Stromule Formation

Abstract

SummaryStromules are highly dynamic stroma-filled tubules extending from the surface of plastids in all multicellular plants. Although stromules may interconnect two, or more, plastids and allow the transfer of stromal proteins as large as Rubisco (∼550 kDa) between plastids, their function is still largely a matter of conjecture. They may increase the plastid surface area to facilitate movement of materials into or out of plastids, be involved in sensing the cellular environment, and/or have signaling functions due to close apposition of stromules and nuclei, plasma membrane and other cell organelles. Stromule formation appears unrelated to chloroplast division or to light-intensity-dependent chloroplast movement. Stromules are most easily observed by confocal microscopy of cells expressing plastid-targeted fluorescent proteins, and the definition of stromules is based on such observations. Identification of stromules in electron microscope images is problematic, and ideally requires examination of thin serial sections. In leaves of both monocots and dicots, stromules are most abundant in epidermal cell-types, such as trichomes, guard cells and pavement cells, and are more difficult to observe in mesophyll cells containing large closely packed chloroplasts. Stromule formation and movement depends on the actin cytoskeleton and requires the ATPase activity of myosin XI proteins. A 42-amino-acid-residue region of myosin XI that directs myosin XI to the chloroplast periphery offers prospects for identification of stromule components required for stromule mobility. Stromules are affected by various environmental factors and by biotic stresses. Stromules are induced by water stress, acting via abscisic acid signaling pathways, and by viral infection. Stromule abundance is also affected by light and temperature.