Root exudate of pmi tomato mutant M161 reduces AM fungal proliferation in vitro

Soluble factors released from roots of the pre-mycorrhizal infection (pmi) myc(-) tomato mutant M161 were analyzed and compared with normal wild-type released factors. Aseptic whole exudates from the M161 mutant retarded the proliferation of Glomus intraradices in vitro. When the whole exudate was further fractionated on a C18 SEPAK cartridge, the 50/70% methanol fraction showed an activity against hyphal tip growth of Gigaspora gigantea and Gl. intraradices. Preliminary characterization of the exudate suggests that the inhibitory moieties are heat labile, bind to PVPP (polyvinyl polypyrrolidone), and are not volatile. This is the first reported instance of the inhibition by a myc(-) plant being ascribed to inhibitory component(s) released in root exudate.     

Absence of carbon transfer between Medicago truncatula plants linked by a mycorrhizal network, demonstrated in an experimental microcosm

Carbon transfer between plants via a common extraradical network of arbuscular mycorrhizal (AM) fungal hyphae has been investigated abundantly, but the results remain equivocal. We studied the transfer of carbon through this fungal network, from a Medicago truncatula donor plant to a receiver (1) M. truncatula plant growing under decreased light conditions and (2) M. truncatula seedling. Autotrophic plants were grown in bicompartmented Petri plates, with their root systems physically separated, but linked by the extraradical network of Glomus intraradices. A control Myc-/Nod- M. truncatula plant was inserted in the same compartment as the receiver plant. Following labeling of the donor plant with 13CO2, 13C was recovered in the donor plant shoots and roots, in the extraradical mycelium and in the receiver plant roots. Fatty acid analysis of the receiver's roots further demonstrated 13C enrichment in the fungal-specific lipids, while almost no label was detected in the plant-specific compounds. We conclude that carbon was transferred from the donor to the receiver plant via the AM fungal network, but that the transferred carbon remained within the intraradical AM fungal structures of the receiver's root and was not transferred to the receiver's plant tissues.     

An active factor from tomato root exudates plays an important role in efficient establishment of mycorrhizal symbiosis

Root exudates play an important role in the early signal exchange between host plants and arbuscular mycorrhizal fungi. M161, a pre-mycorrhizal infection (pmi) mutant of the tomoto (Solanum lycopersicum) cultivar Micro-Tom, fails to establish normal arbuscular mycorrhizal symbioses, and produces exudates that are unable to stimulate hyphal growth and branching of Glomus intraradices. Here, we report the identification of a purified active factor (AF) that is present in the root exudates of wild-type tomato, but absent in those of M161. A complementation assay using the dual root organ culture system showed that the AF could induce fungal growth and branching at the pre-infection stage and, subsequently, the formation of viable new spores in the M161 background. Since the AF-mediated stimulation of hyphal growth and branching requires the presence of the M161 root, our data suggest that the AF is essential but not sufficient for hyphal growth and branching. We propose that the AF, which remains to be chemically determined, represents a plant signal molecule that plays an important role in the efficient establishment of mycorrhizal symbioses.     

Does the presence of arbuscular mycorrhizal fungi influence growth and nutrient uptake of a wild-type tomato cultivar and a mycorrhiza-defective mutant, cultivated with roots sharing the same soil volume?

We investigated the growth and nutrient uptake of the Lycopersicon esculentum symbiosis mycorrhiza-defective plant mutant rmc, challenged with arbuscular mycorrhiza (AM) fungal propagules, in the presence or absence of roots of the commercial wild-type tomato cv. Golden Queen (GQ). Two plants shared the middle (combi) compartment of a horizontal three-compartment split-root pot with one part of their root system; the other part was grown separately in an outer (solo) pot. Combinations of rmc and GQ plants were grown together in soil that was either mycorrhiza-free (-M) or prepared with AM fungal inoculum (+M). Surface colonization of rmc roots was strongly increased in the presence of (+M) GQ roots. AM fungal inoculation increased phosphorus uptake of GQ plants, but decreased growth and P uptake of rmc plants. Growth and P uptake of (+M) GQ plants were reduced when plants were grown in combination with rmc rather than another GQ plant. AM fungi in the (combi) compartment may have preferentially formed hyphae spreading infection rather than functioning in P uptake in (+M) GQ plants grown in combination with rmc. Surface colonization of (+M) rmc roots, in the presence of GQ roots, was probably established at the expense of carbohydrates from associated GQ plants. Possible reasons for a decreased P uptake of rmc plants in response to AM fungal inoculation are proposed.     

