Two phases of an adult plant resistance response in wheat to Puccinia graminis f. sp. tritici
Adult plant resistance (APR) to stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is often conferred by multiple minor genes and has the potential to be durable. A preceding project identified two Kenyan wheat lines (W1406 and W6979) from the Genome Resource Unit (Norwich, UK) that exhibit APR to Pgt. The aim of this study was to investigate the APR response to Pgt race PTKST in W1406 and W6979 compared to 37-07, a susceptible control line. Histological investigation of inoculated flag leaf sheaths indicated a significant and quantifiable decrease in Pgt colony size in the APR lines at 120 hours post inoculation (hpi). Molecular analysis supported the observed fungal biomass decrease in the APR lines at 120 hpi. RNAseq analysis identified 169 transcripts differentially expressed in W1406 and 166 transcripts in W6979 when comparing 24 and 72 hpi to 0 hpi. In W1406 transcripts encoding putative pectinesterases, lipid-transfer proteins and leucine-rich repeat-like proteins were induced at 72 hpi. In W6979 only a corresponding putative pectinesterase encoding transcript was identified. Although the induced defence response in the two APR lines exhibited some dissimilarity, it potentially involves cell wall modification in both lines. Two independent sets of peroxidases were induced at 24 and 72 hpi in both lines, suggesting independent signalling events. Expression analysis suggests the occurrence of two phases of gene expression, one at 24 hpi and another at 72 hpi; the latter seeming to correspond to the inhibition of Pgt growth, manifesting as the observed APR phenotype.