phosphorylation, and the regulation of CWI-related genes, including those involved in biosynthesis of cell wall material, secondary metabolites and tryptophan. Phosphoproteome analyses identified early targets involved in signaling, cellulose synthesis, the endoplasmic reticulum/Golgi secretory pathway, cell wall repair and immune responses. Two conserved phenylalanine residues in the malectin domain are crucial for CORK1 function. We propose that CORK1 is required for CWI and immune responses activated by cellulose breakdown products.

Rhizobia Contribute to Salinity Tolerance in Common Beans (Phaseolus vulgaris L.).

Rhizobia are soil bacteria that induce nodule formation on leguminous plants. In the nodules, they reduce dinitrogen to ammonium that can be utilized by plants. Besides nitrogen fixation, rhizobia have other symbiotic functions in plants including phosphorus and iron mobilization and protection of the plants against various abiotic stresses including salinity. Worldwide, about 20% of cultivable and 33% of irrigation land is saline, and it is estimated that around 50% of the arable land will be saline by 2050. Salinity inhibits plant growth and development, results in senescence, and ultimately plant death. The purpose of this study was to investigate how rhizobia, isolated from Kenyan soils, relieve common beans from salinity stress. The yield loss of common bean plants, which were either not inoculated or inoculated with the commercial R. tropici rhizobia CIAT899 was reduced by 73% when the plants were exposed to 300 mM NaCl, while only 60% yield loss was observed after inoculation with a novel indigenous isolate from Kenyan soil, named S3. Expression profiles showed that genes involved in the transport of mineral ions (such as K+, Ca2+, Fe3+, PO43-, and NO3-) to the host plant, and for the synthesis and transport of osmotolerance molecules (soluble carbohydrates, amino acids, and nucleotides) are highly expressed in S3 bacteroids during salt stress than in the controls. Furthermore, genes for the synthesis and transport of glutathione and γ-aminobutyric acid were upregulated in salt-stressed and S3-inocculated common bean plants. We conclude that microbial osmolytes, mineral ions, and antioxidant molecules from rhizobia enhance salt tolerance in common beans.

So similar yet so different: The distinct contributions of extrafascicular and fascicular phloem to transport and exudation in cucumber plants.

Cucurbits have been used as phloem research models for many decades because their exudates can be accessed with ease. However, cucurbit plants possess two distinct phloem systems known as the fascicular phloem (FP) and extrafascicular phloem (EFP). Therefore, the molecular composition and function of certain exudates can be misinterpreted due to their unclear origin. To characterize the anatomy and function of the different phloem systems more clearly, we generated specific antibodies against marker proteins (PP1 homologs) allowing the clear identification of the EFP at the organ, tissue and cellular levels by immunological staining. We also used detailed microscopy to determine common and unique anatomical features of the FP and EFP sieve elements (SEs) in cucumber (Cucumis sativus). The comparison of exudation rates and the dynamic viscosity, density and sugar content of the exudates from plants grown in the light and dark revealed the consistent composition and behavior of the EFP exudate even when photosynthesis was prevented, thus differing from the properties of the FP exudate. Furthermore, the analysis of phloem transport using a fluorescein disodium salt showed only wound-induced exudation of dye from the EFP, indicating the absence of transport in this tissue. Our results show that it is important to distinguish between the EFP and FP in cucurbits, particularly their differing behaviors in response to wounding.