Multiple resistance phenotypes to Lettuce mosaic virus among Arabidopsis thaliana accessions

With the aim to characterize plant and viral factors involved in the molecular interactions between plants and potyviruses, a Lettuce mosaic virus (LMV)-Arabidopsis thaliana pathosystem was developed. Screening of Arabidopsis accessions with LMV isolates indicated the existence of a large variability in the outcome of the interaction, allowing the classification of Arabidopsis accessions into seven susceptibility groups. Using a reverse genetic approach, the genome-linked protein of LMV, a multifunctional protein shown to be involved in viral genome amplification and movement of potyviruses, was established as the viral determinant responsible for the ability to overcome the resistance of the Niederzenz accession to LMV-0. Preliminary genetic analyses from F2 and recombinant inbred lines available between susceptible and resistant Arabidopsis accessions revealed the existence of at least three resistance phenotypes to LMV with different genetic bases. One dominant resistance gene, designated LLM1, involved in blocking the replication or cell-to-cell movement of the LMV-0 isolate in the Columbia accession, was mapped to chromosome I and shown to be linked to the marker nga280. At the same time, genetic analyses of segregating F2 populations were consistent with the restriction of the systemic movement of the LMV-AF199 isolate in Columbia being controlled by two dominant genes and with the complete resistance to all tested LMV isolates of the Cape Verde islands (Cvi) accession being conferred by a single recessive resistance gene. Sequencing of the eukaryotic translation initiation factor 4E genes from the different LMV-resistant Arabidopsis accessions showed that these factors are not directly involved in the characterized resistance phenotypes.     

Arabidopsis RTM1 and RTM2 Genes Function in Phloem to Restrict Long-Distance Movement of Tobacco Etch Virus

Restriction of long-distance movement of tobacco etch virus (TEV) in Arabidopsis ecotype Col-0 plants requires the function of at least three genes: RTM1 (restricted TEV movement 1), RTM2, and RTM3. The mechanism of TEV movement restriction remains poorly understood, although it does not involve a hypersensitive response or systemic acquired resistance. A functional characterization of RTM1 and RTM2 was done. The RTM1 protein was found to be soluble with the potential to form self-interacting complexes. The regulatory regions of both the RTM1 and RTM2 genes were analyzed using reporter constructs. The regulatory sequences from both genes directed expression of beta-glucuronidase exclusively in phloem-associated cells. Translational fusion proteins containing the green fluorescent protein and RTM1 or RTM2 localized to sieve elements when expressed from their native regulatory sequences. Thus, components of the RTM system may function within phloem, and sieve elements in particular, to restrict TEV long-distance movement.     

Abscisic acid antagonizes ethylene-induced hyponastic growth in Arabidopsis

Ethylene induces enhanced differential growth in petioles of Arabidopsis (Arabidopsis thaliana), resulting in an upward movement of the leaf blades (hyponastic growth). The amplitude of this effect differs between accessions, with Columbia-0 (Col-0) showing a large response, while in Landsberg erecta (Ler), hyponastic growth is minimal. Abscisic acid (ABA) was found to act as an inhibitory factor of this response in both accessions, but the relationship between ethylene and ABA differed between the two; the ability of ABA to inhibit ethylene-induced hyponasty was significantly more pronounced in Col-0. Mutations in ABI1 or ABI3 induced a strong ethylene-regulated hyponastic growth in the less responsive accession Ler, while the response was abolished in the ABA-hypersensitive era1 in Col-0. Modifications in ABA levels altered petiole angles in the absence of applied ethylene, indicating that ABA influences petiole angles also independently from ethylene. A model is proposed whereby the negative effect of ABA on hyponastic growth is overcome by ethylene in Col-0 but not in Ler. However, when ABA signaling is artificially released in Ler, this regulatory mechanism is bypassed, resulting in a strong hyponastic response in this accession.     