Genetic evidence for auxin involvement in arbuscular mycorrhiza initiation

? Formation of arbuscular mycorrhiza (AM) is controlled by a host of small, diffusible signaling molecules, including phytohormones. To test the hypothesis that the plant hormone auxin controls mycorrhiza development, we assessed mycorrhiza formation in two mutants of tomato (Solanum lycopersicum): diageotropica (dgt), an auxin-resistant mutant, and polycotyledon (pct), a mutant with hyperactive polar auxin transport. ? Mutant and wild-type (WT) roots were inoculated with spores of the AM fungus Glomus intraradices. Presymbiotic root-fungus interactions were observed in root organ culture (ROC) and internal fungal colonization was quantified both in ROC and in intact seedlings. ? In ROC, G. intraradices stimulated presymbiotic root branching in pct but not in dgt roots. pct roots stimulated production of hyphal fans indicative of appressorium formation and were colonized more rapidly than WT roots. By contrast, approaching hyphae reversed direction to grow away from cultured dgt roots and failed to colonize them. In intact seedlings, pct and dgt roots were colonized poorly, but development of hyphae, arbuscules, and vesicles was morphologically normal within roots of both mutants. ? We conclude that auxin signaling within host roots is required for the early stages of AM formation, including during presymbiotic signal exchange.     

Influence of an arbuscular mycorrhizal fungus on the interaction of a binucleate Rhizoctonia species with Myc+ and Myc- pea roots

A binucleate Rhizoctonia (BNR) species was isolated from a clay loam soil on the Epoisses experimental station of INRA, Dijon and identified as belonging to the anastomosis Group A (AG-A). The BNR was inoculated to a Myc- Pisum sativum mutant (P53, sym30 locus) and its wild-type parent (cv Frisson) in the presence or absence of the arbuscular mycorrhizal fungus Glomus mosseae. The BNR had no significant effect on plant weight. Myc+ and Myc- roots were equally susceptible towards BNR and showed no localized cellular defense responses. The presence of BNR decreased significantly the percentage of root length colonized by G. mosseae and, inversely, G. mosseae reduced the number of BNR monilioid chains formed in root epidermal cells of the two pea genotypes. The pisatin concentration was increased significantly by BNR in both Myc+ and Myc- roots and by G. mosseae in the wild-type pea plants. The highest accumulation of pisatin was observed in Myc+ roots when both fungi were present.     

水稻超短根突变体ssr1的遗传分析和基因定位

该研究从甲基磺酸乙酯(EMS)诱变的籼稻‘Kasalath’突变体库中筛选到1个根系超短的突变体,命名为ssr1(super short root 1),8d苗龄突变体的主根和不定根长度分别只有野生型的8.89%和2.29%,其不定根发生正常,但侧根的发生和伸长都受到严重抑制,且根毛也非常短。此外,ssr1植株整体矮小,株高不到野生型的一半。遗传分析结果表明,该突变性状由1对隐性核基因控制。利用图位克隆技术将SSR1基因定位在第9染色体的STS(sequence tagged site)分子标记9g7047K和9g7290K之间,物理距离约为243kb,在定位区间共发现39个预测基因,经分析其中没有已克隆的根系发育基因。对SSR1的定位为进一步克隆该基因和阐明水稻根构型的分子机理奠定了基础。     

Characterization and genetic analysis of a lettuce (Lactuca sativa L.) mutant,weary, that exhibits reduced gravitropic response in hypocotyls and inflorescence stems

A lettuce (Lactuca sativa L.) mutant that exhibits a procumbent growth habit was identified and characterized. In two wild type (WT) genetic backgrounds, segregation patterns revealed that the mutant phenotype was controlled by a recessive allele at a single locus, which was designated weary. Hypocotyls and inflorescence stems of plants homozygous for the weary allele exhibited reduced gravitropic responses compared with WT plants, but roots exhibited normal gravitropism. Microscopic analysis revealed differences in the radial distribution of amyloplasts in hypocotyl and inflorescence stem cells of weary and WT plants. Amyloplasts occurred in a single layer of endodermal cells in WT hypocotyls and inflorescence stems. By contrast, amyloplasts were observed in several layers of cortical cells in weary hypocotyls, and weary inflorescence stem cells lacked amyloplasts entirely. These results are consistent with the proposed role of sedimenting amyloplasts in shoot gravitropism of higher plants. The phenotype associated with the weary mutant is similar to that described for the Arabidopsis mutant sgr1/scr, which is defective in radial patterning and gravitropism.     

