Molecular and cellular analysis of adult plant resistance in wheat to Puccinia graminis f. sp. tritici

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Date
2018
Authors
Castelyn, Howard Dean
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Publisher
University of the Free State
Abstract
Wheat (Triticum aestivum L.) production is affected by fungal diseases such as stem rust caused by Puccinia graminis f. sp. tritici Erikss. & Henning (Pgt). Commercially implemented single gene resistance in wheat may break down with the emergence of new virulent races of Pgt. Emergence of the ‘Ug99’ race group reaffirmed the need for durable stem rust resistance. A paradigm shift is therefore required towards durable types of resistance. Adult plant resistance (APR) may remain durable in the presence of new pathogen races. A preceding project identified two Kenyan wheat lines (W1406 and W6979) from the Genome Resource Unit (Norwich, UK) that exhibited APR to Pgt. The aim of this study was to investigate the APR response to Pgt race PTKST in W1406 and W6979 compared to a susceptible control (37-07). Four greenhouse trials were done and Pgt-inoculated flag leaf sheaths were sampled at selected days post inoculation (dpi). Histological observations using scanning electron microscopy and fluorescence microscopy, described the infection process of Pgt on wheat flag leaf sheaths. Fluorescence microscopy in particular showed a significant decrease in Pgt colony sizes in the APR lines at 5 dpi. Fungal biomass quantification with a chitin-binding probe and relative expression of the Pgt β-tubulin gene distinguished the APR lines from line 37-07 at 5 and 10 dpi. Relative expression of a haustorium-associated gene also distinguished the APR lines from the control line at 5 dpi. Taken together, these findings supported phenotypic evidence that W1406 and W6979 inhibited Pgt development, but that the effect was more pronounced in W6979. These results prompted the selection of time points earlier than 5 dpi for RNA-sequencing (RNA-seq) analysis. A time based comparison of differentially expressed (DE) transcripts showed two phases of the APR response at 1 and 3 dpi. The majority of differentially expressed transcripts in W1406 were induced at 3 dpi but in W6979 at 1 dpi. The current hypothesis maintains that the activation of an earlier defence response in W6979 caused a notable decrease of Pgt development in this line. The defence response in W1406 was delayed but still significantly decreased Pgt development. RNA-seq also identified several transcripts encoding putative effectors in Pgt. A comparison of differentially expressed between the APR lines and 37-07 identified a number of candidate APR associated transcripts. The current study therefore delivered a multifaceted description of the APR response in W1406 and W6979 and its effect on Pgt development.
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Keywords
Adult plant resistance, Wheat, Triticum aestivum, Stem rust, Puccinia graminis f. sp. tritici, RNA-seq, Theses (Ph.D. (Plant Sciences))--University of the Free State, 2018
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