Fiji disease resistance in sugarcane: Relationship to cultivar preference in field populations of the planthopper vector Perkinsiella saccharicida

Populations of the planthopper vector Perkinsiella saccharicida on sugarcane cultivars resistant (cvs Q110 and Q87), moderately resistant (cvs Q90 and Q124) and susceptible (evs NCo310 and Q 102) to Fiji disease with known field resistance scores were monitored on the plant (2000-2001) and ratoon (2001-2002) crops. In both crops, the vector population remained very low, reaching its peak in the autumn. The vector population was significantly higher on cultivars susceptible to Fiji disease than on cultivars moderately resistant and resistant to Fiji disease. The number of R saccharicida adults, nymphs and oviposition sites per plant increased with the increase in the Fiji disease susceptibility. The results suggest that under low vector density, cultivar preference by the planthopper vector mediates Fiji disease resistance in sugarcane. To obtain resistance ratings in the glasshouse that reflect field resistance, glasshouse-screening trials should be conducted under both low and high vector densities, and the cultivar preference of the planthopper vector recorded along with Fiji disease incidence.

Elicitors of induced disease resistance in postharvest horticultural crops: a brief review

Increasing loss of conventional fungicides due to pathogen resistance and general unacceptability in terms of public and environmental risk have favoured the introduction of integrated pest management (IPM) programmes. Induction of natural disease resistance (NDR) in harvested horticultural crops using physical, biological and/or chemical elicitors has received increasing attention over recent years, it being considered a preferred strategy for disease management. This article reviews the enhancement of constitutive and inducible antifungal compounds and suppression of postharvest diseases through using elicitors. The effect of timing of pre- and/or postharvest elicitor treatment and environment on the degree of elicitation and the potential for inducing local acquired resistance, systemic acquired resistance and/or induced systemic resistance to reduce postharvest disease is discussed. The review highlights that more applied and basic research is required to understand the role that induced NDR can play in achieving practical suppression of postharvest diseases as part of an IPM approach. (C) 2003 Elsevier B.V. All rights reserved.

Antagonistic interaction between abscisic acid and jasmonate-ethylene signaling pathways modulates defense gene expression and disease resistance in Arabidopsis

The plant hormones abscisic acid (ABA), jasmonic acid (JA), and ethylene are involved in diverse plant processes, including the regulation of gene expression during adaptive responses to abiotic and biotic stresses. Previously, ABA has been implicated in enhancing disease susceptibility in various plant species, but currently very little is known about the molecular mechanisms underlying this phenomenon. In this study, we obtained evidence that a complex interplay between ABA and JA-ethylene signaling pathways regulate plant defense gene expression and disease resistance. First, we showed that exogenous ABA suppressed both basal and JA-ethylene-activated transcription from defense genes. By contrast, ABA deficiency as conditioned by the mutations in the ABA1 and ABA2 genes, which encode enzymes involved in ABA biosynthesis, resulted in upregulation of basal and induced transcription from JA-ethylene responsive defense genes. Second, we found that disruption of AtMYC2 (allelic to JASMONATE INSENSITIVE1 [JIN1]), encoding a basic helix-loop-helix Leu zipper transcription factor, which is a positive regulator of ABA signaling, results in elevated levels of basal and activated transcription from JA-ethylene responsive defense genes. Furthermore, the jin1/myc2 and aba2-1 mutants showed increased resistance to the necrotrophic fungal pathogen Fusarium oxysporum. Finally, using ethylene and ABA signaling mutants, we showed that interaction between ABA and ethylene signaling is mutually antagonistic in vegetative tissues. Collectively, our results indicate that the antagonistic interactions between multiple components of ABA and the JA-ethylene signaling pathways modulate defense and stress responsive gene expression in response to biotic and abiotic stresses.

