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At least 199 records · Page 11Linked to original sources
PubMed2002

Characterization of NADH dehydrogenases of Pseudomonas fluorescens WCS365 and their role in competitive root colonization.

The excellent-root-colonizing Pseudomonas fluorescens WCS365 was selected previously as the parental strain for the isolation of mutants impaired in root colonization. Transposon mutagenesis of WCS365 and testing for root colonization resulted in the isolation of mutant strain PCL1201, which is approximately 100-fold impaired in competitive tomato root colonization. In this manuscript, we provide evidence that shows that the lack of NADH dehydrogenase I, an enzyme of the aerobic respiratory chain encoded by the nuo operon, is responsible for the impaired root-colonization ability of PCL1201. The complete sequence of the nuo operon (ranging from nuoA to nuoN) of P. fluorescens WCS365 was identified, including the promoter region and a transcriptional terminator consensus sequence downstream of nuoN. It was shown biochemically that PCL1201 is lacking NADH dehydrogenase I activity. In addition, the presence and activity of a second NADH dehydrogenase, encoded by the ndh gene, was identified to our knowledge for the first time in the genus Pseudomonas. Since it was assumed that low-oxygen conditions were present in the rhizosphere, we analyzed the activity of the nuo and the ndh promoters at different oxygen tensions. The results showed that both promoters are up-regulated by low concentrations of oxygen and that their levels of expression vary during growth. By using lacZ as a marker, it was shown that both the nuo operon and the ndh gene are expressed in the tomato rhizosphere. In contrast to the nuo mutant PCL1201, an ndh mutant of WCS365 appeared not to be impaired in competitive root tip colonization.

Genes, Bacterial↗
PubMed1995

Genetic analysis of ethylene signal transduction in Arabidopsis thaliana: five novel mutant loci integrated into a stress response pathway.

The response of Arabidopsis thaliana etiolated seedlings to the plant hormone ethylene is a conspicuous phenotype known as the triple response. We have identified genes that are required for ethylene perception and responses by isolating mutants that fail to display a triple response in the presence of exogenous ethylene. Five new complementation groups have been identified. Four of these loci, designated ein4, ein5, ein6 and ein7, are insensitive to ethylene. The fifth complementation group, eir1, is defined by a novel class of mutants that have agravitropic and ethylene-insensitive roots. Double-mutant phenotypes have allowed the positioning of these loci in a genetic pathway for ethylene signal transduction. The ethylene-response pathway is defined by the following loci: ETR1, EIN4, CTR1, EIN2, EIN3, EIN5, EIN6, EIN7, EIR1, AUX1 and HLS1. ctr1-1 is epistatic to etr1-3 and ein4, indicating that CTR1 acts after both ETR1 and EIN4 in the ethylene-response pathway. Mutations at the EIN2, EIN3, EIN5, EIN6 and EIN7 loci are all epistatic to the ctr1 seedling phenotype. The EIR1 and AUX1 loci define a root-specific ethylene response that does not require EIN3 or EIN5 gene activity. HLS1 appears to be required for differential cell growth in the apical hook. The EIR1, AUX1 and HLS1 genes may function in the interactions between ethylene and other plant hormones that occur late in the signaling pathway of this simple gas.

Arabidopsis↗
PubMed2003

Genetic analysis of adventitious root formation with a novel series of temperature-sensitive mutants of Arabidopsis thaliana.

When cultured on media containing the plant growth regulator auxin, hypocotyl explants of Arabidopsis thaliana generate adventitious roots. As a first step to investigate the genetic basis of adventitious organogenesis in plants, we isolated nine temperature-sensitive mutants defective in various stages in the formation of adventitious roots: five root initiation defective (rid1 to rid5) mutants failed to initiate the formation of root primordia; in one root primordium defective (rpd1) mutant, the development of root primordia was arrested; three root growth defective (rgd1, rgd2, and rgd3) mutants were defective in root growth after the establishment of the root apical meristem. The temperature sensitivity of callus formation and lateral root formation revealed further distinctions between the isolated mutants. The rid1 mutant was specifically defective in the reinitiation of cell proliferation from hypocotyl explants, while the rid2 mutant was also defective in the reinitiation of cell proliferation from root explants. These two mutants also exhibited abnormalities in the formation of the root apical meristem when lateral roots were induced at the restrictive temperature. The rgd1 and rgd2 mutants were deficient in root and callus growth, whereas the rgd3 mutation specifically affected root growth. The rid5 mutant required higher auxin concentrations for rooting at the restrictive temperature, implying a deficiency in auxin signaling. The rid5 phenotype was found to result from a mutation in the MOR1/GEM1 gene encoding a microtubule-associated protein. These findings about the rid5 mutant suggest a possible function of the microtubule system in auxin response.

