Abstract: Several factors affect the ability of Trichoderma spp. to provide
systemic disease control. This paper focuses on the role of the substrate in
which plants are grown, resistance of the host to disease, and the ability
of introduced Trichoderma inoculum to spread under commercial
conditions. Several reports reveal that foliar disease control provided by
Trichoderma spp. is more effective on plants grown in compost-amended
media compared with in lower-in-microbial-carrying-capacity sphagnum peat media. In Rhododendron spp., host resistance affects control of
Phytophthora dieback provided by Trichoderma spp. For example,
T. hamatum 382 (T382) significantly (P = 0.05) suppressed the disease on
susceptible cv. Roseum Elegans while plant vigor was increased. The disease was not suppressed, however, on highly susceptible cvs. Aglo and PJM Elite even though the vigor of these plants was increased. Using a strain-specific polymerase chain reaction assay under commercial conditions, it was demonstrated that introduced inoculum of T382 did not spread frequently from inoculated to control compost-amended media. Other Trichoderma isolates typically are abundant in control media within days after potting unless inoculated with a specific Trichoderma isolate. Thus, the low population of isolates that can induce systemic resistance in composting and potting mix environments may explain why most compost-amended substrates do not naturally suppress foliar diseases.
Abstract: Foot rot caused by Phytophthora capsici is the most serious disease of black pepper. Though all varieties of black pepper are susceptible to this pathogen, variations do exist concerning the degree of tolerance and mechanisms of defense. The protein profiles of a relatively tolerant and a susceptible black pepper (Piper nigrum) variety along with that of a resistant wild species (Piper colubrinum) were evaluated to detect variations in the defense related proteins/enzyme expression in response to P. capsici infection. The SDS-PAGE analysis revealed two additional polypeptides of 16.5 and 8 kD in the leaves of the tolerant variety (‘Kalluvally’) on the second day after infection and these proteins expressed only on the fifth day in the susceptible ‘Panniyur-1’. Variety specific proteins of molecular weight 90 and 5.5 kD were also found expressed in ‘Panniyur-1’ while ‘Kalluvally’ had unique protein bands of 14.3, 8.8, and 7.0 kD. However, P. colubrinum with 16 distinct bands had an altogether different banding pattern. The native
protein profile obtained also indicated the expression of two additional proteins in P. nigrum. The over-expressed protein was characterized as b-1,3 glucanase. The intensity of expression was directly related to the level of tolerance. The role of enzymes like phenylalanine ammonia lyase (PAL), chitinase, and peroxidase in defense mechanism was also analyzed. The resistant genotype P.colubrinum, possessed higher enzyme activities than the P. nigrum varieties tested. This study thus confirmed the role of b-1, 3 glucanase and related enzymes in the defense mechanism of black pepper against foot rot disease.
Abstract: Two linkage maps of pepper were constructed and used to identify quantitative trait loci (QTLs) conferring resistance to Phytophthora capsici. Inoculations were done with 7 isolates: 3 from Taiwan, 3 from California, and 1
from New Mexico. The first map was constructed from a set of recombinant inbred lines (RILs) of the PSP-11 (susceptible) × PI201234 (resistant) cross; and the second map was from a set of F2 lines of the Joe E. Parker (susceptible) × Criollo de Morelos 334 (resistant) cross. The RIL map covered 1466.1 cM of the pepper genome, and it consisted of 144 markers — 91 amplified fragment length polymorphisms (AFLPs), 34 random amplified polymorphic DNA (RAPDs), 15 simple sequence repeats (SSRs), 1 sequence characterized amplified region (SCAR), and 3 morphological markers — distributed over 17 linkage groups. The morphological markers mapped on this population were erect fruit habit (up), elongated fruit shape (fsFe), and fasciculate fruit clusters (fa). The FF2 map consisted of 113 markers (51 AFLPs, 45 RAPDs, 14 SSRs, and 3 SCARs) distributed in 16 linkage groups, covering a total of 1089.2 cM of the pepper genome. Resistance to both root rot and foliar blight were evaluated in the RIL population using the 3 Taiwan isolates; the remaining isolates were used for the root-rot test only. Sixteen chromosomal regions of the RIL map contained single QTLs or clusters of resistance QTLs that had an effect on root rot and (or) foliar blight, revealing a complex set of genetics involved in resistance to P. capsici. Five QTLs were detected in the FF2 map that had an effect on resistance to root rot.
