Shu-Zon Wu

Transformation of an ancient cyanobacterium into permanent chloroplasts in photosynthetic eukaryotes had profound effects in altering Earth’s atmosphere and biosphere. Chloroplast biogenesis depends on both protein import and organelle division, yet how their coordination emerged remains unclear. Here, we show that the single septin SEP1 in Chlamydomonas plays roles in both processes. SEP1 forms a filamentous network on the chloroplast surface during interphase and reorganizes into a ring at the chloroplast division site. SEP1-knockout in actin-depleted conditions impairs proper import and localization of the chloroplast-division proteins ARC6 and FTSZ1/2. SEP1 physically associates with evolutionarily related TOC159-family translocon GTPases. Phylogenetic analysis places TOC GTPases within an algal-septin clade, and SEP1 retains chloroplast localization and TOC association in heterologous plant systems. These results identify chloroplast-associated septin as an organizer of protein import and division, and suggest that co-option of an ancestral septin contributed to the early evolution of host control of plastids.

Complex signaling pathways organize cell expansion and proliferation across cells to pattern tissues and organs in plants. The sulfotyrosine peptide hormone family, PLANT PEPTIDE CONTAINING SULFATED TYROSINE (PSY), contributes to these processes. We identified two plasma membrane-localized receptors, PSYR1 and PSYR2, that are necessary for PSY signaling and regulate growth in Physcomitrium patens . Membrane-associated PSYRs accumulate to high levels in a mutant lacking TYROSYL PROTEIN SULFOTRANSFERASE (TPST). Given that a tpst null mutant ( ∆tpst ) is impaired in sulfation, this suggests that in the absence of sulfated peptides, PSYRs accumulate on the membrane. A null mutant of the PSY receptors, ∆psyr1/2 , showed increased growth and was epistatic to ∆tpst , suppressing defects in gametophore formation and early senescence. The transcriptional profiles comparing wild type to ∆psyr1/2 and ∆psyr1/2/∆tpst showed 25 to 30 differentially expressed genes between the receptor null mutants and wild type, with a common signature of cell wall remodeling and stress responses. Similarly, a PSYR1 kinase-inactive mutation rescued ∆tpst and relieved the accumulation of membrane-associated PSYRs. In contrast, overexpression of PSYRs inhibited plant growth, with phenotypic severity correlating with the amount of overexpression. These data are consistent with a constitutive activation model in which membrane-associated PSYRs unbound to PSY serve to inhibit growth through an active kinase. In the presence of the PSY peptide, the kinase is inactivated, promoting growth and driving PSY expression. The relationship between growth-repressive PSYR kinase activity and growth-promoting PSYR kinase inactivation in P. patens serves as a model for optimizing plant growth and development.

In plants, the cell wall fixes the position of each cell; therefore, during development, plants rely on cellular proliferation and expansion for tissue patterning and organ formation. How plant cells communicate with neighboring cells to coordinate expansion for properly patterned tissues and organs is not well understood. In seed plants, organ growth is known to be modulated by sulfotyrosyl peptide signaling. Here, we report that the activity of TYROSYL PROTEIN SULFOTRANSFERASE (TPST), which is responsible for the post-translational modification of sulfotyrosyl peptides, is essential for expansion during development in the non-vascular plant, Physcomitrium patens. Plants that harbor a null mutation in the gene encoding TPST (Δtpst) were smaller, formed fewer caulonemal filaments, and were unable to form expanded gametophores. In Δtpst multiple aspects of gametophore development were affected, including the first division of the gametophore initial, as well as reduced rates of cell division and expansion. Mutational analysis of P. patens TPST identified the residue Histidine 124, a candidate catalytic residue, as essential for TPST function. Notably, addition of the sulfated signaling peptide, PSY1 from either P. patens or Arabidopsis thaliana, rescued all Δtpst developmental deficits. Taken together, these data suggest that TPST functions to sulfate PSY, and this activity is necessary for plant growth and development. Furthermore, since addition of AtPSY fully rescues Δtpst and PpPSY promotes root elongation in Arabidopsis and rice, these findings suggest that PSY signaling is evolutionarily conserved.

