Novel roles for phospholipase C in plant stress signalling and development

In animal cells, phospholipase C (PLC) is known to hydrolyse phosphatidylinositol 4,5-bisphosphate (PIP 2 ) to generate the second messengers, inositol 1,4,5-trisphosphate (IP 3) and diacylglycerol (DAG), which release intracellular Ca 2+ and activate protein kinase C (PKC), respectively, orchestrating a wide range of cellular- and physiological processes. Plants contain PLCs too but lack IP 3 receptors and PKC, and it still remains largely unclear what its physiological role is in plants and what molecular targets it has. The genome of Arabidopsis thaliana encodes 9 PLC genes. Earlier work on PLC2 , PLC3 and PLC9 revealed roles for PLC in gametogenesis, ABA signalling, lateral root formation and heat stress tolerance. Here, we functionally characterised the role of PLC5 . Promoter-GUS analyses revealed that this gene is predominantly expressed in vascular tissue, most likely the phloem, including roots, leaves and flowers, but expression was also detected in the root-apical meristem, in guard cells and in trichomes. We only managed to find one homozygous T-DNA insertion line, plc5-1, which turned-out to be a knock-down mutant, suggesting that a KO mutant is probably lethal. Growth of plc5-1 plants on agar plates consistently exhibited a ~20% reduction in their lateral root formation. The latter was caused by a decrease in initiation rather than emergence of the lateral roots. PLC3 was found to be required for lateral root formation earlier (Chapter 2), but a double plc3plc5 mutant did not intensify the phenotype, indicating the involvement of possible additional redundant PLCs. Complementation of plc5-1 with the PLC5-wt gene, expressed behind its own promoter, restored growth and rescued the lateral root phenotype. UBQ10-Overexpression of PLC5 did not affect lateral root development, but was found to stunt root hair growth, to decrease the stomatal aperture and to increase their tolerance to drought stress. In vivo 32 P i -labeling analyses of PLC's substrate/product lipids revealed