Characterization of early defence responses in rust-infected sunflower

English: Plants are equipped with a surveillance system which enables the plant to recognize an invading pathogen. After recognition, the stress signal is conveyed throughout the plant cell and subsequent defence responses are activated. Protein kinase genes encode proteins which play important roles in the relay of the initial signal. The aim of this study was to characterise some aspects of the defence response of sunflower upon infection with P. helianthi. One such aspect was the identification of a putative protein kinase gene from the resistant sunflower cultivar. The HaLRD15 gene showed good homology (e-value 7e-151) to a Calcineurin B-like protein kinase (CDPK). The virtual protein encoded by the HaLRD15 gene contained various phosphorylation sites and a MAPK docking site. These sites could lead to the protein being phosphorylated thereby conveying the stress signal throughout the plant cell. Further analyses of HaLRD15 in different sunflower cultivars indicated that the gene was inducible expressed in only one resistant cultivar, even though gene copies were present in all the tested cultivars. This led to the conclusion that HaLRD15 could be essential factor needed in the defence response. Furthermore, an attempt was made to obtain putative disease resistance genes which are differentially expressed in sunflower after infection by using primers specific for the nucleotide binding site of resistance genes. One such gene fragment obtained was NBS6. The polypeptide sequence of NBS6 showed homology to a hypothetical protein containing a DUF26 domain. Further analysis of NBS6 showed that the gene is up-regulated by defence response related chemicals such as MeJA and menadione. Literature have implicated that receptor-like protein kinases containing DUF26, were involved in the defence responses in the plant-pathogen interaction, more specifically in the response of plants upon oxidative stress. As photosynthesis related genes were also differentially expressed, the effect of the leaf rust pathogen on the photosynthetic capacity of both resistant and susceptible cultivars was investigated by using chlorophyll fluorescence. The electron transport rate of the infected resistant plants was shown to be much lower than that of the infected susceptible plants. However the energy dissipated as heat (NPQ) was shown to increase over time in the infected susceptible plants. The expression of various genes such as glutathione S-transferase and the large and small subunit of Rubisco were finally analysed. The infected susceptible plants were shown to have a higher down-regulation of the RbcS gene than the infected resistant plants. The higher electron transport rate of the infected susceptible plants in combination with the low induction levels of RbcS as well as a higher induction of GST during the early intervals, led to the conclusion that the susceptible plants increase the electron transport system to produce higher amounts of reactive oxygen species, thereby leading to the higher induction of GST expression.