RTM3, Which Controls Long-Distance Movement of Potyviruses,Is a Member of a New Plant Gene Family Encoding a Meprin and TRAF Homology Domain-Containing Protein

Restriction of long-distance movement of several potyviruses in Arabidopsis (Arabidopsis thaliana) is controlled by at least three dominant restricted TEV movement (RTM) genes, named RTM1, RTM2, and RTM3. RTM1 encodes a protein belonging to the jacalin family, and RTM2 encodes a protein that has similarities to small heat shock proteins. In this article, we describe the positional cloning of RTM3, which encodes a protein belonging to an undescribed protein family of 29 members that has a meprin and TRAF homology (MATH) domain in its amino-terminal region and a coiled-coil domain at its carboxy-terminal end. Involvement in the RTM resistance system is the first biological function experimentally identified for a member of this new gene family in plants. Our analyses showed that the coiled-coil domain is not only highly conserved between RTM3-homologous MATH-containing proteins but also in proteins lacking a MATH domain. The cluster organization of the RTM3 homologs in the Arabidopsis genome suggests the role of duplication events in shaping the evolutionary history of this gene family, including the possibility of deletion or duplication of one or the other domain. Protein-protein interaction experiments revealed RTM3 self-interaction as well as an RTM1-RTM3 interaction. However, no interaction has been detected involving RTM2 or the potyviral coat protein previously shown to be the determinant necessary to overcome the RTM resistance. Taken together, these observations strongly suggest the RTM proteins might form a multiprotein complex in the resistance mechanism to block the long-distance movement of potyviruses.Systemic infection of plants by viruses is the result of compatible interactions between plant and viral factors. These molecular interactions control translation and replication of the viral nucleic acid, assembly of progeny virus particles, and generalized invasion of the host through cell-to-cell and long-distance movements of viral particles or ribonucleoprotein complexes (Carrington and Whitham, 1998; Whitham and Wang, 2004). Plants have developed different mechanisms to resist viruses. Passive resistance generally results from lack of or inappropriate interactions between plant and viral factors, causing a block in one of the viral cycle steps, and is usually controlled by recessive resistance genes (Diaz-Pendon et al., 2004). Active resistance is generally triggered by the recognition of the viruses in plants and can be controlled by at least two types of mechanisms. One well-known mechanism is associated with the hypersensitive response or extreme resistance at initial infection sites and is controlled by dominant resistance genes (R genes) through a gene-for-gene relationship (Soosaar et al., 2005; Maule et al., 2007). The second mechanism concerns the general antiviral defense system of RNA interference, which recognizes and targets the viral nucleic acids (Voinnet, 2005; Maule et al., 2007).Restriction of long-distance movement of several potyviruses controlled by the dominant restricted TEV movement (RTM) genes in Arabidopsis (Arabidopsis thaliana) does not correspond to any of these known resistance mechanisms (Mahajan et al., 1998; Decroocq et al., 2006). Indeed, in this resistance process, viral replication and cell-to-cell movement in inoculated leaves appear unaffected, the hypersensitive response and systemic acquired resistance are not triggered, and salicylic acid is not involved (Mahajan et al., 1998). First identified as specific to Tobacco etch virus (TEV; Whitham et al., 2000), RTM resistance has recently been shown to be active against two other unrelated potyviruses, Lettuce mosaic virus (LMV) and Plum pox virus (PPV; Decroocq et al., 2006), showing that the RTM genes seem to be associated with a general resistance mechanism against potyviruses that blocks their long-distance movement. Whether the RTM genes are also involved in resistance mechanisms against other viral genera remains to be investigated. Genetic characterization from natural ecotype variation and chemically induced mutants has revealed that at least three dominant genes, named RTM1, RTM2, and RTM3, are involved in the restriction of long-distance movement of potyviruses in the Arabidopsis accession Columbia (Col-0; Mahajan et al., 1998; Whitham et al., 1999). Remarkably, a mutation in any of these three genes is sufficient to completely abolish the restriction of long-distance movement, suggesting that these genes act in an interdependent way to block the generalized invasion of the plant (Whitham et al., 1999). RTM1 encodes a protein belonging to the jacalin family, some members of which are involved in defense against insects and fungi (Chisholm et al., 2000). RTM2 encodes a protein that contains a transmembrane domain and has similarities to small heat shock proteins, although its expression is not heat inducible and does not contribute to thermotolerance (Whitham et al., 2000). Although it is not understood how the RTM proteins restrict long-distance movement of potyviruses, it has been shown that both RTM1 and RTM2 are expressed in phloem-associated tissues, that the corresponding proteins localize to phloem sieve elements (Chisholm et al., 2001), and that the N-terminal end of the potyviral capsid protein (CP) is involved in breaking of the RTM resistance (Decroocq et al., 2009), suggesting a direct or indirect interaction between the potyvirus CP and the RTM factors.In this article, we present the positional cloning of RTM3, which encodes a new type of protein containing a meprin and TRAF homology (MATH) domain, and show that RTM3 directly interacts with RTM1.     

