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Bibliography: leaves 218-233. / xiii, 233 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / This thesis identifies a strong relationship between Zn nutrition of medics and severity of disease by R. solani (AG8) which may be a major factor contributing to the current decline in medic pastures observed in Australia. The effect of seed nutritional value had a profound impact on the growth of medics and may also be a key factor in poor medic growth. / Thesis (Ph.D.)--University of Adelaide, Dept. of Plant Science, 1998

Cotton seedling disease complex is caused by a number of pathogens inducing similar symptoms and can lead to thin, uneven stands that grow slowly and yield poorly. Preliminary work indicated that a set of high tannin cotton lines developed and released in 1989 by Texas A&M AgriLife Research, (Smith et al., 1990a, Smith et al., 1990b, Schuster et al., 1990) may possess increased resistance to disease. This evidence, along with additional studies that show a clear role of tannin in disease resistance, suggest that these high tannin cotton lines may prove useful in breeding for increased resistance to cotton seedling disease complex. High tannin cotton lines were screened for their resistance to Rhizoctonia solani and Pythium aphanidermatum. While no high tannin germplasm line was more resistant to R. solani than the resistant control, Tamcot SP 21, the potential for significant gains from selection was demonstrated. Fifteen high tannin lines expressed resistance to P. aphanidermatum equal to the resistant control, Tamcot Sphinx (El-Zik and Thaxton, 1996). This resistance was not shown to be correlated with tannin content, though it is still unclear whether or not this lack of correlation is real or due to limitations in measuring tannin in infected seedlings.

Xanthomonas axonopodis pv. malvacearum(Smith), Pythium aphanidermatum(Edson) and Rhizoctonia solani(Kuhn) have all been shown to cause significant yield losses in cotton. Previous work has demonstrated that a set of high tannin cotton germplasm lines developed and released in 1989 by Texas A&M AgriLife Research may possess resistance to these three diseases. In this research, the usefulness of these high tannin lines in breeding for resistance to these pathogens as well as the role of tannin in conferring this resistance were examined. The high tannin lines were screened for their resistance to Xanthomonas axonopodis pv. malvacearum and five highly resistant lines were subjected to generation means and diallel analysis in order to determine the relative importance of different types of gene action in conferring resistance as well as which lines would be most useful in breeding for resistance. The effect of selection for R. solani resistance was measured and selected lines were subjected to diallel analysis. Lines showing elevated resistant to P. aphanidermatum were subjected to a single cycle of selection and the effect of selection was measured. Lines possessing high degrees of P. aphanidermatum resistance were subjected to generation means and diallel analysis. The importance of tannin content in conferring resistance to X. axonopodis and R. solani was also measured. Four high tannin lines were found to be not different from Tamcot Sphinx in their resistance to Xam. Generation means analysis for Xam resistance showed that in the five high tannin lines tested, additive gene effects were of the greatest importance. No correlation between tannin content and Xam resistance was detected. Under the conditions of this study tannin content was also shown to be unimportant in R. solani resistance. Two rounds of recurrent phenotypic selection were effective in increasing the resistance of selected HT lines to R. solani. One cycle of phenotypic selection for P. aphanidermatum resistance was found to produce significant improvements in seven of the most resistant HT lines. Generation means analysis for P. aphanidermatum resistance showed that in the HT lines tested, additive genetic effects were by far the most important.

Thesis (Ph. D.) in Biochemistry and Molecular Biology--University of Maine, 2001. / Includes vita. Includes bibliographical references (leaves 71-83).

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The chemicals inducers SA, BABA, and BTH were tested as seed treatment and soil drench on a partial-resistant cultivar of sugar beet grown in sand infested with the Rhizoctonia solani AG 2-2IIIB. In another series of experiments, BTH was applied as soil drench on sugar beet plants inoculated with R. solani. The chemical inducers were ineffective in reducing pre-emergence damping-off and post-emergence plant mortality. Despite these results, treatment with BTH altered the levels of expression ratios of four defence encoding genes associated with systemic resistance: chitinase, peroxidase, chalcone isomerase, and chalcone synthase. BTH sensitised sugar beet plants without the necessity of R. solani infection to up-regulate substantially the transcript level ratios of chalcS and chit3, while levels of chalcI were down-regulated levels below 1. Of interest, was the significant increase of transcript levels of chit3 in sugar beet plants infected with R. solani and treated with BTH. In conclusion, sugar beet plants were capable of over expressing selected genes in response to a chemical inducer, but contrary to what had been reported, gene activation in sugar beet as a result of BTH treatment does not confer disease resistance against R. solani.

