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Phycobilisome diversity and evolution

By comparing Synechococcus genomes, candidate genes required for the production of phycobiliproteins, which are part of the lightharvesting antenna complexes called phycobilisomes, were identified. Phylogenetic analyses suggest that the phycobilisome core evolved together with the core genome, whereas rods evolved independently.

Cilia and flagella have essential functions in a wide range of organisms. Cilia assembly is dynamic during development and different types of cilia are found in multicellular organisms. How this dynamic and specific assembly is regulated remains an important question in cilia biology. In metazoans, the regulation of the overall expression level of key components necessary for cilia assembly or function is an important way to achieve ciliogenesis control. The FOXJ1 (forkhead box J1) and RFX (regulatory factor X) family of transcription factors have been shown to be important players in controlling ciliary gene expression. They fulfill a complementary and synergistic function by regulating specific and common target genes. FOXJ1 is essential to allow for the assembly of motile cilia in vertebrates through the regulation of genes specific to motile cilia or necessary for basal body apical transport, whereas RFX proteins are necessary to assemble both primary and motile cilia in metazoans, in particular, by regulating genes involved in intraflagellar transport. Recently, different transcription factors playing specific roles in cilia biogenesis and physiology have also been discovered. All these factors are subject to complex regulation to allow for the dynamic and specific regulation of ciliogenesis in metazoans.

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SummaryRFX3 governs growth and beating efficiency of motile cilia in mouse and controls the expression of genes involved in human ciliopathies

In contrast to vertebrate CBY, which functions in WNT signaling, Drosophila CBY is essential for normal basal body structure and function but dispensable for Wg signaling.

Abstract. Some forest plants adapt to shade by mixotrophy, i.e., they obtain carbon both from photosynthesis and from their root mycorrhizal fungi. Fully achlorophyllous species using exclusively fungal carbon (the so-called mycoheterotrophic plants) have repeatedly evolved from such mixotrophic ancestors. However, adaptations for this evolutionary transition, and the reasons why it has happened a limited number of times, remain unknown. We investigated this using achlorophyllous variants (i.e., albinos) spontaneously occurring in Cephalanthera damasonium, a mixotrophic orchid. In two populations, we compared albinos with co-occurring green individuals in situ. We investigated vegetative traits, namely, shoot phenology, dormancy, CO 2 and H 2 O leaf exchange, mycorrhizal colonization, degree of mycoheterotrophy (using 13 C abundance as a proxy), and susceptibility to pathogens and herbivores. We monitored seed production (in natural or experimental crosses) and seed germination. Albinos displayed (1) more frequent shoot drying at fruiting, possibly due to stomatal dysfunctions, (2) lower basal metabolism, (3) increased sensitivity to pathogens and herbivores, (4) higher dormancy and maladapted sprouting, and, probably due to the previous differences, (5) fewer seeds, with lower germination capacity. Over the growing season, green shoots shifted from using fungal carbon to an increasingly efficient photosynthesis at time of fruiting, when fungal colonization reached its minimum. Conversely, the lack of photosynthesis in fruiting albinos may contribute to carbon limitation, and to the abovementioned trends. With a 10 3 3 fitness reduction, albinos failed a successful transition to mycoheterotrophy because some traits inherited from their green ancestors are maladaptive. Conversely, mycoheterotrophy requires at least degeneration of leaves and stomata, optimization of the temporal pattern of fungal colonization and shoot sprouting, and new defenses against pathogens and herbivores. Transition to mycoheterotrophy likely requires progressive, joint evolution of these traits, while a sudden loss of photosynthesis leads to unfit plants. We provide explanations for the evolutionary stability of mixotrophic nutrition and for the rarity of emergence of carbon sinks in mycorrhizal networks. More broadly, this may explain what prevents the emergence of fully heterotrophic taxa in the numerous other mixotrophic plant or algal lineages recently described.

Chromatic adaptation (CA) in cyanobacteria has provided a model system for the study of the environmental control of photophysiology for several decades. All forms of CA that have been examined so far (types II and III) involve changes in the relative contents of phycoerythrin (PE) and/or phycocyanin when cells are shifted from red to green light and vice versa. However, the chromophore compositions of these polypeptides are not altered. Some marine Synechococcus species strains, which possess two PE forms (PEI and PEII), carry out another type of CA (type IV), occurring during shifts from blue to green or white light. Two chromatically adapting strains of marine Synechococcus recently isolated from the Gulf of Mexico were utilized to elucidate the mechanism of type IV CA. During this process, no change in the relative contents of PEI and PEII was observed. Instead, the ratio of the two chromophores bound to PEII, phycourobilin and phycoerythrobilin, is high under blue light and low under white light. Mass spectroscopy analyses of isolated PEII ␣-and ␤-subunits show that there is a single PEII protein type under all light climates. The CA process seems to specifically affect the chromophorylation of the PEII (and possibly PEI) ␣ chain. We propose a likely process for type IV CA, which involves the enzymatic activity of one or several phycobilin lyases and/or lyase-isomerases differentially controlled by the ambient light quality. Phylogenetic analyses based on the 16S rRNA gene confirm that type IV CA is not limited to a single clade of marine Synechococcus.Members of the genus Synechococcus are ubiquitous and ecologically important in marine ecosystems (19,24,28). This form genus, which includes freshwater, obligatory marine, and halotolerant strains, is clearly polyphyletic. However, based on 16S rRNA gene sequence analysis, most obligatory marine representatives fall into a single monophyletic group called "subcluster 5.1," previously termed "marine cluster A" (8,14). Like most other cyanobacteria, all members of Synechococcus subcluster 5.1 (hereinafter referred to as "marine Synechococcus") utilize phycobilisomes (PBSs) as their major light-harvesting antenna system. PBSs are large macromolecular complexes that consist of a core in direct contact with the stromal surface of the thylakoid membrane, and this core is surrounded by six radiating rods. The major constituents of PBSs are several classes of chromophore-binding proteins called phycobiliproteins (PBPs). In all PBS-containing cyanobacteria, the PBP components of the core and proximal parts of the rods are allophycocyanin (AP) and phycocyanin (PC), respectively. The PBP components constituting the distal parts of rods are more varied. All marine Synechococcus sp. strains but those belonging to clades VI and VIII (8) are thought to possess two different phycoerythrin (PE) forms, termed PEI and PEII. The latter is specific to this cyanobacterial group and is located at a distal position within the rods. Each PBP binds one or two types of chromophores (p...