Identification of quantitative trait loci controlling symptom development during viral infection in Arabidopsis thaliana

In compatible interactions between plants and viruses that result in systemic infection, symptom development is a major phenotypic trait. However, host determinants governing this trait are mostly unknown, and the mechanisms underlying it are still poorly understood. In a previous study on the Arabidopsis thaliana-Plum pox virus (PPV) pathosystem, we showed a large degree of variation in symptom development among susceptible accessions. In particular, Cvi-1 (Cape Verde islands) accumulates viral particules but remains symptomless, Col-0 (Columbia) sometimes shows weak symptoms compared with Ler (Landsberg erecta), which always shows severe symptoms. Genetic analyses of Col x Ler and Cvi x Ler F2 and recombinant inbred line (RIL) populations suggested that symptom development as well as viral accumulation traits are polygenic and quantitative. Three of the symptom quantitative trait loci (QTL) identified could be confirmed in near-isogenic lines, including PSI1 (PPV symptom induction 1), which was identified on the distal part of chromosome 1 in both RIL populations. With respect to viral accumulation, several factors have been detected and, interestingly, in the Col x Ler population, two out of three viral accumulation QTL colocalized with loci controlling symptom development, although correlation analysis showed weak linearity between symptom severity and virus accumulation. In addition, in the Cvi x Ler RIL population, a digenic recessive determinant controlling PPV infection was identified.     

ERECTA, an LRR receptor-like kinase protein controlling development pleiotropically affects resistance to bacterial wilt

Bacterial wilt, one of the most devastating bacterial diseases of plants worldwide, is caused by Ralstonia solanacearum and affects many important crop species. We show that several strains isolated from solanaceous crops in Europe are pathogenic in different accessions of Arabidopsis thaliana. One of these strains, 14.25, causes wilting symptoms in A. thaliana accession Landsberg erecta (Ler) and no apparent symptoms in accession Columbia (Col-0). Disease development and bacterial multiplication in the susceptible Ler accession depend on functional hypersensitive response and pathogenicity (hrp) genes, key elements for bacterial pathogenicity. Genetic analysis using Ler x Col-0 recombinant inbred lines showed that resistance is governed by at least three loci: QRS1 (Quantitative Resistance to R. solanacearum) and QRS2 on chromosome 2, and QRS3 on chromosome 5. These loci explain about 90% of the resistance carried by the Col-0 accession. The ERECTA gene, which encodes a leucine-rich repeat receptor-like kinase (LRR-RLK) and affects development of aerial organs, is dimorphic in our population and lies close to QRS1. Susceptible Ler plants transformed with a wild-type ERECTA gene, and the LER line showed increased disease resistance to R. solanacearum as indicated by reduced wilt symptoms and impaired bacterial growth, suggesting unexpected cross-talk between resistance and developmental pathways.     

Arabidopsis RTM2 gene is necessary for specific restriction of tobacco etch virus and encodes an unusual small heat shock-like protein

Arabidopsis plants have a system to specifically restrict the long-distance movement of tobacco etch potyvirus (TEV) without involving either hypersensitive cell death or systemic acquired resistance. At least two dominant genes, RTM1 and RTM2, are necessary for this restriction. Through a series of coinfection experiments with heterologous viruses, the RTM1/RTM2-mediated restriction was shown to be highly specific for TEV. The RTM2 gene was isolated by a map-based cloning strategy. Isolation of RTM2 was confirmed by transgenic complementation and sequence analysis of wild-type and mutant alleles. The RTM2 gene product is a multidomain protein containing an N-terminal region with high similarity to plant small heat shock proteins (HSPs). Phylogenetic analysis revealed that the RTM2 small HSP-like domain is evolutionarily distinct from each of the five known classes of plant small HSPs. Unlike most other plant genes encoding small HSPs, expression of the RTM2 gene was not induced by high temperature and did not contribute to thermotolerance of seedlings. The RTM2 gene product was also shown to contain a large C-terminal region with multiple repeating sequences.     

