The discovery of florigen will lead the studies on floral transition, especially from the morphological point of view. The Japanese Society of Plant Morphology held the symposium namely “ From flowering to inflorescence and flower bud formation ” during the 71 st Annual Meeting of the Botanical Society of Japan as the co-organized meeting of the two societies. The symposium was aimed to connect physiological processes with morphological events during and after flowering. This special issue includes the mini-reviews related to this symposium.

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After long pursuit for 70 years, the flowering signal named “ florigen” turned out to be FT protein, a 20-kDasoluble protein of the PEBP/RKIP family. FT protein is expressed in the companion cells of leaf vasculatures under favorable external(e.g.photoperiod, light quality, temperature)and internal(e.g.age, size)conditions, and is transported from leaves to shoot apex via phloem. At the shoot apex, FT protein acts with transcription factors such as FD to activate the floral program. FT is conserved among seed plants, and its expression is regulated by a conserved pathway(“ CO/FT module”)as well as other pathways unique to specific groups of plants. Recent findings that FT protein is involved in environmental(photoperiod and temperature)regulation of tuberization in potato, and bud set in angiosperm and conifer trees may suggest its universal nature of regulatory signal possibly acting across a long distance.

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In vernalization, the effect of coldness which the plants perceived at early developmental stage emerges after they matured. The stable memory like this effect of coldness in vernalization is mediated by epigenetics which is regulated by DNA methylation and/or histone modification in the related genes followed by stable change in chromatin structure and gene expression.The expression of vernalization-integral gene, FLOWERING LOCUS C(FLC)of Arabidopsis thaliana is controlled by various epigenetical regulations. On the other hand, the involvement of epigenetics was not assumed for photoperiodic flowering because the flowering state in most of the plant species does not last long. In Perilla frutescens, however, once-induced flowering state is quite stable as the effect of inductive photoperiod lasts long even after the induced plants were moved to under non-inductive photoperiodic condition. This memory of inductive photoperiodic cycle suggests involvement of the mechanism similar to that in vernalization, namely epigenetics. Our study revealed that the treatment with a DNA demethylating reagent induced the flowering of P. frutescens and some other plant species, suggesting that photoperiodic flowering is also regulated by epigenetical mechanism. We summarize epigenetics in vernalization and also in the photoperiodic flowering.

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Post-embryonic morphogenesis largely depends on generation of new meristems, called axillary meristems. Generation of axillary meristems allow plants the continued elaboration of their morphology. Inflorescences of grass species are composed of different types of branches, which are generated as axillary meristems. Therefore, patterns of axillary meristem formation, branch identity determination and gowth of branches define the basic architecture of grass inflorescences. Here, I overview the current state of our knowledge on genetic mechanisms controlling branching of grass species.

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The inflorescence architecture in Arabidopsis thaliana accession Columbia is a raceme with individualflowers developing acropetally. The accession Landsberg erecta(er)displays a corymb-like inflorescence. To understand the regulation of inflorescence architecture, we screened corymb-like inflorescence mutants, corymbosal(crm1)and crm2. We focused crm1 mutant and found that reduction of cell elongation in crml pedicels and stem internodes. crml is additive with er or crm2, suggesting CRM1, ER, and CRM2 control inflorescence development independently. Fine mapping and complementation test confirmed that crm1-1is a new allele of BIG, which is required for normal auxin transport. Histochemical analysis and in situ hybridization revealed that CRM1/BIG is highly expressed in inflorescence meristem, floral metistem and vasculature in pedicels and internodes. The decreased levels of auxin-responsive DRS and increased levels of PIN-FORMED1 (PIN1)were observed in crm1-1and wild type treated with auxin transport inhibitor. Furthermore, auxin transport inhibitor treatment also led to the elongation defects of pedicel cells, suggesting that the defect of CRM1/BIG-mediated auxin transport causes these changes. Expression analysis revealed that LEAFY(LFY)and CUP-SHAPED COTYLEDON2(CUC2) were increased in crm1-1mutant. These data strongly suggested the possibility that the cause of morphological change in crm1is not only due to the defect in auxin transport and requirement of examination of relationship between CRM1/BIG and these genes.