Identification of a novel genetically controlled step in mycorrhizal colonization: plant resistance to infection by fungal spores but not extra-radical hyphae

Vesicular arbuscular mycorrhizal fungi infect plants by means of both spores and vegetative hyphae at early stages of symbiosis. Using 2500 M2 fast-neutron-mutagenized seeds of the miniature tomato (Lycopersicon esculentum) cultivar, Micro-Tom, we isolated a mutant, M161, that is able to resist colonization in the presence of Glomus intraradices spores. The myc(-) phenotype of the mutant was stable for nine generations, and found to segregate as a single Mendelian recessive locus. The mutant exhibited morphological and growth-pattern characteristics similar to those of wild-type plants. Alterations of light intensity and day/night temperatures did not eliminate the myc(-) characteristic. Resistance to mycorrhizal fungal infection and colonization was also evident following inoculation with the fungi Glomus mosseae and Gigaspora margarita. Normal colonization of M161 was evident when mutant plants were grown together with arbuscular mycorrhizal-inoculated wild-type plants in the same growth medium. During evaluation of the pre-infection stages in the mutant rhizosphere, spore germination and appressoria formation of G. intraradices were lower by 45 and 70%, respectively, than the rates obtained with wild-type plants. These results reveal a novel, genetically controlled step in the arbuscular mycorrhizal colonization process, governed by at least one gene, which significantly reduces key steps in pre-mycorrhizal infection stages.     

The shoot controls zeatin riboside export from pea roots. Evidence from the branching mutant rms4

The rms4 mutant of pea ( Pisum sativum L.) was used in grafting studies and cytokinin analyses of the root xylem sap to provide evidence that, at least for pea, the shoot can modify the import of cytokinins from the root. The rms4 mutation, which confers a phenotype with increased branching in the shoot, causes a very substantial decrease (down to 40-fold less) in the concentration of zeatin riboside (ZR) in the xylem sap of the roots. Results from grafts between wild-type (WT) and rms4 plants indicate that the concentration of cytokinins in the xylem sap of the roots is determined almost entirely by the genotype of the shoot. WT scions normalize the cytokinin concentration in the sap of rms4 mutant roots, whereas mutant scions cause WT roots to behave like those of self-grafted mutant plants. The mechanism whereby rms4 shoots of pea cause a down-regulation in the export of cytokinins from the roots is unknown at this time. However, our data provide evidence that the shoot transmits a signal to the roots and thereby controls processes involved in the regulation of cytokinin biosynthesis in the root.     

Defects in root development and gravity response in the aem1 mutant of rice are associated with reduced auxin efflux

The phytohormone auxin is involved in the regulation of a variety of developmental processes. In this report, we describe how the processes of lateral root and root hair formations and root gravity response in rice are controlled by auxin. We use a rice mutant aem1 (auxin efflux mutant) because the mutant is defective in these characters. The aem1 line was originally isolated as a short lateral root mutant, but we found that the mutant has a defect in auxin efflux in roots. The acropetal and basipetal indole-3-acetic acid (IAA) transports were reduced in aem1 roots compared to wild type (WT). Furthermore, gravitropic bending as well as efflux of radioactive IAA was impaired in the mutant roots. We also propose a unique distribution of endogenous IAA in aem1 roots. An immunoassay revealed a 4-fold-endogenous IAA content in the aem1 roots compared to WT, and the application of IAA to the shoot of WT seedlings mimicked the short lateral root phenotype of aem1, suggesting that the high content of IAA in aem1 roots impaired the elongation of lateral roots. However, the high level of IAA in aem1 roots contradicts the auxin requirement for root hair formation in the epidermis of mutant roots. Since the reduced development in root hairs of aem1 roots was rescued by exogenous auxin, the auxin level in the epidermis is likely to be sub-optimum in aem1 roots. This discrepancy can be solved by the ideas that IAA level is higher in the stele and lower in the epidermis of aem1 roots compared to WT and that the unique distribution of IAA in aem1 roots is induced by the defect in auxin efflux. All these results suggest that AEM1 may encode a component of auxin efflux carrier in rice and that the defects in lateral roots, root hair formation and root gravity response in aem1 mutant are due to the altered auxin efflux in roots.     

环境因子对西藏高原草地植物丛枝菌根真菌的影响

对西藏高原不同草地类型建群种植物的研究结果表明,寄主植物根围土壤AM真菌孢子密度与菌根侵染率之间无相关性;不同海拔条件下温度、降水量等的显著变化对草地植物AM真菌的发育和侵染具有重要影响,不同草地类型、土壤质地对AM真菌的影响亦较明显;在一定范围内,孢子密度随土壤pH、有机质含量的提高分别呈显著增加(r=0.5319^*,n=20)和下降趋势(r=-0.1973,n=20),菌根侵染率与土壤pH、有机质含量间则分别呈一定程度的负相关和正相关;高磷土壤环境对AM真菌的产孢和侵染均具有不同程度的抑制作用;最适AM真菌发育和产孢的土壤pH、有机质和有效磷含量范围分别为8.0～8.7、3.8～4.8g·kg和7.8～10.1mg·kg^-1;中度特别是重度退化草地对AM真菌的繁殖和侵染均具有不利影响,适度放牧对AM真菌关键种的保持具有重要意义;AM真菌对沙生苔草、矮生嵩草和扁穗莎草根系均具有良好的侵染效应.     

