Session 6. New Vistas in Plant Molecular Biology L6.2 Phytohormone-binding proteins in legume plants

Plant immunity responses are initiated upon recognition of conserved microbe-associated molecular patterns (MAMPs) that trigger a network of cellular signalling events. Our research emphasis is on two aspects of the cellular signal transduction, viz. calcium and mitogen-activated protein kinase (MAPK) signalling. Calcium flux is one of the earliest events following MAMP perception. Several known signalling components acting concomitantly with cellular Ca2+ elevation were tested for an influence on calcium signalling. Among the investigated components, BAK1, a receptor-like kinase associated with several receptors, appears to differentially amplify the signals to attain stimuli-specific calcium amplitudes. Additionally, the overall “calcium signature” is shaped by reactive oxygen species accumulation. To further search for novel components regulating this crucial signalling step, a genetic screen was performed to isolate mutants with altered calcium signature after application of the MAMP, flg22. This led to several novel alleles of the flg22 receptor, FLS2 and its associated partner, BAK1, as well as several new mutants with changed calcium elevation (cce). Downstream of calcium, mitogen-activated protein kinase (MAPK) cascades control signal transduction pathways in eukaryotes through protein phosphorylation. As several MAPK components are shared in multiple distinct pathways, specificity must somehow be maintained to prevent erroneous signalling. We propose that signal fidelity of MAPK pathways is, in part, maintained through the repertoire and combination of diverse MAPK substrates. Towards this end, we have identified a number of MAPK-interacting proteins and putative substrates by yeast-two-hybrid analysis or a protein-array-based kinase screen. These potential MAPK substrate targets include potential transcription factors, RNA-binding proteins, enzymes or proteins of yet unknown functions. We are in the process to verify how some of the isolated MAPK substrates are involved in plant immunity or other processes. Session 6. New Vistas in Plant Molecular Biology