Type B phosphatidylinositol-4-phosphate 5-kinases mediate Arabidopsis and Nicotiana tabacum pollen tube growth by regulating apical pectin secretion

Function and regulation of phospholipid signalling in plants

As an important metabolic pathway, phosphatidylinositol metabolism generates both constitutive and signalling molecules that are crucial for plant growth and development. Recent studies using genetic and molecular approaches reveal the important roles of phospholipid molecules and signalling in multiple processes of higher plants, including root growth, pollen and vascular development, hormone effects and cell responses to environmental stimuli plants. The present review summarizes the current progress in our understanding of the functional mechanism of phospholipid signalling, with an emphasis on the regulation of Ins(1,4,5)P3-Ca2+ oscillation, the second messenger molecule phosphatidic acid and the cytoskeleton.

Uncovering hidden treasures in pollen tube growth mechanics

The long-standing model of tip growth in pollen tubes considers that exocytosis and growth occur at the apex and that the pool of very small vesicles in the apical dome contains secretory (exocytic) vesicles. However, recent work on vesicle trafficking dynamics in tobacco pollen tubes shows that exocytosis occurs in the subapical region. Taking these and other new results into account, we set out to resolve specific problems that are endemic in current models and present a two-part ACE (apical cap extension)-H (hydrodynamics) growth model. The ACE model involves delivery and recycling of materials required for new cell synthesis and the H model involves mechanisms that integrate and regulate key cellular pathways and drive cell elongation during growth.

Using genetic tools to understand plant phosphoinositide signalling

Phosphoinositides (PIs) are regulatory lipids that control various physiological processes in eukaryotic organisms. As in other eukaryotes, the plant PI system is a central regulator of metabolism. The analysis of mutant plants that lack certain PI species has revealed their physiological relevance; however, knowledge of the factors controlling the distribution of PIs and the effects on their target proteins is still limited. To understand PI functions better, genetic approaches should be combined with biochemical analyses and cell biology, as has been done in several recent publications. Here, I highlight plant-specific physiological processes that are controlled by PIs and suggest future avenues of research. A detailed understanding of the functions and effects of PIs might offer new opportunities for modulating plant growth and hardiness against environmental influences.

Exocytosis and cell polarity in plants - exocyst and recycling domains

In plants, exocytosis is a central mechanism of cell morphogenesis. We still know surprisingly little about some aspects of this process, starting with exocytotic vesicle formation, which may take place at the trans-Golgi network even without coat assistance, facilitated by the local regulation of membrane lipid organization. The RabA4b guanosine triphosphatase (GTPase), recruiting phosphatidylinositol-4-kinase to the trans-Golgi network, is a candidate vesicle formation organizer. However, in plant cells, there are obviously additional endosomal source compartments for secretory vesicles. The Rho/Rop GTPase regulatory module is central for the initiation of exocytotically active domains in plant cell cortex (activated cortical domains). Most plant cells exhibit several distinct plasma membrane domains, established and maintained by endocytosis-driven membrane recycling. We propose the concept of a 'recycling domain', uniting the activated cortical domain and the connected endosomal compartments, as a dynamic spatiotemporal entity. We have recently described the exocyst tethering complex in plant cells. As a result of the multiplicity of its putative Exo70 subunits, this complex may belong to core regulators of recycling domain organization, including the generation of multiple recycling domains within a single cell. The conventional textbook concept that the plant secretory pathway is largely constitutive is misleading.