Arabidopsis↗
PubMed2006

Induction of lateral root structure formation on petunia roots: A novel effect of GMI1000 Ralstonia solanacearum infection impaired in Hrp mutants.

Ralstonia solanacearum is a soilborne plant pathogen that invades its host via roots. As in many gram-negative bacterial plant pathogens, the R. solanacearum Hrp type III secretion system is essential for interactions of the bacterium with plants; however, the related mechanisms involved in disease expression are largely unknown. In this work, we examined the effects of infection by R. solanacearum GMI1000 and Hrp mutants on the root system of petunia plants. Both the wild-type and mutant strains disturbed the petunia root architecture development by inhibiting lateral root elongation and provoking swelling of the root tips. In addition, GMI100 but not the Hrp mutants induced the formation of new root lateral structures (RLS). This ability is shared by other, but not all, R. solanacearum strains tested. Like lateral roots, these new structures arise from divisions of pericycle founder cells which, nevertheless, exhibit an abnormal morphology. These RLS are efficient colonization sites allowing extensive bacterial multiplication. However, they are not required for the bacterial vascular invasion that leads to the systemic spread of the bacterium through the whole plant, indicating that, instead, they might play a role in the rhizosphere-related stages of the R. solanacearum life cycle.

Bacterial Proteins↗
PubMed2004

Development of ectopic roots from abortive nodule primordia.

The symbiotic phenotype of five Tn5-induced mutants of Rhizobium etli affected in different anabolic pathways (namely, gluconeogenesis and biosynthesis of lysine, purine, or pyrimidine) was analyzed. These mutants induced, on the root of Phaseolus vulgaris, a normal early sequence of morphogenetics events, including root hair deformation and development of nodule primordia. Later on, however, from the resulting root outgrowths, instead of nodules, one or more ectopic roots (spaced closely related and agravitropic) emerged. Therefore, this group of mutant was collectively called "root inducer" (RIND). It was observed that the RIND-induced infection threads aborted early inside the invaded root hair, and that the resulting abortive nodules lack induction of late nodulin genes. Moreover, experiments performed using a conditional mutant (a methionine-requiring invader) revealed that bacterial invasion plays a key role in the maintenance of the program of nodule development and, in particular, in the differentiation of the most specific symbiotic tissue of globose nodules, the central tissue. These data indicate that, in P. vulgaris, the nodule primordium is a root-specified pro-meristematic tissue.

Base Sequence↗
PubMed1997

Structural and genetic analysis of epidermal cell differentiation in Arabidopsis primary roots.

In a screen designed to identify genes in the specification of epidermal cell fate in Arabidopsis primary roots we have isolated 8 new mutants that fall into 6 complementation groups corresponding to the 'root hairless' genes RHL1, RHL2 and RHL3 and the 'ectopic root hair' genes ERH1, ERH2 and ERH3. The erh2 mutant is allelic to pom1, a conditional root expansion mutant, and reveals a possible link between epidermal root hair initiation and radial cell expansion. Apart from erh1 the mutants also show defects in shoot development, indicating a complex role for the affected genes. Mutant phenotypes in the patterning and shape of leaf trichomes in rhl1, rhl2, rhl3 and erh3 were particularly obvious. The root hairless mutants are only partly responsive to increased ethylene concentrations, while the ectopic root hair mutants are fully responsive to reduced concentrations of ethylene, a permissive regulator of root hair initiation. This result and the analysis of double mutants suggest a complex pathway leading to root hair initiation that requires the RHL and ERH genes for correct differentiation.

