This article is dedicated to the memory of Nora Ausmees (19602015), a pioneering Streptomyces cell biologist and a wonderful, exuberantspirit.

Streptomycetes are abundant and important members of the microbiota of soil and many other environments. These Gram-positive bacteria are found in two distinct forms: a multicellular mycelium and dormant unicellular spores. Underlying the interconversion between these two modes of existence is a truly fascinating and complex developmental life cycle (FIG.1).

The vegetative mycelium of streptomycetes comprises branching hyphae that scavenge for nutrients, grow and proliferate (FIG.1). Hyphae grow by tip extension, and this strictly apical mode of growth is directed by a cytoplasmic protein complex the polarisome that determines cell polarity and organizes assembly of the peptidoglycan cell wall at the hyphal tips (BOX1). New sites of growth arise by branching from the lateral cell wall, thereby generat-ing a ramifying mycelial network and creating an almost exponential mode of growth. The resulting mycelium is multicellular and consists of long and branching fila-mentous cells delimited by occasional hyphal cross-walls (BOX2). Remarkably, these cross-walls are not essential, as mutants not competent for cell division are viable and can still proliferate by tip extension and branching1 (BOX2).

The spores of streptomycetes allow the bacteria to survive harsh conditions. As the mycelium is non-motile

and unable to reach new habitats, sporulation may also have evolved as a means to enable efficient dispersal to new environments, which is probably a feature of large adaptive value for these organisms. Sporulation involves profound reorganization of the growth, morphology and physiology of the cells. First, hyphal branches differenti-ate into specialized sporogenic structures called aerial hyphae, typically emerging as a hair-like layer on the sur-face of colonies. In order for the aerial hyphae to grow into the air, the reproductive structures are covered in an extremely hydrophobic sheath, which breaks the surface tension to enable escape from the aqueous environment of the vegetative mycelium. This sheath is composed of two developmentally regulated protein families the chaplins and the rodlins29. On certain media, escape from the aqueous environment also requires the secretion of a surfactant peptide called SapB912.

Following erection of the aerial hyphae, chromo-some replication, segregation, cell division, cell wall assembly and various other fundamental processes are controlled in such a way that each multigenomic aerial hypha becomes synchronously divided by multiple septa into a long chain of unigenomic pre-spores (BOXES2,3; FIG.1). Finally, these box-like compartments further differentiate and mature before being released into the environment as dormant, thick-walled spores. When encountering suitable conditions, the spores germinate and grow out to produce a new vegetative mycelium.

MyceliumThe collective term for a mass of hyphae, such as the vegetative mycelium within the medium or the reproductive aerial mycelium on the colony surface.

Surfactant peptideA peptide that functions to reduce the surface tension of water.

cdiGMP signalling and the regulation of developmental transitions in streptomycetesMatthew J.Bush1, Natalia Tschowri2, Susan Schlimpert1, Klas Flrdh3 and Mark J.Buttner1

Abstract | The complex life cycle of streptomycetes involves two distinct filamentous cell forms: the growing (or vegetative) hyphae and the reproductive (or aerial) hyphae, which differentiate into long chains of spores. Until recently, little was known about the signalling pathways that regulate the developmental transitions leading to sporulation. In this Review, we discuss important new insights into these pathways that have led to the emergence of a coherent regulatory network, focusing on the erection of aerial hyphae and the synchronous cell division event that produces dozens of unigenomic spores. In particular, we highlight the role of cyclic di-GMP (c-di-GMP) in controlling the initiation of development, and the role of the master regulator BldD in mediating c-di-GMP signalling.

1Department of Molecular Microbiology, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK.2Department of Microbiology, Institute for Biology, Humboldt University, 10115 Berlin, Germany.3Department of Biology, Lund University, Slvegatan 35, 223 62 Lund, Sweden.Correspondence to M.J.Buttner e-mail: mark.buttner@jic.ac.ukdoi:10.1038/nrmicro3546Published online 26 October 2015

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Nature Reviews | Microbiology

Spore maturation Spore germination

Spore dispersal

Erection of aerial hyphae

Vegetative growthSporulation septation and chromosome segregation

SiderophoreOne of a class of small, high-affinity iron-chelating compounds that are secreted by bacteria and other microorganisms.