Repressor- and activator-type ethylene response factors functioning in jasmonate signaling and disease resistance identified via a genome-wide screen of Arabidopsis transcription factor gene expression

To identify transcription factors (TFs) involved in jasmonate (JA) signaling and plant defense, we screened 1,534 Arabidopsis (Arabidopsis thaliana) TFs by real-time quantitative reverse transcription-PCR for their altered transcript at 6 h following either methyl JA treatment or inoculation with the incompatible pathogen Alternaria brassicicola. We identified 134 TFs that showed a significant change in expression, including many APETALA2/ethylene response factor (AP2/ERF), MYB, WRKY, and NACTF genes with unknown functions. Twenty TF genes were induced by both the pathogen and methyl JA and these included 10 members of the AP2/ERF TF family, primarily from the B1a and B3 subclusters. Functional analysis of the B1a TF AtERF4 revealed that AtERF4 acts as a novel negative regulator of JA-responsive defense gene expression and resistance to the necrotrophic fungal pathogen Fusarium oxysporum and antagonizes JA inhibition of root elongation. In contrast, functional analysis of the B3 TF AtERF2 showed that AtERF2 is a positive regulator of JA-responsive defense genes and resistance to F. oxysporum and enhances JA inhibition of root elongation. Our results suggest that plants coordinately express multiple repressor-and activator-type AP2/ERFs during pathogen challenge to modulate defense gene expression and disease resistance.

Pathogen-responsive expression of a putative ATP-binding cassette transporter gene conferring resistance to the diterpenoid sclareol is regulated by multiple defense signaling pathways in arabidopsis

The ATP-binding cassette (ABC) transporters are encoded by large gene families in plants. Although these proteins are potentially involved in a number of diverse plant processes, currently, very little is known about their actual functions. In this paper, through a cDNA microarray screening of anonymous cDNA clones from a subtractive library, we identified an Arabidopsis gene (AtPDR12) putatively encoding a member of the pleiotropic drug resistance (PDR) subfamily of ABC transporters. AtPDR12 displayed distinct induction profiles after inoculation of plants with compatible and incompatible fungal pathogens and treatments with salicylic acid, ethylene, or methyl jasmonate. Analysis of AtPDR12 expression in a number of Arabidopsis defense signaling mutants further revealed that salicylic acid accumulation, NPR1. function, and sensitivity to jasmonates and ethylene were all required for pathogen-responsive expression of AtPDR12. Germination assays using seeds from an AtPDR12 insertion line in the presence of sclareol resulted in lower germination rates and much stronger inhibition of root elongation in the AtPDR12 insertion line than in wild-type plants. These results suggest that AtPDR12 may be functionally related to the previously identified ABC transporters SpTUR2 and NpABC1, which transport sclareol. Our data also point to a potential role for terpenoids in the Arabidopsis defensive armory.

Homologues of the maize rust resistance gene Rp1-D are genetically associated with a major rust resistance QTL in sorghum

As part of a comparative mapping study between sugarcane and sorghum, a sugarcane cDNA clone with homology to the maize Rp1-D rust resistance gene was mapped in sorghum. The cDNA probe hybridised to multiple loci, including one on sorghum linkage group (LG) E in a region where a major rust resistance QTL had been previously mapped. Partial sorghum Rp1-D homologues were isolated from genomic DNA of rust-resistant and -susceptible progeny selected from a sorghum mapping population. Sequencing of the Rp1-D homologues revealed five discrete sequence classes: three from resistant progeny and two from susceptible progeny. PCR primers specific to each sequence class were used to amplify products from the progeny and confirmed that the five sequence classes mapped to the same locus on LG E. Cluster analysis of these sorghum sequences and available sugarcane, maize and sorghum Rp1-D homologue sequences showed that the maize Rp1-D sequence and the partial sugarcane Rp1-D homologue were clustered with one of the sorghum resistant progeny sequence classes, while previously published sorghum Rp1-D homologue sequences clustered with the susceptible progeny sequence classes. Full-length sequence information was obtained for one member of a resistant progeny sequence class (Rp1-SO) and compared with the maize Rp1-D sequence and a previously identified sorghum Rp1 homologue (Rph1-2). There was considerable similarity between the two sorghum sequences and less similarity between the sorghum and maize sequences. These results suggest a conservation of function and gene sequence homology at the Rp1 loci of maize and sorghum and provide a basis for convenient PCR-based screening tools for putative rust resistance alleles in sorghum.