Arabidopsis↗
PubMed1995

A cadmium-sensitive, glutathione-deficient mutant of Arabidopsis thaliana.

The roots of the cadmium-sensitive mutant of Arabidopsis thaliana, cad1-1, become brown in the presence of cadmium. A new cadmium-sensitive mutant affected at a second locus, cad2, has been identified using this phenotype. Genetic analysis has grown that the sensitive phenotype is recessive to the wild type and segregates as a single Mendelian locus. Assays of cadmium accumulation by intact plants indicated that the mutant is deficient in its ability to sequester cadmium. Undifferentiated callus tissue was also cadmium sensitive, suggesting that the mutant phenotype is expressed at the cellular level. The level of cadmium-binding complexes formed in vivo was decreased compared with the wild type and accumulation of phytochelatins was about 10% of that in the wild type. The level of glutathione, the substrate for phytochelatin biosynthesis, in tissues of the mutant was decreased to about 15 to 30% of that in the wild type. Thus, the deficiency in phytochelatin biosynthesis can be explained by a deficiency in glutathione.

Arabidopsis↗
PubMed2006

Molecular cloning and characterization of beta-expansin gene related to root hair formation in barley.

Root hairs are specialized epidermal cells that play a role in the uptake of water and nutrients from the rhizosphere and serve as a site of interaction with soil microorganisms. The process of root hair formation is well characterized in Arabidopsis (Arabidopsis thaliana); however, there is a very little information about the genetic and molecular basis of root hair development in monocots. Here, we report on isolation and cloning of the beta-expansin (EXPB) gene HvEXPB1, tightly related to root hair initiation in barley (Hordeum vulgare). Using root transcriptome differentiation in the wild-type/root-hairless mutant system, a cDNA fragment present in roots of wild-type plants only was identified. After cloning of full-length cDNA and genomic sequences flanking the identified fragment, the subsequent bioinformatics analyses revealed homology of the protein coded by the identified gene to the EXPB family. Reverse transcription-PCR showed that expression of HvEXPB1 cosegregated with the root hair phenotype in F2 progeny of the cross between the hairless mutant rhl1.a and the wild-type Karat parent variety. Expression of the HvEXPB1 gene was root specific; it was expressed in roots of wild-type forms, but not in coleoptiles, leaves, tillers, and spikes. The identified gene was active in roots of two other analyzed root hair mutants: rhp1.a developing root hair primordia only and rhs1.a with very short root hairs. Contrary to this, a complete lack of HvEXPB1 expression was observed in roots of the spontaneous root-hairless mutant bald root barley. All these observations suggest a role of the HvEXPB1 gene in the process of root hair formation in barley.

Base Sequence↗
PubMed2002

The novel symbiotic phenotype of enhanced-nodulating mutant of Lotus japonicus: astray mutant is an early nodulating mutant with wider nodulation zone.

We have isolated a novel enhanced-nodulating mutant astray (Ljsym77) from Lotus japonicus. The name astray derives from the non-symbiotic phenotype of this mutant, agravitropic lateral roots that go "astray" against gravity. In this report we evaluated the symbiotic aspects of this mutant in detail. The astray mutant developed approximately twice the number of nodules on a wider area of roots compared with the wild type. Furthermore, the astray mutant demonstrated early initiation of nodule development, which is an unprecedented symbiotic phenotype. The astray seedlings showed normal sensitivity to the general inhibitors of nodulation such as ethylene and nitrate. These results indicate that the astray mutant is distinct from the hypernodulating mutants reported previously, and that the ASTRAY gene acts as an early and negative regulator in the cascade of nodule development.

Amino Acids, Cyclic↗
PubMed2007

Functional ecology of a blue light photoreceptor: effects of phototropin-1 on root growth enhance drought tolerance in Arabidopsis thaliana.

