The Role of Light Signalling in Plant Immunity

Light is essential for regulating every aspect of plant growth and development. Recent studies have revealed that light quality and duration can influence plant immune response onset as well as the virulence of some pathogens. The oomycete Phytophthora infestans, the causal agent of potato late blight, is still the most devastating pathogen of potato worldwide. The genome of P. infestans encodes a family of effector proteins characterised by an RXLR motif. These effectors are translocated into host cells during pathogen attack to manipulate the host to the benefit of the pathogen.
Specific objectives for 0-18 months:
1. Impact of light quality, intensity and duration on Phytophthora infestans infection in potato and Nicotiana.
a. The student will utilize a number of transgenic Solanum tuberosums that have been generated to express pathogen effectors from P. infestans (PITG_02860 and PITG_06099), plus silencing and overexpression of host target proteins. All plants and the oomycete pathogens will be assessed for light-mediated physiological and molecular responses in response to red, blue, far-red, UV-B light and circadian timing such as hypocotyl elongation, chloroplast movement in plants and the ability to sporulate for oomcyetes (Suetsugu et al., 2016).
b. The student will inoculate and monitor disease progression in transgenic plants infected with pathogenic sporangia of P. infestans (potato/tomato specialist) and P. capsici (a broad host range hemibiotrophic crop pathogen). The infection process will be monitored under specific light qualities (using blue/red light filters in front of normal light sources and some cabinets with special arrangements of LED lights at JHI primarily. Stably transformed lines will be used to alter plant light signalling components, such as phyB and NRL.
c. Virus-induced gene silencing (VIGS) and agrobacterium based transient overexpression will be utilised as a Solanaceae model for certain experiments, particularly where good expression levels of particular tagged proteins are required for some immune assays and westerns blots. Outcome: Findings will clearly define how light impacts infection in crop plants and potentially define components involved.
2. Genetically define molecular signalling components involved in light-infection integration in model plant Arabidopsis.
a. The student will challenge mutant and transgenic Arabidopsis of interest with triggers of both Pamp-triggered Immunity (PTI) and NB-LRR based resistances, known as Effector triggered immunity (ETI). Both PTI and ETI can trigger signalling events such as ROS bursts, phosphorylation cascades and gene expression changes that can lead to a visible and quantifiable cell death response in the host.
b. The student will inoculate mutant and transgenic Arabidopsis with pathogenic sporangia of broad host range hemibiotrophic crop pathogen and model infecting Oomcyetes Phytophthora parasitica and Phytophthora capsici and the biotrophic Arabidopsis specialist Hyaloperonosprora arabidopsidis to monitor disease resistance/susceptibility of wild-type, mutant and overexpressing Arabidopsis lines of photoreceptor (phyA, phyB, phot1, phot2, cry1, cry2, UVR8) and light signalling components (PIFs, HY5, NPH3, COP1) in response to different wavelengths of light. Immune responses will be monitored using a series of histochemical and gene expression assays which can be used model to extrapolate findings to phytophtora research. Outcome: extend knowledge of how light impacts infection processes and potentially define components involved in Arabidopsis.
3. Direct interactions between effector proteins and light signalling components.
a. Given preliminary evidence the student will confirm the interactions between P. infestans effectors and host targets from crop plants with putative roles in light-mediated responses using protein-protein interaction techniques such as BiFC, Yeast-2-Hybrid and co-immunoprecipitation. Potential effector-triggered changes in post-translational modifications, stability or localization changes of target proteins will be examined.
b. The student will examine a series of light-mediated physiological and molecular responses (hypocotyl elongation, flowering initiation, chloloplast photo-relocation, as well as gene expression analysis) in response to red, blue, far-red and UV-B light on transgenic lines overexpressing P. infestans protein effectors (PITG_02860 and PITG_06099) that are known to target putative light signalling components in potato. Outcome: The student will elucidate if the pathogen effectors are positively or negatively manipulating their protein targets to aid disease progression.
4. Does light directly affect pathogenicity. Pseudomonas and Phytophthora contain potential light receptor proteins. Does genetic manipulation of these components or prior light treatment affect infectivity and if so can the infection process be altered by modifying these parameters.