Analysis of natural allelic variation at seed dormancy loci of Arabidopsis thaliana

Arabidopsis accessions differ largely in their seed dormancy behavior. To understand the genetic basis of this intraspecific variation we analyzed two accessions: the laboratory strain Landsberg erecta (Ler) with low dormancy and the strong-dormancy accession Cape Verde Islands (Cvi). We used a quantitative trait loci (QTL) mapping approach to identify loci affecting the after-ripening requirement measured as the number of days of seed dry storage required to reach 50% germination. Thus, seven QTL were identified and named delay of germination (DOG) 1-7. To confirm and characterize these loci, we developed 12 near-isogenic lines carrying single and double Cvi introgression fragments in a Ler genetic background. The analysis of these lines for germination in water confirmed four QTL (DOG1, DOG2, DOG3, and DOG6) as showing large additive effects in Ler background. In addition, it was found that DOG1 and DOG3 genetically interact, the strong dormancy determined by DOG1-Cvi alleles depending on DOG3-Ler alleles. These genotypes were further characterized for seed dormancy/germination behavior in five other test conditions, including seed coat removal, gibberellins, and an abscisic acid biosynthesis inhibitor. The role of the Ler/Cvi allelic variation in affecting dormancy is discussed in the context of current knowledge of Arabidopsis germination.     

A member of a new plant gene family encoding a meprin and TRAF homology (MATH) domain-containing protein is involved in restriction of long distance movement of plant viruses

Restriction of long distance movement of several potyviruses in Arabidopsis thaliana is controlled by at least three dominant restricted TEV movement (RTM) genes, named RTM1, RTM2 and RTM3 and acts as a non-conventional resistance. RTM1 encodes a protein belonging to the jacalin family and RTM2 encodes a protein which has similarities to small heat shock proteins. The recent cloning of RTM3 which encodes a protein belonging to an unknown protein family of 29 members that has a meprin and TRAF homology (MATH) domain in its N-terminal region and a coiled-coil (CC) domain at its C-terminal end is an important breakthrough for a better understanding of this resistance process. Not only the third gene involved in this resistance has been identified and has allowed revealing a new gene family in plant but the discovery that the RTM3 protein interacts directly with RTM1 strongly suggests that the RTM proteins form a multimeric complex. However, these data also highlight striking similarities of the RTM resistance with the well known R-gene mediated resistance.Key words: plant virus, potyvirus, resistance, long distance movement, RTM genes, Arabidopsis thaliana     

Genetic characterization of five powdery mildew disease resistance loci in Arabidopsis thaliana

This paper reports on six Arabidopsis accessions that show resistance to a wild isolate of the powdery mildew pathogen, Erysiphe cichoracearum . Resistance at 7 days post-inoculation in these accessions was characterized by limited fungal growth and sporadic development of chlorotic or necrotic lesions at inoculation sites. Three accessions, Wa-1, Kas-1 and SI-0, were highly resistant, while the other accessions permitted some fungal growth and conidiation. Papilla formation was a frequent host response; however, cell death appeared to be neither a rapid nor a common response to infection. To determine the genetic basis of resistance, segregation analyses of progeny from crosses between each of the resistant accessions and Columbia ( gl1 ), which is susceptible to the powdery mildew pathogen, were performed. For all accessions except SI-0, resistance was conferred by a single locus. SI-0 was unique in that two unlinked loci controlled the disease reaction phenotype. In accessions Wa-1, Kas-1, Stw-0 and Su-0, powdery mildew resistance was encoded by a semi-dominant allele. However, susceptibility was dominant to resistance in accessions Te-0 and SI-0. Mapping studies revealed that powdery mildew resistances in Kas-1, Wa-1, Te-0, Su-0 and Stw-0 were controlled by five independent loci. This study suggests that the Arabidopsis powdery mildew disease will be a suitable model system in which to investigate powdery mildew diseases.     

