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Key messageSQUAMOSA promoter-binding protein-like 7 mediates copper deficiency response in the presence of high nitrogen even with the sufficient level of copper in Arabidopsis thaliana.Under copper (Cu) deficiency, accumulation of mRNA encoding two Cu/Zn superoxide dismutases, CSD1 and CSD2, is downregulated to save Cu for plastocyanin. This downregulation depends on miR398 and is under the control of SQUAMOSA promoter-binding protein-like7 (SPL7). Arabidopsis seedlings are routinely cultured on Murashige and Skoog medium. However, the high nitrogen (N) content of the medium (60 mM) has been shown to induce a similar response to Cu deficiency. The mRNA and protein levels of CSD1 and CSD2 are reduced under high N conditions, even if the Cu concentration in the medium is sufficient (0.1–0.5 µM). In this study, we show that this symptom, similar to the Cu deficiency, occurred in the presence of high N largely depending on SPL7, suggesting that plants actually sensed Cu deficiency. However, a change in N concentration in the medium did not influence the total Cu concentration in shoots or roots. High N did not increase the protein content in leaves but facilitated rapid seedling growth. We speculate that this rapid growth causes a continuous Cu deficiency mainly because of high Cu uptake by mesophyll cells in the leaves. This idea was supported by the observation that plastocyanin did not overaccumulate at the range of 0.1–0.5 µM Cu with 30 mM N. In contrast, in the presence of 5 µM Cu with 30 mM N, plants accumulate more Cu in plastocyanin in the thylakoid lumen, resulting in a slight Cu deficiency in the chloroplast stroma. This process is independent of SPL7.

Ultrasmall algae have attracted the attention of biologists investigating the basic mechanisms underlying living systems. Their potential as effective organisms for producing useful substances is also of interest in bioindustry. Although genomic information is indispensable for elucidating metabolism and promoting molecular breeding, many ultrasmall algae remain genetically uncharacterized. Here, we present the nuclear genome sequence of an ultrasmall green alga of freshwater habitats, Medakamo hakoo . Evolutionary analyses suggest that this species belongs to a new genus within the class Trebouxiophyceae. Sequencing analyses revealed that its genome, comprising 15.8 Mbp and 7629 genes, is among the smallest known genomes in the Viridiplantae. Its genome has relatively few genes associated with genetic information processing, basal transcription factors, and RNA transport. Comparative analyses revealed that 1263 orthogroups were shared among 15 ultrasmall algae from distinct phylogenetic lineages. The shared gene sets will enable identification of genes essential for algal metabolism and cellular functions. Multi-omic and evolutionary analyses of the ultrasmall green algae, Medakomo hakoo , suggest that it might belong to a new genus within the class Trebouxiophyceae and provide further insight into the essential genes for microalgae.

BackgroundVolvox carteri f. nagariensis is a model taxon that has been studied extensively at the cellular and molecular level. The most distinctive morphological attribute of V. carteri f. nagariensis within V. carteri is the production of sexual male spheroids with only a 1:1 ratio of somatic cells to sperm packets or androgonidia (sperm packet initials). However, the morphology of male spheroids of V. carteri f. nagariensis has been examined only in Japanese strains. In addition, V. carteri f. nagariensis has heterothallic sexuality; male and female sexes are determined by the sex-determining chromosomal region or mating-type locus composed of a > 1 Mbp linear chromosome. Fifteen sex-specific genes and many sex-based divergent shared genes (gametologs) are present within this region. Thus far, such genes have not been identified in natural populations of this species.ResultsDuring a recent fieldwork in Taiwan, we encountered natural populations of V. carteri that had not previously been recorded from Taiwan. In total, 33 strains of this species were established from water samples collected in Northern Taiwan. Based on sequences of the internal transcribed spacer 2 region of nuclear ribosomal DNA and the presence of asexual spheroids with up to 16 gonidia, the species was clearly identified as V. carteri f. nagariensis. However, the sexual male spheroids of the Taiwanese strains generally exhibited a 1:1 to > 50:1 ratio of somatic cells to androgonidia. We also investigated the presence or absence of several sex-specific genes and the sex-based divergent genes MAT3m, MAT3f and LEU1Sm. We did not identify recombination or deletion of such genes between the male and female mating-type locus haplotypes in 32 of the 33 strains. In one putative female strain, the female-specific gene HMG1f was not amplified by genomic polymerase chain reaction. When sexually induced, apparently normal female sexual spheroids developed in this strain.ConclusionsMale spheroids are actually variable within V. carteri f. nagariensis. Therefore, the minimum ratio of somatic cells to androgonidia in male spheroids and the maximum number of gonidia in asexual spheroids may be diagnostic for V. carteri f. nagariensis. HMG1f may not be directly related to the formation of female spheroids in this taxon.

Co-ordination between plastids and mitochondria is so essential that there should be extensive inter-organellar crosstalk during development of plant cells. Indeed, chloroplast dysfunction in white leaves of plastid ribosome-deficient mutant barley, albostrians, is reportedly accompanied by increases in the levels of mitochondrial DNA and mitochondrial transcripts, suggesting that (i) developmental/physiological status of plastids (or heterotrophic growth condition of albino leaves) can affect the status of mitochondrial genome, and (ii) the function of the affected mitochondria may also be up-regulated accordingly. However, functional aspects of the mitochondria affected by chloroplast dysfunction have not yet been examined in detail. Here, we examined the effects of chloroplast dysfunction on mitochondrial DNA level and dark respiration rate, by comparing white and green sectors within individual variegated leaves, using 12 ornamental plants as experimental materials. The pattern of leaf variegation differed from species to species, suggesting that different mechanisms were involved in the formation of white sectors in different species. Quantitative hybridization analysis revealed that mitochondrial DNA levels were generally higher in white sectors than in green sectors. In spite of the elevated mitochondrial DNA levels, however, dark respiration rates in white sectors were generally lower than those in green sectors. Several possible mechanisms for elevation of mitochondrial DNA level and suppression of dark respiration rates in white sectors are discussed.