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The explanation for why sex is so common as a reproductive strategy continues to resist understanding despite many years of theoretical and experimental work. Our recent empirical works have addressed key questions in this field, especially regarding 1)a novel mitochondrial plasmid named mF, which is a mitochondrial linear plasmid of Physarum polycephalum, 2)sexual asymmetry of cell fusion site and/or mating structure positions for determining gamete mating type of Ulva compressa, 3)sexual modifications of a host plant, Silene latifolia, by a parasite, Microbotryum violaceum, which is known as the dimorphic smut fungus. Mitochondrial DNA is inherited maternally in most eukaryotes. The mF evades uniparental mitochondrial inheritance, the parental mitochondria fused to yield giant mitochondria with two or more mitochondrial nucleoids. The mF appears to exchange mitochondria from the recipient(paternal)to the donor(maternal)by promoting mitochondrial fusion. Such features remind us also of the bacterial conjugative plasmids such as F plasmid. Conjugation between cells was the initial stage in the evolution of sex. The “ molecular symbiont” of selfish DNA(such as the mF and F plasmids)derive a major selective advantage from conjugation and sexual outbreeding. Such selfish molecular-symbiont theory is directly relevant only to the origin of sex, and it also has implicationsfor understanding the subsequent stages in the evolution of sex. At the same time, in this issue, new ideas about origin and evolution of sex, including asymmetrical features of male and female of U. compressa and a parasite disturbing sexuality of S. latifolia, are introduced and discussed intriguing an important interplay between sex and genetic architecture in their own evolution.

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Summary: Eukaryotic sex was initially isogametic and oogamy has evolved independently in many lineages including animals, land plants and volvocine green algae. A number of questions have been proposed for evolution of oogamy from isogamy. However, the point had not been reached where either question has been answered with molecular-genetic data until our recent study of the male-specific gene“ OTOKOGI”(PlestMID). In that study, we have opened a highly promising new route for elucidating the molecular nature of the pathway leading from isogamy to oogamy in a group of volvocine green algae. In the close relative Chlamydomonas reinhardtii, sexual reproduction is isogametic with mating-types plus(MT+)and minus(MT-). MT-represents a“ dominant sex” because MID (“ minus-dominance”)gene of C. reinhardtii is both necessary and sufficient to cause the cells to differentiate as MT-isogametes. However, no sex-specific genes had been identified in the volvocine green algae until we successively cloned the MID gene of an anisogamous or oogamous volvocine alga, Pleodorina. starrii. This“ OTOKOGI” gene is present only in the male genome, and encodes a protein localized abundantly in the nuclei of mature male gametes(sperm). Thus, P. starrii maleness evolved from the dominant sex(MT-)of its isogamous ancestor. This breakthrough provides an opportunity to address any number of extremely interesting questions regarding evolution of oogamy and male-female dichotomy.

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Summary: The axial growth comprise the development of plant form. For comprehensive analysis on the axial growth, it is necessary to approach both of its diversity and basis. Studies on specific axial growths, which is variation in shoot organization in Prunus, development and structure of trichotomous branching in Edgeworthia chrysantha, and floral development of Ceratophyllum demersum, show the diversity of axial growth and may give clues to re-evaluate its classical model. Kinematic studies on root growth in Arabidopsis thaliana have elucidated the cellular basis of axial growth. Notably, the kinematic analysis combined with a simple mathematical model dissects the coordination of cell proliferation and volume growth, and is an effective tool of quantitatively analyzing essential factors responsible for growth change by environmental and genetic factors.

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Summary: Aggregation of the cellular slime mould Dictyostelium discoideum is known to be triggered by starvation of the nutrients. Ax-2, an axenically growing strain, can be maintained with the liquid medium containing peptone, yeast extracts and glucose. Peptone and yeast extracts are known to be essential nutrients. However, in the case of shaking culture in the liquid medium with no glucose, cells are able to grow, although the speed of growth is somewhat reduced. In this study, we tested the effect of glucose starvation with static culture in the culture dishes. We found that Ax-2 cells start aggregating over the density of 7.5 X 105 cells/ml in the glucose-deprived liquid medium. Application of glucose in the medium quickly interrupted the process and aggregates were rapidly dissociated. Finally, the cells remained as single vegetative amoebae. These results suggest that glucose-starvation can trigger aggregation of vegetative cells in certain culture conditions. In order to find the genes involved in the process of aggregation triggered by glucose-starvation, we isolated a REMI-mutant clone that does not aggregate in the glucose-deprived medium and identified the mutated gene as abpE. AbpE has been reported to be involved in the process of pseudopod formation. It implies that proper pseudopod formation is necessary for cells to aggregate in the nutrient medium.

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