New Arabidopsis recombinant inbred line populations genotyped using SNPWave and their use for mapping flowering-time quantitative trait loci

The SNPWave marker system, based on SNPs between the reference accessions Colombia-0 and Landsberg erecta (Ler), was used to distinguish a set of 92 Arabidopsis accessions from various parts of the world. In addition, we used these markers to genotype three new recombinant inbred line populations for Arabidopsis, having Ler as a common parent that was crossed with the accessions Antwerp-1, Kashmir-2, and Kondara. The benefit of using multiple populations that contain many similar markers and the fact that all markers are linked to the physical map of Arabidopsis facilitates the quantitative comparison of maps. Flowering-time variation was analyzed in the three recombinant inbred line populations. Per population, four to eight quantitative trait loci (QTL) were detected. The comparison of the QTL positions related to the physical map allowed the estimate of 12 different QTL segregating for flowering time for which Ler has an allele different from one, two, or three of the other accessions.     

Genetic and physical mapping of the RPP13 locus, in Arabidopsis, responsible for specific recognition of several Peronospora parasitica (downy mildew) isolates.

Fifteen isolates of the biotrophic oomycete Peronospora parasitica (downy mildew) were obtained from a population of Arabidopsis thaliana plants that established naturally in a garden the previous year. They exhibited phenotypic variation in a set of 12 Arabidopsis accessions that suggested that the parasite population consisted of at least six pathotypes. One isolate, Maks9, elicited an interaction phenotype of flecking necrosis and no sporulation (FN) in the Arabidopsis accession Nd-1, and more extensive pitting necrosis with no sporulation (PN) in the accession Ws-2. RPP13 was designated as the locus for a single dominant resistance gene associated with the resistance in Nd-1 and mapped to an interval of approximately 60 kb on a bacterial artificial chromosome (BAC) contig on the lower arm of chromosome 3. This locus is approximately 6 cM telomeric to RPP1, which was previously described as the locus for the PN interaction with five Peronospora isolates, including resistance to Maks9 in Ws-2. New Peronospora isolates were obtained from four other geographically distinct populations of P. parasitica. Four isolates were characterized that elicited an FN phenotype in Nd-1 and mapped resistance to the RPP13 locus. This suggests that the RPP13 locus contains either a single gene capable of multiple isolate recognition or a group of tightly linked genes. Further analysis suggests that the RPP11 gene in the accession Rld-0 may be allelic to RPP13 but results in a different recognition capability.     

The Arabidopsis ISR1 Locus is Required for Rhizobacteria-Mediated Induced Systemic Resistance Against Different Pathogens

Abstract: In Arabidopsis thaliana, non-pathogenic, root-colonizing Pseudomonas fluorescens WCS417r bacteria trigger an induced systemic resistance (ISR) that is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). In contrast to SAR, WCS417r-mediated ISR is controlled by a salicylic acid (SA)-independent signalling pathway that requires an intact response to the plant hormones jasmonic acid (JA) and ethylene (ET). Arabidopsis accessions RLD1 and Ws-0 fail to express ISR against Pseudomonas syringae pv. tomato and show enhanced disease susceptibility to this pathogen. Genetic analysis of progeny from crosses between WCS417r-responsive and non-responsive accessions demonstrated that ISR inducibility and basal resistance against P. syringae pv. tomato are controlled by a single dominant locus (ISR1) on chromosome III (Ton et al., 1999^[294]). Here, we investigated the role of the ISR1 locus in ISR, SAR and basal resistance against three additional pathogens: Xanthomonas campestris pv. armoraciae, Peronospora parasitica and turnip crinkle virus (TCV), using accessions Col-0 (ISR1), RLD1 (isr1) and Ws-0 (isr1) as host plants.     