Light regulates the expression and function of aquaporins, which are involved in water and solute transport. In Arabidopsis thaliana, mRNA levels of one of the aquaporin genes, TIP2;2, increase during dark adaptation and decrease under far-red light illumination, but the effects of light at the protein level and on the mechanism of light regulation remain unknown. Numerous studies have described the light regulation of aquaporin genes, but none have identified the regulatory mechanisms behind this regulation via specific photoreceptor signaling. In this paper, we focus on the role of phytochrome A (phyA) signaling in the regulation of the TIP2;2 protein. We generated Arabidopsis transgenic plants expressing a TIP2;2-GFP fusion protein driven by its own promoter, and showed several differences in TIP2;2 behavior between wild type and the phyA mutant. Fluorescence of TIP2;2-GFP protein in the endodermis of roots in the wild-type seedlings increased during dark adaptation, but not in the phyA mutant. The amount of the TIP2;2-GFP protein in wild-type seedlings decreased rapidly under far-red light illumination, and a delay in reduction of TIP2;2-GFP was observed in the phyA mutant. Our results imply that phyA, cooperating with other photoreceptors, modulates the level of TIP2;2 in Arabidopsis roots.

Compaction of bulky DNA is a universal issue for all DNA-based life forms. Chloroplast nucleoids (chloroplast DNA–protein complexes) are critical for chloroplast DNA maintenance and transcription, thereby supporting photosynthesis, but their detailed structure remains enigmatic. Our proteomic analysis of chloroplast nucleoids of the green alga Chlamydomonas reinhardtii identified a protein (HBD1) with a tandem repeat of two DNA-binding high mobility group box (HMG-box) domains, which is structurally similar to major mitochondrial nucleoid proteins transcription factor A, mitochondrial (TFAM), and ARS binding factor 2 protein (Abf2p). Disruption of the HBD1 gene by CRISPR-Cas9–mediated genome editing resulted in the scattering of chloroplast nucleoids. This phenotype was complemented when intact HBD1 was reintroduced, whereas a truncated HBD1 with a single HMG-box domain failed to complement the phenotype. Furthermore, ectopic expression of HBD1 in the mitochondria of yeast Δabf2 mutant successfully complemented the defects, suggesting functional similarity between HBD1 and Abf2p. Furthermore, in vitro assays of HBD1, including the electrophoretic mobility shift assay and DNA origami/atomic force microscopy, showed that HBD1 is capable of introducing U-turns and cross-strand bridges, indicating that proteins with two HMG-box domains would function as DNA clips to compact DNA in both chloroplast and mitochondrial nucleoids.

Pleodorina sphaerica Iyengar was considered to be a phylogenetic link between Volvox and the type species Pleodorina californica Shaw because it has small somatic cells distributed from the anterior to posterior poles in 64- or 128-celled vegetative colonies. However, cultural studies and molecular and ultrastructural data are lacking in P. sphaerica, and this species has not been recorded since 1951. Here, we performed light and electron microscopy and molecular phylogeny of P. sphaerica based on newly established culture strains originating from Thailand. Morphological features of the present Thai species agreed well with those of the previous studies of the Indian material of P. sphaerica and with those of the current concept of the advanced members of the Volvocaceae. The present P. sphaerica strains exhibited homothallic sexuality; male and facultative female colonies developed within a single clonal culture. Chloroplast multigene phylogeny demonstrated that P. sphaerica was sister to two other species of Pleodorina (P. californica and Pleodorina japonica Nozaki) without posterior somatic cells, and these three species of Pleodorina formed a robust clade, which was positioned distally in the large monophyletic group including nine taxa of Volvox sect. Merrillosphaera and Volvox (sect. Janetosphaera) aureus Ehrenberg. Based on the present phylogenetic results, evolutionary losses of posterior somatic cells might have occurred in the ancestor of P. californica and P. japonica. Thus, P. sphaerica might represent an ancestral morphology of Pleodorina, rather than of Volvox.

Here we report the complete organellar genome sequences of Medakamo hakoo, a green alga identified in freshwater in Japan. It has 90.8-kb plastid and 36.5-kb mitochondrial genomes containing 80 and 33 putative protein coding genes, respectively, representing the smallest organellar genome among currently known core Trebouxiophyceae.

Iyengar was considered to be a phylogenetic link between and the type species Shaw because it has small somatic cells distributed from the anterior to posterior poles in 64- or 128-celled vegetative colonies. However, cultural studies and molecular and ultrastructural data are lacking in , and this species has not been recorded since 1951. Here, we performed light and electron microscopy and molecular phylogeny of based on newly established culture strains originating from Thailand. Morphological features of the present Thai species agreed well with those of the previous studies of the Indian material of and with those of the current concept of the advanced members of the Volvocaceae. The present strains exhibited homothallic sexuality; male and facultative female colonies developed within a single clonal culture. Chloroplast multigene phylogeny demonstrated that was sister to two other species of ( and Nozaki) without posterior somatic cells, and these three species of formed a robust clade, which was positioned distally in the large monophyletic group including nine taxa of sect. and (sect. ) Ehrenberg. Based on the present phylogenetic results, evolutionary losses of posterior somatic cells might have occurred in the ancestor of and . Thus, might represent an ancestral morphology of , rather than of .