Plant Responses to Drought Stress and Exogenous ABA Application are Modulated Differently by Mycorrhization in Tomato and an ABA-deficient Mutant (<Emphasis Type="Italic">Sitiens</Emphasis>)

The aims of the present study are to find out whether the effects of arbuscular mycorrhizal (AM) symbiosis on plant resistance to water deficit are mediated by the endogenous abscisic acid (ABA) content of the host plant and whether the exogenous ABA application modifies such effects. The ABA-deficient tomato mutant sitiens and its near-isogenic wild-type parental line were used. Plant development, physiology, and expression of plant genes expected to be modulated by AM symbiosis, drought, and ABA were studied. Results showed that only wild-type tomato plants responded positively to mycorrhizal inoculation, while AM symbiosis was not observed to have any effect on plant development in sitiens plants grown under well-watered conditions. The application of ABA to sitiens plants enhanced plant growth both under well-watered and drought stress conditions. In respect to sitiens plants subjected to drought stress, the addition of ABA had a cumulative effect in relation to that of inoculation with G. intraradices. Most of the genes analyzed in this study showed different regulation patterns in wild-type and sitiens plants, suggesting that their gene expression is modulated by the plant ABA phenotype. In the same way, the colonization of roots with the AM fungus G. intraradices differently regulated the expression of these genes in wild-type and in sitiens plants, which could explain the distinctive effect of the symbiosis on each plant ABA phenotype. This also suggests that the effects of the AM symbiosis on plant responses and resistance to water deficit are mediated by the plant ABA phenotype.     

AM fungi might affect the root morphology of maize by increasing indole-3-butyric acid biosynthesis

Inoculation of maize ( Zea mays L.) with the arbuscular mycorrhizal (AM) fungus Glomus intraradices resulted in a distinct root phenotype ca 10 days after inoculation. Although the fresh weight of inoculated and control roots was about the same, the AM-inoculated roots showed a significant increase in the percentage of lateral fine roots. This increase coincided with an increase in free indole-3-butyric acid (IBA) as well as an increase in IBA synthesis. At later time points (31 days after inoculation), the free IBA content was not increased in infected roots; however, the fraction of bound IBA increased compared to controls. The phenotype of mycorrhizal maize roots could be mimicked by IBA applied exogenously to non-mycorrhizal roots. Addition of trifluoro-IBA (TFIBA), an inhibitor of IBA-induced root growth and lateral root induction, simultaneously with IBA resulted in a phenotype resembling that of untreated controls. In roots treated with TFIBA the inoculation with AM fungi did not increase the formation of fine roots. The TFIBA treatment also reduced endogenous free IBA and the AM infection rate in mycorrhizal roots. The results are discussed with respect to a possible role of IBA in the establishment of AM symbiosis.     

The Lotus japonicus LjSym4 gene is required for the successful symbiotic infection of root epidermal cells

The role of the Lotus japonicus LjSym4 gene during the symbiotic interaction with Mesorhizobium loti and arbuscular mycorrhizal (AM) fungi was analyzed with two mutant alleles conferring phenotypes of different strength. Ljsym4-1 and Ljsym4-2 mutants do not form nodules with M. loti. Normal root hair curling and infection threads are not observed, while a nodC-dependent deformation of root hair tips indicates that nodulation factors are still perceived by Ljsym4 mutants. Fungal infection attempts on the mutants generally abort within the epidermis, but Ljsym4-1 mutants allow rare, successful, infection events, leading to delayed arbuscule formation. On roots of mutants homozygous for the Ljsym4-2 allele, arbuscule formation was never observed upon inoculation with either of the two AM fungi, Glomus intraradices or Gigaspora margarita. The strategy of epidermal penetration by G. margarita was identical for Ljsym4-2 mutants and the parental line, with appressoria, hyphae growing between two epidermal cells, penetration of epidermal cells through their anticlinal wall. These observations define a novel, genetically controlled step in AM colonization. Although rhizobia penetrate the tip of root hairs and AM fungi access an entry site near the base of epidermal cells, the LjSym4 gene is necessary for the appropriate response of this cell type to both microsymbionts. We propose that LjSym4 is required for the initiation or coordinated expression of the host plant cell's accommodation program, allowing the passage of both microsymbionts through the epidermis layer.