Identification of resistance to Peanut bud necrosis virus (PBNV) in wild Arachis germplasm

Eighty three wild Arachis germplasm accessions, belonging to 24 species of five sections and one natural hybrid derivative of a cross between the cultivated and a wild Arachis species, were evaluated along with a susceptible groundnut cultivar for resistance to Peanut bud necrosis virus (PBNV) in a replicated field trial at ICRISAT, Patancheru, India. Thirty days after sowing, the percentage of infected plants were recorded for all the accessions and subsequently young leaflets from all these accessions were tested for the presence of the virus by enzyme linked immunosorbent assay (ELISA). One accession each of A. benensis and A. cardenasii, and two accessions of A. villosa, in the section Arachis, two accessions of A. appressipila in the section Procumbentes, and one accession of A. triseminata under section Triseminatae were not infected by PBNV. These seven field‐resistant accessions were tested under glasshouse conditions for virus resistance by mechanical sap inoculations. One accession of A. cardenasii and two accessions of A. villosa did not show systemic infection. Similarly, in another glasshouse test, where 13 A. cardenasii accessions of section Arachis were evaluated, two accessions did not show systemic infection. In all these resistant accessions, the inoculated leaves showed infection, but the systemic leaves did not show the presence of virus in spite of repeated mechanical sap inoculations. So, the resistance in these accessions appears to be due to a block in systemic movement of the virus. To our knowledge this is the first report on the identification of resistance to PBNV in wild Arachis species. Since both A. cardenasii and A. villosa are the progenitors of cultivated groundnut and can be hybridised with the latter, the resistant accessions are being utilised in conventional breeding programmes to transfer PBNV resistance to widely adapted groundnut cultivars.     

Effects of host plant development and genetic determinants on the long-distance movement of cauliflower mosaic virus in Arabidopsis.

During systemic infections, viruses move long distances through the plant vascular system. The long-distance movement of cauliflower mosaic virus (CaMV) in Arabidopsis has been examined using a whole plant in situ hybridization technique called plant skeleton hybridization. CaMV moves long distance through the phloem largely following the flow of photoassimilates from source to sink leaves. During the course of plant development, sink-source relationships change and the region of the plant that CaMV can invade is progressively reduced. In Arabidopsis, we have found that conditions that influence the rate of plant development dramatically impact the long-distance movement of CaMV, because under normal conditions the rate of plant development is closely matched to the kinetics of virus movement. Ecotypes and mutants of Arabidopsis that flower early show a form of resistance to systemic CaMV infection, which we call "developmental resistance." Developmental resistance results from the fact that the rosette leaves mature early in the life of an early flowering plant and become inaccessible to virus. On the other hand, if the development of early flowering plants is retarded by suboptimal growth conditions, inoculated plants appear more susceptible to the virus and systemic infections become more widespread. We have found that other Arabidopsis ecotypes, such as Enkheim-2 (En-2), show another form of resistance to virus movement that is not based on developmental or growth conditions. The virus resistance in ecotype En-2 is largely conditioned by a dominant trait at a single locus.     

VPg of tobacco etch potyvirus is a host genotype-specific determinant for long-distance movement.

The V20 cultivar of Nicotiana tabacum was shown previously to exhibit a strain-specific restriction of long-distance movement of tobacco etch potyvirus (TEV). In V20, both TEV-HAT and TEV-Oxnard strains are capable of genome amplification and cell-to-cell movement, but only TEV-Oxnard is capable of systemic infection by vasculature-dependent long-distance movement. To investigate the basis for host-specific movement of TEV, chimeric virus genomes were assembled from TEV-HAT and TEV-Oxnard. Viruses containing the TEV-Oxnard coding regions for HC-Pro and/or capsid protein (CP), two proteins that are known to be essential for TEV long-distance movement, failed to infect V20 systemically. In contrast, chimeric viruses encoding the TEV-Oxnard VPg domain of NIa were able to infect V20 systemically. The critical region controlling the infection phenotype in V20 was mapped to a 67-nucleotide segment containing 10-nucleotide differences, but only five amino acid differences, between TEV-HAT and TEV-Oxnard. In V20 coinfection experiments, a restricted strain had no effect on systemic infection by a long-distance movement-competent chimeric strain, suggesting that the restricted strain was not inducing a generalized systemic resistance response. These data suggest that the VPg domain, which is covalently attached to the 5' end of genomic RNA, interacts either directly or indirectly with host components to facilitate long-distance movement.     

