Takashi Araki

Flagella and cilia are conserved eukaryotic organelles whose formation depends on membrane trafficking, but how this is coupled to axonemal architecture and functional diversification remains unclear. RAB23, a small GTPase known for its role in Sonic hedgehog signaling, has an elusive function in ciliogenesis because its loss is embryonic lethal in animals. Here we analyze RAB23 in the liverwort Marchantia polymorpha, where motile flagella are confined to spermatozoids. Loss of MpRAB23 causes severe defects in spermatozoid morphology, motility, and 9 + 2 axonemal organization, while leaving vegetative growth unaffected. During spermiogenesis, MpRAB23 localizes to elongating flagella and is required for flagellar membrane organization, including polarized localization of MpPOFCAP, a calcium pump confined to the posterior flagellum. Disruption of MpPOFCAP reduces spermatozoid motility and fertility without affecting axonemal structure. These findings identify RAB23 as a regulator of flagellar biogenesis and uncover a specific mechanism by which asymmetric membrane protein targeting differentiates structurally similar flagella within a single cell.

Never-in-mitosis A-related kinases (NEKs) are conserved protein kinases in eukaryotes that regulate cell division and elongation. In Arabidopsis thaliana, NEK6 has been shown to control anisotropic growth through cortical microtubule depolymerization, but the functions of other NEK family members remain largely unknown. Here, we show that Arabidopsis NEKs redundantly regulate organ growth and flowering through phosphorylation-dependent mechanisms. Expression analyses revealed overlapping promoter activities of NEK genes in meristems, vascular tissues, and floral organs. While most single mutants showed no obvious phenotype, multiple mutants exhibited defects in root growth direction, leaf elongation, vascular formation, and floral transition. Biochemical analysis showed that NEK3 and NEK6 phosphorylate both β-tubulin and γ-tubulin, indicating a conserved role in microtubule regulation. In addition, several NEKs interacted with the florigen FLOWERING LOCUS T (FT), and NEK3 phosphorylated FT in vitro and inhibited the FT–FD interaction in transient assays, suggesting a role in flowering regulation. These findings show that Arabidopsis NEKs redundantly coordinate vegetative and reproductive development through phosphorylation-dependent regulation of microtubules and the flowering pathway.

Gametogenesis, which is essential to the sexual reproductive system, has drastically changed during plant evolution. Bryophytes, lycophytes and ferns develop reproductive organs called gametangia—antheridia and archegonia for sperm and egg production, respectively. However, the molecular mechanism of early gametangium development remains unclear. Here we identified a ‘non-canonical’ type of BZR/BES transcription factor, MpBZR3, as a regulator of gametangium development in a model bryophyte, Marchantia polymorpha. Interestingly, overexpression of MpBZR3 induced ectopic gametangia. Genetic analysis revealed that MpBZR3 promotes the early phase of antheridium development in male plants. By contrast, MpBZR3 is required for the late phase of archegonium development in female plants. We demonstrate that MpBZR3 is necessary for the successful development of both antheridia and archegonia but functions in a different manner between the two sexes. Together, the functional specialization of this ‘non-canonical’ type of BZR/BES member may have contributed to the evolution of reproductive systems.

A revision of the genus Apterodela Rivalier, 1950 is presented. The new subgenus Protoapterodela subgen. nov. with Apterodela shirakii (W. Horn, 1927) as its type species is established. The lectotype and the paralectotype of Cicindela ovipennis Bates, 1883 and Cicindela shirakii W. Horn, 1927 as well as the neotype of Cicindela lobipennis Bates, 1888 are designated. Apterodela (s. str.) bivirgulata (Fairmaire, 1889), stat. rest. is extracted from the synonym of Apterodela (s. str.) lobipennis (Bates, 1888) and is considered as a separate species. Two new species and one new subspecies from China are described as well: Apterodela (s. str.) alopecomma sp. nov.—from Sichuan Province, Apterodela (s. str.) latissima sp. nov.—from Yunnan Province, and Apterodela (s. str.) bivirgulata occidentalis ssp. nov.—from Qinghai, Gansu, Shaanxi, Henan, Shanxi, Sichuan Provinces, and Inner Mongolia Autonomus Region. A key to identifying the species of the genus Apterodela is provided, and its taxonomic status is examined.

