The colonization of land by plants was a key event in the evolution of life. Here we report the draft genome sequence of the filamentous terrestrial alga Klebsormidium flaccidum (Division Charophyta, Order Klebsormidiales) to elucidate the early transition step from aquatic algae to land plants. Comparison of the genome sequence with that of other algae and land plants demonstrate that K. flaccidum acquired many genes specific to land plants. We demonstrate that K. flaccidum indeed produces several plant hormones and homologues of some of the signalling intermediates required for hormone actions in higher plants. The K. flaccidum genome also encodes a primitive system to protect against the harmful effects of high-intensity light. The presence of these plant-related systems in K. flaccidum suggests that, during evolution, this alga acquired the fundamental machinery required for adaptation to terrestrial environments.

SummaryIt has been reported that eukaryotic organisms have a nonsense-mediated mRNA decay (NMD) system to exclude aberrant mRNAs that produce truncated proteins. NMD is an RNA surveillance pathway that degrades mRNAs possessing premature translation termination codons (PTCs), thus avoiding production of possibly toxic truncated proteins. Three interacting proteins, UPF1, UPF2 and UPF3, are required for NMD in mammals and yeasts, and their amino acid sequences are well conserved among most eukaryotes, including plants. In this study, 'The Arabidopsis Information Resource' database was searched for mRNAs with premature termination codons. We selected five of these mRNAs and checked for the presence of PTCs in these mRNAs when translated in vivo. As a result we identified aberrant mRNAs produced by alternative splicing for each gene. These genes produced at least one alternative splicing variant including a PTC (PTCþ) and another variant without a PTC (PTC)). We analyzed their PTCþ/PTC) ratios in wild-type Arabidopsis and upf3 mutant plants and showed that the PTCþ/PTC) ratios were higher in atupf3 mutant plants than wild-type plants and that the atupf3 mutant was less able to degrade mRNAs with premature termination codons than wild-type plants. This indicated that the AtUPF3 gene is required by the plant NMD system to obviate aberrantly spliced mRNA.

Sulfide was used as an electron donor early in the evolution of photosynthesis, with many extant photosynthetic bacteria still capable of using sulfur compounds such as hydrogen sulfide (H 2 S) as a photosynthetic electron donor. Although enzymes involved in H 2 S oxidation have been characterized, mechanisms of regulation of sulfide-dependent photosynthesis have not been elucidated. In this study, we have identified a sulfide-responsive transcriptional repressor, SqrR, that functions as a master regulator of sulfide-dependent gene expression in the purple photosynthetic bacterium Rhodobacter capsulatus. SqrR has three cysteine residues, two of which, C41 and C107, are conserved in SqrR homologs from other bacteria. Analysis with liquid chromatography coupled with an electrospray-interface tandem-mass spectrometer reveals that SqrR forms an intramolecular tetrasulfide bond between C41 and C107 when incubated with the sulfur donor glutathione persulfide. SqrR is oxidized in sulfidestressed cells, and tetrasulfide-cross-linked SqrR binds more weakly to a target promoter relative to unmodified SqrR. C41S and C107S R. capsulatus SqrRs lack the ability to respond to sulfide, and constitutively repress target gene expression in cells. These results establish that SqrR is a sensor of H 2 S-derived reactive sulfur species that maintain sulfide homeostasis in this photosynthetic bacterium and reveal the mechanism of sulfide-dependent transcriptional derepression of genes involved in sulfide metabolism.sulfide sensor | photosynthesis regulation | reactive sulfur species | purple bacteria | Rhodobacter T he discovery of ∼550 deep-sea hydrothermal vents more than 30 y ago (1) has led to the theory that energy metabolism in early ancestral organisms may have arisen from deep-sea hydrothermal vents where simple inorganic molecules such as hydrogen sulfide or hydrogen gas, as well as methane, exist (2-4). Such ancient energy metabolism has been assumed to be similar to that of extant chemolithotrophs, which obtain energy from these molecules. Indeed, various chemolithoautotrophic microbes thrive in deep-sea hydrothermal vents and are capable of oxidizing sulfides, methane, and/or hydrogen gas for use as energy sources and electron donors (5). Some photosynthetic bacteria have also been isolated from deep-sea hydrothermal vents that can grow photosynthetically using sulfide as an electron donor and geothermal radiation as an energy source instead of solar radiation (6), as hypothesized for ancestral phototrophs.Many purple photosynthetic bacteria have remarkable metabolic versatility required to meet the energy demands of sulfidedependent and -independent photosynthesis as well as aerobic and anaerobic respiration. These bacteria tightly control the synthesis of their electron transfer proteins involved in each growth mode in response to a specific electron donor, oxygen tension, and light intensity (7, 8). Among these regulatory systems, oxygen-and lightsensing mechanisms have been well-studied; however, mechanisms used to sen...

The SNF1-related protein kinase 2 (SnRK2) family includes key regulators of osmostress and abscisic acid (ABA) responses in angiosperms and can be classified into three subclasses. Subclass III SnRK2s act in the ABA response while ABA-nonresponsive subclass I SnRK2s are regulated through osmostress. Here we report that an ancient subclass III SnRK2-based signalling module including ABA and an upstream Raf-like kinase (ARK) exclusively protects the moss Physcomitrella patens from drought. Subclass III SnRK2s from both Arabidopsis and from the semiterrestrial alga Klebsormidium nitens, which contains all the components of ABA signalling except ABA receptors, complement Physcomitrella snrk2− mutants, whereas Arabidopsis subclass I SnRK2 cannot. We propose that the earliest land plants developed the ABA/ARK/subclass III SnRK2 signalling module by recruiting ABA to regulate a pre-existing dehydration response and that subsequently a novel subclass I SnRK2 system evolved in vascular plants conferring osmostress protection independently from the ancient system.