Abstract:Phytophthora capsici causes devastating disease on many crop species, including Capsicum. Resistance in Capsicum annuum is genetically and physiologically complex. A panel of Capsicum germplasm that included
genotypes from both C. annuum and C. chinense showing highly resistant, highly susceptible and intermediate or tolerant responses to the pathogen, respectively, was screened with a series of randomly amplified polymorphic sequence primers to determine which genomic regions contribute to the highest level of resistance. One primer, OpD04, amplified a single band only in those C.annuum and C. chinense genotypes showing the highest
level of resistance. The amplified product was cloned, sequenced and used to design longer primers in order to generate a sequence characterized amplified region marker which was then mapped in a reference mapping population and a screened population segregating for resistance to P. capsici. These primers were observed to define a locus on pepper chromosome 5 tightly linked to Phyto.5.2, one of six quantitative trait loci (QTL) previously reported to contribute to P. capsici resistance. These results indicate that the Phyto.5.2 QTL may be widely distributed in highly resistant germplasm and provide improved resolution for this QTL. This work also defines the first breeding tools for this system, allowing for the rapid selection of genotypes likely to be highly resistant to P. capsici.
Abstract:Phytophthora capsici Leonian, known as the causal agent of the stem, collar and root rot, is one of the most serious problems limiting the pepper crop in many areas in the world. Genetic resistance to the parasite displays complex inheritance. Quantitative trait locus (QTL) analysis was performed in three intraspecific pepper populations, each involving an unrelated resistant
accession. Resistance was evaluated by artificial inoculations of roots and stems, allowing the measurement of four components involved in different steps of the plantpathogen interaction. The three genetic maps were aligned using common markers, which enabled the detection of QTLs involved in each resistance component and the comparison of resistance factors existing among the three resistant accessions. The major resistance factor was found to be common to the three populations. Another resistance factor was found conserved between two populations, the others being specific to a single cross. This comparison across intraspecific germplasm revealed a large variability for quantitative resistance loci to P. capsici. It also provided insights both into the allelic relationships between QTLs across pepper germplasm and for the comparative mapping of resistance factors across the Solanaceae.
Abstract: Criollo de Morelos 334 (CM334) is one of the most promising sources of resistance to Phytophthora capsici in pepper. This Mexican accession is distantly related to bell pepper and its resistance displays a complex
inheritance. The QTLs involved in resistance to P. capsici were previously mapped. In order to transfer the resistance factors from CM334 into a bell pepper genetic background, a modified, recurrent breeding scheme was initiated. The breeding population was divided into three subpopulations which were screened by distinct phenotypic tests of increasing severity. The plants from the first subpopulation were screened with low-severity tests and backcrossed to the susceptible bell pepper; the plants from the second and third sub-populations were screened by more severe resistance tests and crossed with the plants from the first and second sub-populations, respectively. In this study, the phenotypic data for the three sub-populations during five screening/intermating cycles were analysed. In parallel, the changes in allelic frequencies at molecular markers linked to the resistance QTLs were reported. The resistance phenotype and allelic frequencies strongly depended on the sub-population and screening severity. Regarding allelic frequency changes across the selection cycles, a loss of resistant QTL alleles was observed in the first sub-population, particularly for the low-effect QTLs, whereas a better conservation of the resistant QTL alleles was observed in the two other subpopulations. The same trend was observed in the phenotypic data with an increasing resistance level from the first to the third sub-populations. The changes in the allelic frequencies of loci not linked to resistance QTLs and for horticultural traits across the breeding process indicated that the recovery of the recipient parent genome was not significantly affected by the selection for resistance.
Abstract: Conserved domains or motifs shared by most known resistance (R) genes have been extensively exploited to identify unknown R-gene analogs (RGAs). In an attempt to isolate all potential RGAs from the maize genome, we adopted the following three methods: modified amplified fragment length polymorphism (AFLP), modified rapid amplification of cDNA ends (RACE), and data mining. The first two methods
involved PCR-based isolations of RGAs with degenerate primers designed based on the conserved NBS domain; while the third method involved mining of RGAs from
the maize EST database using full-length R-gene sequences. A total of 23 and 12 RGAs were obtained from the modified AFLP and RACE methods, respectively; while, as many as 109 unigenes and 77 singletons with high homology to known R-genes were recovered via data-mining. Moreover, R-gene-like ESTs (or RGAs)
identified from the data-mining method could cover all RACE-derived RGAs and nearly half AFLP-derived RGAs. Totally, the three methods resulted in 199 non-
redundant RGAs. Of them, at least 186 were derived from putative expressed R-genes. RGA-tagged markers were developed for 55 unique RGAs, including 16 STS
and 39 CAPS markers.