Use of Arabidopsis recombinant inbred lines reveals a monogenic and a novel digenic resistance mechanism to Xanthomonas campestris pv campestris

Infiltration of the Arabidopsis thaliana accession Landsberg erecta (Ler) with Xanthomonas campestris pv campestris isolate 2D520 results in extensive necrosis and limited chlorosis within 5–6 days post-inoculation (d.p.i.), which can lead to systemic necrosis within 23 d.p.i. In contrast, the accession Columbia (Col) remains asymptomatic after infiltration. Although both accessions support bacterial growth, 5–28-fold more bacteria are present in Ler than in Col leaf tissue. Inheritance studies indicate that three independent, dominant or partially dominant, nuclear genes condition resistance to X. c. campestris 2D520. The major gene, termed RXC2, conditions monogenic resistance to X. c. campestris and was mapped to a 5.5 cM interval of chromosome V. Segregation data indicate that the locus RXC3 in conjunction with RXC4 confers digenic resistance to X. c. campestris. The combined action of RXC3 and RXC4 is correlated with a suppression of in planta bacterial levels and a suppression of symptoms relative to Ler. The RXC3 + RXC4-mediated resistance is novel in that although the Col allele of RXC4 contributes positively to resistance, it is the Ler and not the Col allele of RXC3 that contributes positively to resistance. RXC3 was mapped to the bottom arm of chromosome V in a 2.7 cM interval within the major recognition gene complex MRC-J, a cluster of genes involved in disease resistance. RXC4 was mapped to a 12 cM interval on chromosome II that also contains RXC1, a gene conferring tolerance to X. c. campestris.     

Root architecture of Arabidopsis is affected by competition with neighbouring plants

How roots detect and respond to the presence of neighbors is relevant to understand plant belowground interactions. The aim of the present work was to evaluate the effect of the presence of neighboring plants and the limited availability of phosphorus on root architecture. A target plant of Arabidopsis thaliana (Ler or Col) was surrounded by combinations of two individuals (Ler and Col), and subjected to different growth conditions (levels of activated charcoal (AC) and phosphorus). Both accessions consistently concentrated their roots towards the competition zone shared with a neighbor of the same accession, avoiding the side shared with the other accession. All these competition strategies disappeared when plants were limited by phosphorus or when activated charcoal was added to the growth media. Plants produced consistently fewer but longer lateral roots when activated charcoal was added to the growth media irrespective of the neighbors. Our results indicate a direct role of secondary metabolites present in the root exudates and phosphorus availability in the response of presence and identity of neighboring roots.     

New Arabidopsis recombinant inbred lines (Landsberg erecta x Nossen) reveal natural variation in phytochrome-mediated responses

We used 52 Arabidopsis (Arabidopsis thaliana) accessions and developed a new set of 137 recombinant inbred lines between Landsberg erecta (Ler) and Nossen (No-0) to explore the genetic basis of phytochrome-mediated responses during deetiolation. Unexpectedly, most accessions showed weak or moderate hypocotyl growth and cotyledon unfolding responses to pulses of far-red light (FR). Crosses between Columbia and No-0, two accessions with poor response, segregated seedlings with unfolded cotyledons under pulsed FR, suggesting the occurrence of accession-specific loci in the repression of morphological responses to weak light signals. Confirming the latter expectation, mapping of responses to pulsed FR in the Ler x No-0 lines identified novel loci. Despite its weak response to pulsed FR, No-0 showed a response to continuous FR stronger than that observed in Ler. By mapping the differential effect of pulsed versus continuous FR, we identified two high-irradiance response loci that account for the steeper response to continuous FR in No-0. This underscores the potential of the methodology to identify loci involved in the regulation of the shape of signal input-output relationships. Loci specific for a given phytochrome-mediated response were more frequent than pleiotropic loci. Segregation of these specific loci is predicted to yield different combinations of seedling responsivity to light. Such flexibility in combination of responses is observed among accessions and could aid in the adjustment to different microenvironments.