* The blue light photoreceptor phototropin-1 has been shown to enhance fitness in Arabidosis thaliana under field conditions. Here, we ask whether performance consequences of phototropin-1 reflect its impact on root growth and drought tolerance. * We used a PHOT1-GFP gene construct to test whether phototropin-1 abundance in roots is highest at shallow soil depths where light penetration is greatest. We then compared root growth efficiency and size at maturity between individuals with and without functional phototropin-1. Comparisons were made under wet and dry conditions to assess the impact of phototropin-1 on drought tolerance. * Phototropin-1 was most abundant in upper root regions and its impact on root growth efficiency decreased with soil depth. Roots of plants with functional phototropin-1 made fewer random turns and traveled further for a given length (higher efficiency) than roots of phot1 mutants. In dry (but not wet) soil, enhancement of root growth efficiency by phototropin-1 increased plant size at maturity. * Results indicate that phototropin-1 enhances performance under drought by mediating plastic increases in root growth efficiency near the soil surface.

Arabidopsis↗
PubMed1989

Gravitropism in a starchless mutant of Arabidopsis: implications for the starch-statolith theory of gravity sensing.

The starch-statolith theory of gravity reception has been tested with a mutant of Arabidopsis thaliana (L.) Heynh. which, lacking plastid phosphoglucomutase (EC 2.7.5.1) activity, does not synthesize starch. The hypocotyls and seedling roots of the mutant were examined by light and electron microscopy to confirm that they did not contain starch. In upright wild-type (WT) seedlings, starch-filled plastids in the starch sheath of the hypocotyl and in three of the five columellar layers of the root cap were piled on the cell floors, and sedimented to the ceilings when the plants were inverted. However, starchless plastids of the mutant were not significantly sedimented in these cells in either upright or inverted seedlings. Gravitropism of light-grown seedling roots was vigorous: e.g., 10 degrees curvature developed in mutants rotated on a clinostat following a 5 min induction at 1 g, compared with 14 degrees in the WT. Curvatures induced during intervals from 2.5 to 30 min were 70% as great in the mutant as the WT. Thus under these conditions the presence of starch and the sedimentation of plastids are unnecessary for reception of gravity by Arabidopsis roots. Gravitropism by hypocotyls of light-grown seedlings was less vigorous than that by roots, but the mutant hypocotyls exhibited an average of 70-80% as much curvature as the WT. Roots and hypocotyls of etiolated seedlings and flower stalks of mature plants were also gravitropic, although in these cases the mutant was generally less closely comparable to the WT. Thus, starch is also unnecessary for gravity reception in these tissues.

Arabidopsis↗
PubMed1998

The HOBBIT gene is required for formation of the root meristem in the Arabidopsis embryo.

In Arabidopsis, the root meristem originates from the hypophyseal cell and from an adjoining cell tier that is distinct at the heart stage of embryogenesis. We have analysed mutations in the HOBBIT (HBT) gene that is essential for root meristem formation. hbt embryos display incorrect hypophyseal cell development from the quadrant stage onward. At the heart stage, the adjoining cell tier of hbt embryos develops abnormally, in that the activation of cell division and the formation of a lateral root cap layer are disturbed. Strong hbt mutants give rise to seedlings that lack an anatomically recognisable quiescent centre and differentiated columella root cap cells, the cell types derived from the wild-type hypophysis. Furthermore, they have no mitotically active root meristem and lack a differentiated lateral root cap. Secondary roots of hbt mutants and roots obtained from cultured cells of hbt mutants have similar defects. Therefore the HBT gene is required for root meristem formation in different developmental contexts.

Arabidopsis↗
PubMed1996

Arabidopsis mutants with increased sensitivity to aluminum.

