Monosporascus cannonballus Pollack & Uecker, a soilborne root-infecting ascomycete, an economically important pathogen of melons (Cucumis melo L.) and watermelons (Citrullus lanatus L.). The fungus causes root rot and/or vine decline in different geographical areas worldwide. Little is known about the biology of this fungus and the mechanism by which the pathogen induces wilting of infected plants. This dissertation focuses on the biology, epidemiology, and etiology, as well as management strategies of both the pathogen and the disease. Microscopic examination of the xylem vessels of wilted plants revealed heavy occlusion by tyloses. Measurement of hydraulic conductivity indicated a 20-fold reduction in flow rate of plugged vessels, suggesting that tyloses contribute to the rapidity of vine decline in infected plants. The onset of symptoms coincide with high soil temperatures (25°C and above) and although vine decline occurs late in the growing season (i.e. generally two weeks before harvest), plants could be infected as early as nine days after planting. This dissertation also reports for the first time the existence of strains of M. cannonballus. Phenotypic differences such as cultural characteristics and growth rate were observed among the isolates and results indicate that genetically isolated strains, based on mycelial incompatibility, exist within M. cannonballus. Furthermore, local field populations of M. cannonballus can be heterogenous according to the number of mycelial compatibility groups identified. Ascospores of M. cannonballus germinate only in the rhizosphere/rhizoplane of melon and watermelon hosts in five soil. This result suggests a very limited host range of ascospore germination of M. cannonballus. Finally, field tolerance against M. cannonballus exists among cantaloupe cultivars and breeding lines. This will facilitate breeding for resistance within the cantaloupe type of melon. The knowledge derived from these studies contribute to our understanding of the biology and epidemiology of M. cannonballus and will serve as the basis for control or management of vine decline in the future.

Plant exudates have been implicated as a driving force for rhizosphere interactions, the molecular mechanisms of root exudation and release of plant signal molecules remain unknown. Molecular dissection of the process of root exudation may eventually lead to the genetic engineering of plants to manage rhizosphere interactions. In this study, the release of microbial gene inducers was examined by manipulating border cell separation. Specifically, the hypothesis to be tested is whether border cell separation contributes to the release of nodulation (nod) gene inducers. The model system was pea and Rhizobium leguminosarum bv viciae. The experimental approach was to identify gene that play a role in border cell separation, which can be used as a tool to manipulate the process experimentally. Molecular cloning and genetic manipulation by antisense mutagenesis of rcpme1 was carried out to test whether expression of rcpme1 in root tips of pea is required for border cell separation. The cDNA and genomic copy of rcpme1 were cloned and characterized. The rcpme1 promoter was isolated and analyzed by expression of a GUS reporter genes fused to the promoter. Using Agrobacterium rhizogenes-mediated transgenic hairy roots, the effect of PME on border cell separation was examined by expressing antisense rcpme1 mRNA driven by its own promoter. Transgenic hairy roots of pea expressing antisense rcpme1 mRNA showed various phenotypes including incomplete separation of root border cells, decreased border cell number, abnormal root tip morphology, and stunted hairy root development. To test the hypothesis that the process of border cell separation plays a role in root-microbe interactions, pea and its symbiotic partner Rhizobium leguminosarum bv viciae were used. Results from this study indicated that transgenic hairy roots with reduced border cell separation resulted in reduced nod gene induction, while physiological treatments that increase border cell separation activity resulted in enhanced nod gene induction. Increased nod gene induction was correlated with increased nodulation on pea roots. These results are consistent with the hypothesis that the process of border cell separation from root tips of pea is important in border cell separation and consequent release of root exudate-derived nod inducers.

<p> Plant endogenous small RNA pathways generate non-coding regulatory RNAs that regulate gene expression through target mRNA cleavage, translation inhibition or chromosomal modifications. Regulation of small RNAs and their targets during pathogen infection is tightly controlled to promote defensive mechanisms against disease progression. The oomycete pathogen, <i>Phytophthora sojae</i> is a principal infectious agent of soybean. To date, there is limited information on small RNAs that regulate defense responsive genes against <i>P. sojae </i>. </p><p> Infection response in plants is evidently regulated in part by small RNAs. High-throughput sequencing of small RNA libraries constructed from <i> P. sojae</i>-infected and mock-infected soybean roots and subsequent computational analysis revealed approximately 324 known soybean miRNAs and 109 potential novel soybean miRNAs that differentially accumulate between the <i>P. sojae</i>-infected and mock-infected samples. Of these, 8 conserved miRNAs and 2 novel miRNAs were verified by Northern blot analysis. Targets of the miRNAs displayed abundance changes respective to their complementary miRNA's levels. </p><p> The down-regulation of the conserved miR393 by target mimicry points to a positive regulatory role for miR393 during pathogen response. In addition, we noted the induction of miRNA-directed expression of phasiRNAs from multiple NB-LRR loci. These results indicate a pool of miRNAs specific in responding to <i>P. sojae</i> infection. Our study identified multiple conserved and novel soybean miRNAs with potential defensive roles against <i>P. sojae</i>. Our data demonstrates that plant response to pathogen infection is complex and multi-layered. Further study of small RNAs involved in defense regulation may contribute to combating <i>Phytophthora</i> diseases. </p>