Stachybotrys elegans is a mycoparasite of the soilborne plant pathogen fungus Rhizoctonia solani. The mycoparasitic activity of S. elegans is correlated with the production of cell wall-degrading enzymes such as chitinases. This study details the cloning and characterization of the cDNA, sechi44, that encodes an extracellular endochitinase. The expression regulation of sechi44 was altered when S. elegans was in interaction with its host, R. solani, and also when the mycoparasite was grown on minimal media amended with different carbon and nitrogen sources. Direct contact with R. solani significantly upregulated sechi44 expression which followed a cyclical pattern suggesting that this gene has a role not only in mycoparasitism, but also in linear growth of the mycoparasite. The addition of high concentrations of glucose and ammonium triggered a decrease of sechi44 expression suggesting that sechi44 is subject to glucose and ammonium repression. In a separate study, several genes (1016 clones) whose transcription was substantially up-regulated during the mycoparasitic interaction were identified using SSH and microarray analysis. Twenty-five percent (261 clones) of these were sequenced and assigned to putative functions. Among them, 15 expressed sequence tags (ESTs) were identified in R. solani whose functions were related to defense while the majority of ESTs were identified in S. elegans and assigned functions related to toxin metabolism, pathogenic process, stress response., multidrug resistance, apoptosis, transport, ATP synthesis, replication, transcription and DNA repair, translation, transduction, protein degradation, and ribosomal protein. The overexpression of 13 selected genes of S. elegans was validated and confirmed using quantitative reverse transcription polymerase chain reaction (QRT-PCR). The temporal gene expression of nine genes was monitored when the mycoparasite was grown on R. solani (host) and Sclerotinia sclerotiorum (non-host) mycelia and sclerotia. Some genes such as seglu, selec, and se151 were completely inhibited by the presence of non-host hyphae suggesting that these genes play an important role during mycoparasitism. Also, the absence of these corresponding transcripts suggests that the non-host produces transcription inhibitors. As expected, gene expression of cytochrome P450 was highly up-regulated early after germination of S. elegans conidia. This is in agreement with our finding in the EST data mining study, in which a role in toxin production was assigned to cytochrome P450.

The mycoparasite Stachybotrys elegans represents a potential biocontrol agent of Rhizoctonia solani, the causal organism of potato stem canker. The differential expression of two mycoparasitism-induced genes was monitored in S. elegans during interactions with its host, Rhizoctonia solani, and a non-host, Sclerotinia sclerotiorum. Using real-time reverse-transcription polymerase chain reaction (QRT-PCR), comparative analyses demonstrated that hyphal and sclerotial forms triggered different patterns of gene expression in the mycoparasite, as did the presence of the host or non-host. The calmodulin gene did not appear to be involved in conidial germination or appressoria formation of S. elegans. Potential roles of calmodulin during mycoparasitism are suggested, but further studies are required. The expression of the endochitinase-encoding gene, sechi44, was susbstantial only at later stages of interaction with living host sclerotia. Host defense mechanisms probably play a role in regulating sechi44 expression. Knowledge of the genetic mechanisms underlying this mycoparasitic relationship will further our knowledge on the potential use of S. elegans in biocontrol strategies. / Le mycoparasite Stachybotrys elegans est un agent potentiel de lutte biologiquepour le contrôle de Rhizoctonia solani, un phytopathogène causant le chancre dela tige et des stolons chez la pomme de terre (Solanum tuberosum). L'expressionde deux gènes induits pendant le mycoparasitisme a été étudiée chez S. elegans,alors que le mycoparasite était en interaction avec son hôte, R. solani, et un nonhôte,Sclerotinia sclerotiorum en utilisant la PCR quantitative en temps réel. Desanalyses comparatives ont démontré que les différentes formes d'hyphes et desc1érotes, ainsi que la présence de l'hôte ou du non-hôte, induisent différentspatrons d'expression. Le gène codant pour la calmodulin (calmodulin) ne semblepas être impliqué dans la germination des conidia ou dans la formation desappressoria chez S. elegans. Des rôles possibles de calmodulin sont suggérés,mais des études plus poussées demeurent nécessaires. L'expression du gènesechi44, codant pour une endochitinase, est importante dans des stades plusavancés du mycoparasitisme sur les sc1érotes vivants de 1 'hôte. Des mécanismesde défense de l'hôte jouent probablement un rôle important dans la régulation del'expression de sechi44. Une meilleure connaissance de la régulation génétiquelors du mycoparasitisme pourrait nous aider à évaluer le potentiel de S. elegansdans des stratégies de biocontrôle.