Al-sensitive (als) mutants of Arabidopsis were isolated and characterized with the aim of defining mechanisms of Al toxicity and resistance. Most als mutants selected on the basis of root growth sensitivity to Al were recessive, and together the mutants constituted eight complementation groups. Also, in most als mutants, Al sensitivity appeared to be specific for Al relative to La (another trivalent cation), except als2, which was more sensitive to La than wild type. The tendency of roots on mutant seedlings to accumulate Al was examined by staining with morin and hematoxylin, dyes used to indicate the presence of Al. A significant increase in morin staining was observed in als5, consistent with its increased sensitivity to Al. Unexpectedly, als7 and als4 showed less morin staining, suggesting that the roots on these mutants accumulate less Al than wild type seedlings after exposure to Al-containing solutions. Roots of wild-type seedlings produce callose in response to AlCl3 concentrations that inhibit root growth. Only als5 accumulated more callose than wild type in response to low levels (25 mu M) of AICI3 However, als4 and als7 did not accumulate callose at this AlCl3 concentration even though root growth was significantly inhibited. The lack of callose accumulation in als4 and als7 suggests that there is not an obligatory relationship between callose deposition and Al-induced inhibition of root growth.

Aluminum↗
PubMed2006

An improved grafting technique for mature Arabidopsis plants demonstrates long-distance shoot-to-root transport of phytochelatins in Arabidopsis.

Phytochelatins (PCs) are peptides that function in heavy-metal chelation and detoxification in plants and fungi. A recent study showed that PCs have the ability to undergo long-distance transport in a root-to-shoot direction in transgenic Arabidopsis (Arabidopsis thaliana). To determine whether long-distance transport of PCs can occur in the opposite direction, from shoots to roots, the wheat (Triticum aestivum) PC synthase (TaPCS1) gene was expressed under the control of a shoot-specific promoter (CAB2) in an Arabidopsis PC-deficient mutant, cad1-3 (CAB2TaPCS1/cad1-3). Analyses demonstrated that TaPCS1 is expressed only in shoots and that CAB2TaPCS1/cad1-3 lines complement the cadmium (Cd) and arsenic metal sensitivity of cad1-3 shoots. CAB2TaPCS1/cad1-3 plants exhibited higher Cd accumulation in roots and lower Cd accumulation in shoots compared to wild type. Fluorescence HPLC coupled to mass spectrometry analyses directly detected PC2 in the roots of CAB2:TaPCS1/cad1-3 but not in cad1-3 controls, suggesting that PC2 is transported over long distances in the shoot-to-root direction. In addition, wild-type shoot tissues were grafted onto PC synthase cad1-3 atpcs2-1 double loss-of-function mutant root tissues. An Arabidopsis grafting technique for mature plants was modified to obtain an 84% success rate, significantly greater than a previous rate of approximately 11%. Fluorescence HPLC-mass spectrometry showed the presence of PC2, PC3, and PC4 in the root tissue of grafts between wild-type shoots and cad1-3 atpcs2-1 double-mutant roots, demonstrating that PCs are transported over long distances from shoots to roots in Arabidopsis.

Arabidopsis↗
PubMed2006

The dctA gene of Pseudomonas chlororaphis O6 is under RpoN control and is required for effective root colonization and induction of systemic resistance.

Transcription from the dctA gene, which encodes an organic acid transporter in the root-colonizing bacterium Pseudomonas chlororaphis O6, is under complex regulatory control. Promoter sequence analysis revealed an RpoN binding site. The regulation of transcript accumulation by the level of ammonium ions in the growth medium confirmed RpoN regulation, even in the presence of glucose. A dctA mutant colonized tobacco roots to a lesser extent than the wild-type mutant during early seedling development. Colonization by the dctA mutant, as compared to the wild type, also reduced the level of systemically induced resistance against the soft rot pathogen Erwinia carotovora SCC1. We ascribe this reduced colonization to the inability of the mutant to utilize certain organic acid components in the root exudates. The dctA mutant failed to grow on succinate and fumarate, and showed reduced growth on malate. All altered properties of the mutant were complemented by the full-length dctA gene. We propose that organic acids in root exudates may provide important nutrient sources for the beneficial root-colonizing pseudomonad.

Ammonium Chloride↗
PubMed2006

Opposite root growth phenotypes of hy5 versus hy5 hyh mutants correlate with increased constitutive auxin signaling.