<p> Fusarium wilt of cotton, caused by the soilborne fungus <i>Fusarium oxysporum</i> f. sp. <i>vasinfectum,</i> is a widespread disease occurring in most cotton-growing regions of the world. Fusarium wilt occurs in all domesticated cotton. Currently, six nominal races are recognized: 1, 2, 3, 4, 6, and 8, as well as many un-named genotypes worldwide. Many are widespread in the U.S., but race 4, which is highly virulent, is apparently restricted to California. Race 4 is found in an increasing number of fields in California due in part to seed-borne dissemination. The first aim of this study was to evaluate the efficacy of hot water treatments alone or in conjunction with fungicides and other treatments to reduce the viability of FOV race 4 in infected cotton seed. The second aim was to develop and evaluate a rapid and reliable molecular diagnostic assay, the AmplifyRP^&reg; Acceler8&trade;, for the direct detection of FOV race 4 in cotton tissue. In the seed treatment assay, a 1 hour immersion of seed in water or sterile 30% potato dextrose broth (PDB) at 24&deg;C followed by a 20 minute immersion in a 60&deg;C solution containing four fungicides (azoxystrobin, fludioxonil, thiabendazole, and thiophanate) or thiophanate alone were the most effective pretreatment-treatment combinations in reducing FOV in seed and avoiding loss of seed germination and vigor. The incidence of FOV in the seed was reduced by approximately 86% without reducing seed germination and vigor based on recovery of the fungus on petri plates and greenhouse grow-out assays. FOV was completely eliminated from infected seed when the seed was pretreated in water at 24&deg;C followed by a 20 minute immersion in a solution of thiophanate heated to 70&deg;C. With this treatment, seed germination was reduced by 36% and vigor was reduced by 38%. The AmplifyRP^&reg; Acceler8&trade; diagnostic assay consistently detected FOV race 4 from all infected tissue samples. The test is rapid, simple and more sensitive than conventional PCR. The AmplifyRP^&reg; Acceler8&trade; diagnostic assay detected DNA from FOV race 4 at concentrations of 1 ng/&micro;L and above. In addition, it did not amplify DNA from other known FOV races (races 1, 2, 3, 6, and 8). The whole process from sample preparation to reading the results was completed in as little as 30 minutes. The test detected FOV race 4 in cotton taproots, petioles, and stems.</p>

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 68-11, Section: B, page: 7040. Adviser: Steven J. Clough. Includes supplementary digital materials. Includes bibliographical references (leaves 86-98) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--Indiana University, Dept. of Biology, 2008. / Title from PDF t.p. (viewed on Jul 23, 2009). Source: Dissertation Abstracts International, Volume: 69-10, Section: B, page: 5865. Adviser: Roger W. Innes.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-11, Section: B, page: 6778. Adviser: Hans J. Bohnert. Includes bibliographical references (leaves 87-104). Available on microfilm from Pro Quest Information and Learning.

Abstract not available.

The presence on wheat crops of the fungal pathogen Fusarium graminearum poses serious financial and health-related problems in Canada. The fungus causes a significant decrease in yield and quality of the crop, in addition to producing mycotoxins, which can survive the milling process. Signaling processes involving protein kinases are thought to play a major role in the activation of a host defense response. A research project employing western detection experiments and kinase assays was initiated to determine whether the presence of F. graminearum caused changes in the level of protein kinases in cultivars Frontana (resistant) and Roblin (susceptible). The results indicated that F. graminearum had modest effects on phosphorylation levels of threonine and tyrosine residues. Moreover, differences occurred between the resistant and susceptible cultivar. Kinase assays did not show any variation in activity in either cultivar. A MAPK (w&barbelow;heat M&barbelow;APK h&barbelow;omologue-1&barbelow;) homologue was cloned, sequenced and shown to be identical to the wck-1 sequence found in the public database. Preliminary results show an increase in the level of transcript in the Fusarium-treated Frontana sample. The implications of the results are discussed.

Wheat (Triticum aestivum) kernel texture (hardness) is the most important determinant of milling and end-product quality. Recent data indicate that the only difference between soft and hard textured wheat is a single amino acid mutation in one protein, puroindoline-b (PIN-b). A rare tryptophan-rich domain in this protein consists of five tryptophan residues among a stretch of seven amino acid residues. To understand how this single mutation makes hard wheat possible, thus enabling bread making, it is crucial to have a high-resolution three-dimensional structure of this protein. The prerequisite for structural elucidation of any protein is the high-quality sample preparation. In this thesis PIN-b from a diploid wheat (Triticum monococcum ) was chosen as the model system because its grain is soft and it has the simplest genome of all wheats. The coding sequence of PIN-b was amplified from the diploid wheat using PIN-b specific primers. It was cloned into a protein expression vector. PIN-b was expressed as a protein behind the thioredoxin (TRX-a) tag. The TRX-a-PIN-b fusion protein was purified using nickel chelating chromatography. The immunological identity of the fusion protein was confirmed by Western blot. The PIN-b was released from the fusion protein by enterokinase proteolysis and purified using ion exchange chromatography. After glutathione treatment to facilitate full formation of the potential five disulfide bonds, PIN-b demonstrated higher fungicidal activity when compared to the non-treated PIN-b.