Fine mapping of the tomato I-3 gene for fusarium wilt resistance and elimination of a co-segregating resistance gene analogue as a candidate for I-3

The I-3 gene from the wild tomato species Lycopersicon pennellii confers resistance to race 3 of the devastating vascular wilt pathogen Fusarium oxysporum f. sp. lycopersici. As an initial step in a positional cloning strategy for the isolation of I-3, we converted restriction fragment length polymorphism and conserved orthologue set markers, known genes and a resistance gene analogue (RGA) mapping to the I-3 region into PCR-based sequence characterised amplified region (SCAR) and cleaved amplified polymorphic sequence (CAPS) markers. Additional PCR-based markers in the I-3 region were generated using the randomly amplified DNA fingerprinting (RAF) technique. SCAR, CAPS and RAF markers were used for high-resolution mapping around the I-3 locus. The I-3 gene was localised to a 0.3-cM region containing a RAF marker, eO6, and an RGA, RGA332. RGA332 was cloned and found to correspond to a putative pseudogene with at least two loss-of-function mutations. The predicted pseudogene belongs to the Toll interleukin-1 receptor-nucleotide-binding site-leucine-rich-repeat sub-class of plant disease resistance genes. Despite the presence of two RGA332 homologues in L. esculentum, DNA gel blot and PCR analysis suggests that no other homologues are present in lines carrying I-3 that could be alternative candidates for the gene.

A genetic linkage map in autotetraploid lucerne adapted to northern Australia, and use of the map to identify DNA markers linked to resistance to Phytophthora medicaginis

Phytophthora root rot, caused by Phytophthora medicaginis, is a major limitation to lucerne ( Medicago sativa L.) production in Australia and North America. Quantitative trait loci (QTLs) involved in resistance to P. medicaginis were identified in a lucerne backcross population of 120 individuals. A genetic linkage map was constructed for tetraploid lucerne using 50 RAPD ( randomly amplified polymorphic DNA), 104 AFLP (amplified fragment length polymorphism) markers, and one SSR ( simple sequence repeat or microsatellite) marker, which originated from the resistant parent (W116); 13 markers remain unlinked. The linkage map contains 18 linkage groups covering 2136.5 cM, with an average distance of 15.0 cM between markers. Four of the linkage groups contained only either 2 or 3 markers. Using duplex markers and repulsion phase linkages the map condensed to 7 homology groups and 2 unassigned linkage groups. Three regions located on linkage groups 2, 14, and 18, were identified as associated with root reaction and the QTLs explained 6 - 15% of the phenotypic variation. The research also indicates that different resistance QTLs are involved in conferring resistance in different organs. Two QTLs were identified as associated with disease resistance expressed after inoculation of detached leaves. The marker, W11-2 on group 18, identified as associated with root reaction, contributed 7% of the phenotypic variation in leaf response in our population. This marker appears to be linked to a QTL encoding a resistance factor contributing to both root and leaf reaction. One other QTL, not identified as associated with root reaction, was positioned on group 1 and contributed to 6% of the variation. This genetic linkage map provides an entry point for future molecular-based improvement of lucerne in Australia, and markers linked to the QTLs we have reported should be useful for marker-assisted selection for partial resistance to P. medicaginis in lucerne.

