Pollen tube growth and guidance: Occam’s razor sharpened on a molecular AGP Rosetta Stone

Abstract

Occams Razor suggests a new model of pollen tube tip growth based on a novel Hechtian oscillator that integrates: (1) a periplasmic AGP-Ca2+ calcium capacitor with tip-localised arabinogalactan glycoproteins (AGPs); (2) tip-focussed cytosolic Ca2+oscillations; (3) Hechtian strands evidence of adhesion between the plasma membrane and the cell wall of the growing tip. Thus Hechtian adhesion, as a piconewton force transducer, couples the internal stress of a rapidly growing wall to the plasma membrane. Such Hechtian transduction via stretch-activated Ca2+ channels and H+-ATPase proton efflux dissociating periplasmic AGP-Ca2+, creates a Ca2+ influx that activates exocytosis of wall precursors. In effect a highly simplified primary cell wall regulates its own synthesis and a Hechtian growth oscillator regulates overall tip growth. By analogy with the Rosetta Stone that translates trilingual inscriptions as a single identical proclamation, the Hechtian Hypothesis translates classical AGPs and their roles as a Ca2+ capacitor, pollen tube guide and wall plasticiser into a simple but widely applicable model of tip growth. Even wider ramifications of the Hechtian oscillator may implicate AGPs in osmosensing or gravisensing and other tropisms, leading us yet further towards the Holy Grail of plant growth.