RegulonsSets of genes, each of which is under the direct control of a given transcription factor.

Second messengerOne of a class of small, intracellular molecules that relay a signal perceived from the outside of the cell to an effector protein (or riboswitch) within the cytoplasm.

PlanktonicA description for single cells that can float or swim in water.

The transition from vegetative mycelial growth to the formation of a reproductive aerial mycelium involves complex developmental decision making that is poorly understood. The decision is influenced by a range of extrinsic and intrinsic cues, including environmental conditions, nutrient availability, metabolism and extra-cellular signalling (for example, quorum sensing via -butyrolactones). Several previous reviews have dis-cussed the cues that affect the initiation of development in streptomycetes2,1318. Most recently, iron availability and siderophore production have been shown to have an important influence on the onset of differentiation19,20.

Once initiated, morphological differentiation pro-gresses along a genetically programmed developmental pathway, from the formation of the sporogenic aerial hyphae via multiple septum formation to the final release of dormant spores. The key developmental regu-lators that orchestrate this pathway have been identified and fall into two classes: Bld (bald) and Whi (white). Bld regulators are required for the formation of the hair-like reproductive aerial hyphae, and so deleterious mutations

in bld loci result in a bald phenotype that lacks these structures. Whi regulators are required for the differen-tiation of aerial hyphae into mature spores; these regula-tors are named for the colour of the colonies formed by whi mutants, which lack the spore pigment that gives mature streptomycete colonies their characteristic dark colour. Although these developmental regulators have been known for a long time, the regulons they control had remained largely uncharacterized. Therefore, the global signalling networks that control streptomycete development could not be elucidated and, as we noted in a previous review2, it remained unknown how the Bld and Whi regulators control the intriguing cell biological processes underlying morphogenesis in streptomycetes. For example, although it has long been known that the hydrophobic sheath is required for aerial mycelium for-mation3,4,8,12, the mechanism of activation of chaplin and rodlin expression was unclear. Since then, substantial progress has been made in mapping the regulatory net-works underpinning streptomycete development, and we can now, for the first time, connect the key developmen-tal regulators into a coherent regulatory network and associate specific regulators with defined cell biological and morphogenetic processes. In particular, substantial insights have been gained into the regulation of the two most striking stages of the developmental cycle, namely the erection of aerial hyphae and the division of aerial hyphae into unigenomic spores. A key advance was the discovery that the nucleotide second messenger cyclic di-GMP (c-di-GMP) functions as a major and overarching regulatory device controlling the switch from vegetative growth to sporulation in streptomycetes. The study of c-di-GMP as a second messenger is usually confined to unicellular Gram-negative bacteria, in which it controls the transition from a surface-associated, sessile lifestyle to a planktonic, motile lifestyle (stick or swim)2123. The finding that c-di-GMP has an entirely different role in streptomycetes, functioning as a signal to control mul-ticellular development2426, thus has broad implications.

In this Review, we place c-di-GMP into the context of the global network of factors that regulate the transitions between developmental stages in streptomycetes. We describe the unusual mechanism that partners c-di-GMP to the master regulator BldD at the top of the sporula-tion regulatory network, and we describe recent molecu-lar insights into the regulatory cascade downstream of BldD(c-di-GMP) that controls the developmental tran-sitions from vegetative mycelium to reproductive aerial hyphae and from aerial growth to sporulation. Finally, we consider the main outstanding questions that need to be addressed if we are to gain a comprehensive understand-ing of the molecular mechanisms underlying develop-mental transitions in streptomycetes. Much of the recent progress summarized here is derived from studies of the new model organism Streptomyces venezuelae (BOX3). By integrating results from S.venezuelae with studies from the two classical model systems Streptomyces coelicolor and Streptomyces griseus, we aim to provide a general description of development in streptomycetes; h