Dissecting plant immune responses mediated by the PI4Kβs

Plants are essential for human survival. They possess a complex immune system to defend against pathogens that threaten their growth and productivity. While this immunity is essential, its regulation is equally important to ensure a balanced immune output for optimal plant health and productivity. A thorough understanding of immunity and its regulation is vital for the generation of disease resistant plants. The ubiquitin-proteasome system (UPS) plays an important role in plant immune regulation. In a reverse genetic screen to identify immune regulating E3 ligases, Tubby Like Protein 6 (TLP6), an F-box protein that is part of an SCF E3 ligase was identified as a positive immune regulator. Further analysis showed that TLP6 targets two phosphatidylinositol 4-phosphate (PI4P) biosynthesis enzymes, Phosphatidylinositol 4-Kinase (PI4K) β1 and β2 for degradation. TLP6 overexpression led to ubiquitination and reduction of PI4Kβ levels and plants overexpressing TLP6 displayed autoimmunity and short roots like the pi4kβ1,2 mutant. Overall, the data suggests that TLP6 regulates immunity by modulating PI4Kβ levels. An ethyl methanesulfonate mutagenesis based forward genetic screen was conducted in the pi4kβ1,2 T-DNA double mutant background to understand the source of its autoimmunity and further our understanding of PI4Kβs’ role in plant immunity. Dominant suppressors of pi4kβ1,2 autoimmunity and root length defect were identified. Analysis showed that they are intragenic suppressors containing deletions within the PI4Kβ1 T-DNA insertion that allow for PI4Kβ1 transcription. Further, suppressors mutated in Signal Peptide Peptidase (SPP), and Orosomucoid-like 1 (ORM1) that encodes a negative regulator of sphingolipid biosynthesis were identified. They displayed suppression of pi4kβ1,2’s immunity and root defects. pi4kβ1,2 displayed high levels of ER stress and mutations in SPP and ORM1 suppressed it. The orm1 suppressors also revealed a link between sphingolipids and phosphatidylinositols in pi4kβ1,2 phenotypes. Overall, my research revealed: a novel role for the UPS in plant immune regulation by modulating PI4Kβ levels, a new mechanism of T-DNA mutant reversion, and the role of SPP and ORM1 in pi4kβ1,2 phenotypes . These findings add to the existing body of knowledge on plant immunity and its regulation and may be relevant for future research in developing disease resistant plants.