Abstract: Late blight, caused by the oomycete pathogen Phytophthora infestans, is the most devastating potato disease in the world. Control of late blight in the United States and other developed countries relies extensively on fungicide application. We previously demonstrated that the wild diploid potato species Solanum bulbocastanumis highly resistant to all known races of P. infestans. Potato germplasm derived from S. bulbocastanum has shown
durable and effective resistance in the field. Here we report the cloning of the major resistance gene RB in S. bulbocastanum by using a map-based approach in combination with a long-range (LR)-PCR strategy. A cluster of four resistance genes of the CC-NBS-LRR (coiled coil–nucleotide binding site–Leu-rich repeat) class was found within the genetically mapped RB region. Transgenic plants containing a LR-PCR product of one of these four genes displayed broad spectrumlate blight resistance. The cloned RB gene provides a new resource for developing late blight-resistant potato varieties. Our results also demonstrate that LR-PCR is a valuable approach to isolate genes that cannot be maintained in the bacterial artificial chromosome system.
Abstract: Late blight is the most serious potato disease world-wide. The most effective and environmentally sound way for controlling late blight is to incorporate natural resistance into potato cultivars. Several late blight resistance genes have been cloned recently. However, there is almost no information available about the resistance pathways mediated by any of those genes. We previously cloned a late blight resistance gene, RB, from a diploid wild potato species Solanum bulbocastanum. Transgenic potato lines containing a single RB gene showed a rate-limiting resistance against all known races of Phytophthora infestans, the late blight pathogen. To better understand the RB-mediated resistance we silenced the potato Rar1 and Sgt1 genes that have been implicated in mediating disease resistance responses against various plant pathogens and pests. The Rar1 and Sgt1 genes of a RB-containing potato clone were silenced using a RNA interference (RNAi)-based approach. All of the silenced potato plants displayed phenotypically normal growth. The late blight resistance of the Rar1 and Sgt1 silenced lines were evaluated by a traditional greenhouse inoculation method and quantified using a GFP-tagged P. infestans strain. The resistance of the Rar1-silenced plants was not affected. However, silencing of the Sgt1 gene abolished the RB-mediated resistance. Our study shows that silencing of the Sgt1 gene in potato does not result in lethality. However, the Sgt1 gene is essential for the RB-mediated late blight resistance. In contrast, the Rar1 gene is not required for RB-mediated resistance. These results provide additional evidence for the universal role of the Sgt1 gene in various R gene-mediated plant defense responses.
Abstract: The broad-spectrum rice blast resistance gene Pi9 was cloned using a map-based cloning strategy. Sequencing of a 76-kb bacterial artificial chromosome (BAC) contig spanning the Pi9 locus led to identification of six tandemly arranged resistance-like genes with a nucleotide-binding site (NBS) and leucine-rich repeats (LRRs) (Nbs1-Pi9–Nbs6-Pi9). Analysis of selected Pi9 deletionmutants and transformation of a 45-kb fragment from the BAC contig into the susceptible rice cultivar TP309 narrowed down Pi9 to the candidate genes Nbs2-Pi9 and Nbs3-Pi9. Disease evaluation of the transgenic lines carrying the individual candidate genes confirmed that Nbs2-Pi9 is the Pi9 gene. Sequence comparison analysis revealed that the six paralogs at the Pi9 locus belong to four classes and gene duplication might be one of the major evolutionary forces contributing to the formation of the NBS–LRR gene cluster. Semiquantitative reverse transcriptase (RT)–PCR analysis showed that Pi9 was constitutively expressed in the Pi9-resistant plants and was not induced by blast infection. The cloned Pi9 gene provides a starting point to elucidate the molecular basis of the broad-spectrum disease resistance and the evolutionary mechanisms of blast resistance gene clusters in rice.
Abstract