The Arabidopsis transcription factor HY5 controls light-induced gene expression downstream of photoreceptors and plays an important role in the switch of seedling shoots from dark-adapted to light-adapted development. In addition, HY5 has been implicated in plant hormone signaling, accounting for the accelerated root system growth phenotype of hy5 mutants. Mutants in the close HY5 homolog HYH resemble wild-type, despite the largely similar expression patterns and levels of HY5 and HYH, and the functional equivalence of the respective proteins. Moreover, the relative contribution of HYH to the overall activity of the gene pair is increased by an alternative HYH transcript, which encodes a stabilized protein. Consistent with the enhanced root system growth observed in hy5 loss-of-function mutants, constitutively overexpressed alternative HYH inhibits root system growth. Paradoxically, however, in double mutants carrying hy5 and hyh null alleles, the hy5 root growth phenotype is suppressed rather than enhanced. Even more surprisingly, compared to wild-type, root system growth is diminished in hy5 hyh double mutants. In addition, the double mutants display novel shoot phenotypes that are absent from either single mutant. These include cotyledon fusions and defective vasculature, which are typical for mutants in genes involved in the transcriptional response to the plant hormone auxin. Indeed, many auxin-responsive and auxin signaling genes are misexpressed in hy5 mutants, and at a higher number and magnitude in hy5 hyh mutants. Therefore, auxin-induced transcription is constitutively activated at different levels in the two mutant backgrounds. Our data support the hypothesis that the opposite root system phenotypes of hy5 single and hy5 hyh double mutants represent the morphological response to a quantitative gradient in the same molecular process, that is gradually increased constitutive auxin signaling. The data also suggest that HY5 and HYH are important negative regulators of auxin signaling amplitude in embryogenesis and seedling development.

Alternative Splicing↗
PubMed2005

Arabidopsis cytokinin receptor mutants reveal functions in shoot growth, leaf senescence, seed size, germination, root development, and cytokinin metabolism.

We used loss-of-function mutants to study three Arabidopsis thaliana sensor histidine kinases, AHK2, AHK3, and CRE1/AHK4, known to be cytokinin receptors. Mutant seeds had more rapid germination, reduced requirement for light, and decreased far-red light sensitivity, unraveling cytokinin functions in seed germination control. Triple mutant seeds were more than twice as large as wild-type seeds. Genetic analysis indicated a cytokinin-dependent endospermal and/or maternal control of embryo size. Unchanged red light sensitivity of mutant hypocotyl elongation suggests that previously reported modulation of red light signaling by A-type response regulators may not depend on cytokinin. Combined loss of AHK2 and AHK3 led to the most prominent changes during vegetative development. Leaves of ahk2 ahk3 mutants formed fewer cells, had reduced chlorophyll content, and lacked the cytokinin-dependent inhibition of dark-induced chlorophyll loss, indicating a prominent role of AHK2 and, particularly, AHK3 in the control of leaf development. ahk2 ahk3 double mutants developed a strongly enhanced root system through faster growth of the primary root and, more importantly, increased branching. This result supports a negative regulatory role for cytokinin in root growth regulation. Increased cytokinin content of receptor mutants indicates a homeostatic control of steady state cytokinin levels through signaling. Together, the analyses reveal partially redundant functions of the cytokinin receptors and prominent roles for the AHK2/AHK3 receptor combination in quantitative control of organ growth in plants, with opposite regulatory functions in roots and shoots.

Alleles↗
PubMed2002

Root system architecture determines fitness in an Arabidopsis mutant in competition for immobile phosphate ions but not for nitrate ions.

Plant root systems often have complex branching patterns. Models indicate that a complex architecture is only required for the acquisition of immobile resources, such as phosphate; mobile ions, notably nitrate, can be effectively taken up by very restricted root systems. We have tested this prediction using the axr4 mutation of Arabidopsis thaliana, the principal phenotypic effect of which is to reduce the number of lateral roots. Arabidopsis thaliana is not a host for mycorrhizal fungi and so acquires all its nutrients through the root system. In both a pot experiment and a field experiment conducted under natural conditions for A. thaliana, we found that only phosphate, and not nitrate, affected the fitness of the mutant relative to the isogenic wild-type line, Columbia. These results confirm model predictions and have implications both for the evolution of complex root systems and for the design of efficient root systems for crops.

Arabidopsis↗