Resistance gene analogues in sugarcane and sorghum and their association with quantitative trait loci for rust resistance

Fifty-four different sugarcane resistance gene analogue (RGA) sequences were isolated, characterized, and used to identify molecular markers linked to major disease-resistance loci in sugarcane. Ten RGAs were identified from a sugarcane stem expressed sequence tag (EST) library; the remaining 44 were isolated from sugarcane stem, leaf, and root tissue using primers designed to conserved RGA motifs. The map location of 31 of the RGAs was determined in sugarcane and compared with the location of quantitative trait loci (QTL) for brown rust resistance. After 2 years of phenotyping, 3 RGAs were shown to generate markers that were significantly associated with resistance to this disease. To assist in the understanding of the complex genetic structure of sugarcane, 17 of the 31 RGAs were also mapped in sorghum. Comparative mapping between sugarcane and sorghum revealed syntenic localization of several RGA clusters. The 3 brown rust associated RGAs were shown to map to the same linkage group (LG) in sorghum with 2 mapping to one region and the third to a region previously shown to contain a major rust-resistance QTL in sorghum. These results illustrate the value of using RGAs for the identification of markers linked to disease resistance loci and the value of simultaneous mapping in sugarcane and sorghum.

Analysing diversity in sugarcane resistance gene analogues

As resistance genes have been shown to contain conserved motifs and cluster in many plant genomes, the identification of resistance gene analogues can be used as a strategy for both the discovery of DNA markers linked to disease resistance loci and the map-based cloning of disease resistance genes. Sugarcane suffers from many important diseases and an analysis of resistance gene analogues offers a means to identify DNA markers linked to resistance loci. However, sugarcane has the most complex genome of any crop plant and initially it is important to understand the extent of resistance gene analogue diversity in the sugarcane genome before genetic analysis. We review herein how more than 100 expressed sequence tags with homology to different resistance genes have been identified in sugarcane with many mapped as single-dose restriction fragment length polymorphism markers. Importantly, some of these resistance gene analogues have been shown to be linked to disease resistance genes or disease quantitative trait loci. In an attempt to more efficiently analyse additional resistance gene analogues in sugarcane, we report on experiments aimed at investigating the molecular diversity of several resistance gene analogue families using a modified form of a technique termed Ecotilling. Using Ecotilling, we were able to rapidly detect single nucleotide polymorphisms in fragments amplified by PCR from four different resistance gene analogue families, SoRP1D, SoPTO, SoXa21 and SoHs1pro-1. An analysis of a diverse set of sugarcane varieties, including modern sugarcane cultivars and several S. officinarum and S. spontaneum clones, indicated that all amplicons, apart from SoHs1pro-1, contained significant polymorphism within the gene region studied. However, a comparison among these sugarcane clones, including between the parents of two sugarcane mapping populations, indicated that most polymorphisms were multi-dose, not single-dose, preventing their genetic map location or association with disease susceptibility or resistance from being determined.

Mapping the I-3 gene for resistance to Fusarium wilt in tomato: application of an I-3 marker in tomato improvement and progress towards the cloning of I-3

Fusarium wilt of tomato, caused by the fungal pathogen, Fusarium oxysporum f. sp. lycopersici (Fol), is an economically damaging disease that results in huge losses in Australia and other countries worldwide. The I-3 gene, which confers resistance to Fol race 3, has been described in wild tomato, Lycopersicon pennellii, accessions LA716 and PI414773. We are pursuing the isolation of I-3 from LA716 by map-based cloning. We have constructed a high-resolution map of the I-3 region and have identified markers closely flanking I-3 as well as markers co-segregating with I-3. In addition, construction of a physical map based on these markers has been initiated. This review describes the context of our research and our progress towards isolating the I-3 gene. It also describes some important practical outcomes of our work, including the development and use of a PCR-based marker for marker-assisted selection for I-3, and the finding that the I-3 gene from LA716 is different to that from PI1414773, which we have now designated I-7. Tomato varieties combining I-3 and I-7 have been developed and are currently being introduced into commercial production to further safeguard tomato crops against Fusarium wilt.