Daylength is perceived as a seasonal cue to induce growth-phase transition at a proper time of a year. The core of the mechanism of daylength measurement in angiosperms lies in the circadian clock-controlled expression of regulators of growth-phase transition. However, the roles of the circadian clock in daylength measurement in basal land plants remain largely unknown. In this study, we investigated the contribution of circadian clock to daylength measurement in a basal land plant, the liverwort Marchantia polymorpha. In M. polymorpha, transition from vegetative to reproductive phase under long-day conditions results in differentiation of sexual branches called gametangiophores which harbor gametangia. First, we showed that a widely used wild-type accession Takaragaike-1 is an obligate long-day plant with a critical daylength of about 10 hours and requires multiple long days. Then, we compared the timing of gametangiophore formation between wild type and circadian clock mutants in long-day and short-day conditions. Mutations in two clock genes, MpTIMING OF CAB EXPRESSION 1 and MpPSEUDO-RESPONSE REGULATOR, had no significant effects on the timing of gametangiophore formation. In addition, when M. polymorpha plants were treated with a chemical which lengthens circadian period, there was no significant effect on the timing of gametangiophore formation, either. We next observed the timing of gametangiophore formation under various non-24-h light/dark cycles to examine the effect of phase alteration in circadian rhythms. The results suggest that daylength measurement in M. polymorpha is based on the relative amount of light and darkness within a cycle rather than the intrinsic rhythms generated by circadian clock. Our findings suggest that M. polymorpha has a daylength measurement system which is different from that of angiosperms centered on the circadian clock function.

The circadian clock allows plants to anticipate and adapt to periodic environmental changes. Organ- and tissue-specific properties of the circadian clock as well as shoot-to-root circadian signaling have been reported. While this long-distance signaling is thought to coordinate physiological functions across tissues, little is known about the feedback regulation of root clock on the shoot clock in the hierarchical circadian network. Here, we show that the plant circadian clock conveys circadian information between shoots and roots through sucrose and K⁺. We also demonstrate that K⁺ transport from roots suppress the variance of period length in shoots and then improve the accuracy of shoot circadian clock. Sucrose measurements and qPCR showed that root sucrose accumulation was regulated by the circadian clock. Furthermore, root circadian clock genes, including PSEUDO-RESPONSE REGULATOR7 (PRR7), were regulated by sucrose, suggesting the involvement of sucrose from the shoot in the regulation of root clock gene expression. Therefore, we performed time-series measurements of xylem sap and micrografting experiments using prr7 mutants and showed that root PRR7 regulates K⁺ transport and suppresses variance of period length in the shoot. Our modeling analysis supports the idea that root-to-shoot signaling contributes to the precision of the shoot circadian clock. We performed micrografting experiments that illustrated how root PRR7 plays key roles in maintaining the accuracy of shoot circadian rhythms. We thus present a novel directional signaling pathway for circadian information from roots to shoots and propose that plants modulate physiological events in a timely manner through various timekeeping mechanisms.

Key message The liverwort Marchantia polymorpha regulates gametangia and gametangiophore development by using evolutionarily conserved regulatory modules that are shared with angiosperm mechanisms regulating flowering and germ cell differentiation. Abstract Bryophytes, the earliest diverged lineage of land plants comprised of liverworts, mosses, and hornworts, produce gametes in gametangia, reproductive organs evolutionarily conserved but lost in extant angiosperms. Initiation of gametangium development is dependent on environmental factors such as light, although the underlying mechanisms remain elusive. Recent studies showed that the liverwort Marchantia polymorpha regulates development of gametangia and stalked receptacles called gametangiophores by using conserved regulatory modules which, in angiosperms, are involved in light signaling, microRNA-mediated flowering regulation, and germ cell differentiation. These findings suggest that these modules were acquired by a common ancestor of land plants before divergence of bryophytes, and were later recruited to flowering mechanism in angiosperms.