Chloroplast biogenesis depends on both protein import and organelle division, yet how their coordination emerged during evolution remains unclear. Here, we show that the single septin SEP1 links these pathways in the green alga Chlamydomonas reinhardtii . SEP1 forms a filamentous network on the chloroplast envelope during interphase and reorganizes into a ring at the chloroplast division site during cytokinesis. Loss of SEP1 selectively impairs import of chloroplast-division proteins and causes mispositioning of the division ring, without impairing bulk chloroplast protein import. SEP1 physically associates with outer-envelope TOC GTPases through evolutionarily related GTPase domains. Phylogenetic analysis places TOC GTPases within an algal septin-derived clade, and heterologous expression of SEP1 in land plants, in which septins are absent, shows conservation of its chloroplast targeting and TOC binding. Together, these findings identify septins as coordinators of plastid protein import and division and suggest that this coupling emerged early in chloroplast evolution.

Because plant cells cannot migrate, their position is fixed with respect to their neighbors. Thus, placement of the new cell plate is critical for determining cell shape, cell identity, and tissue patterning. Here, we show that TONNEAU1 (TON1), a protein essential for preprophase band formation, plays a role in phragmoplast guidance even in cells that do not have a microtubule-based preprophase band. TON1 is part of a complex that contains TONNEAU2/FASS, PROTEIN PHOSPHATASE 2A (PP2A), and the TON1 RECRUITING MOTIF (TRM) proteins, known as the TON1-TRM-PP2A (TTP) complex. We found that TON1 localizes specifically to the plasma membrane along the branch-adjacent wall in a branching cell in Physcomitrium patens juvenile tissue. Knocking out FASS does not affect TON1 localization, suggesting that TON1 recruitment to the plasma membrane is independent of FASS. A novel point mutation in TON1 disrupts dimerization and localization to the plasma membrane. In mutants lacking TTP function, myosin VIII localization is altered, resulting in phragmoplast guidance defects. Our data suggest a model whereby myosin VIII guides the phragmoplast in a TTP-dependent manner to the branch-adjacent wall facilitating efficient insertion at that site.

Polarized growth drives the morphogenesis of elongated cellular structures. In plants, polarized growth depends on actin and a tip focused ionic calcium gradient. How the calcium gradient is maintained remains unclear. We discovered that autoinhibitory calcium ATPases (ACAs) redundantly contribute to the steepness of the calcium gradient. ACA1 and ACA2 localize to the subapical plasma membrane and ACA5 to the vacuole membrane, providing spatial regulation of calcium efflux. Tip-growing plant cells also exhibit apical calcium fluctuations. Even though Δaca1/2/5 cells have a diminished calcium gradient, they exhibit normal fluctuations and actin but have significantly reduced apical secretion. Furthermore, cells lacking apical actin retain a strong calcium gradient but have reduced apical secretion. Suppression of both the calcium gradient and apical actin dramatically impairs growth, supporting a model where two independent and parallel processes, the calcium gradient and apical actin, promote rapid polarized growth.

Similar to cellulose synthases (CESAs), cellulose synthase–like D (CSLD) proteins synthesize β-1,4-glucan in plants. CSLDs are important for tip growth and cytokinesis, but it was unknown whether they form membrane complexes in vivo or produce microfibrillar cellulose. We produced viable CESA-deficient mutants of the moss Physcomitrium patens to investigate CSLD function without interfering CESA activity. Microscopy and spectroscopy showed that CESA-deficient mutants synthesize cellulose microfibrils that are indistinguishable from those in vascular plants. Correspondingly, freeze-fracture electron microscopy revealed rosette-shaped particle assemblies in the plasma membrane that are indistinguishable from CESA-containing rosette cellulose synthesis complexes (CSCs). Our data show that proteins other than CESAs, most likely CSLDs, produce cellulose microfibrils in P. patens protonemal filaments. The data suggest that the specialized roles of CSLDs in cytokinesis and tip growth are based on differential expression and different interactions with microtubules and possibly Ca ²⁺ , rather than structural differences in the microfibrils they produce.

Cellulose Synthase-Like D (CSLD) proteins, important for tip growth and cell division, are known to generate β-1,4-glucan. However, whether they are propelled in the membrane as the glucan chains they produce assemble into microfibrils is unknown. To address this, we endogenously tagged all eight CSLDs in Physcomitrium patens and discovered that they all localize to the apex of tip-growing cells and to the cell plate during cytokinesis. Actin is required to target CSLD to cell tips concomitant with cell expansion, but not to cell plates, which depend on actin and CSLD for structural support. Like Cellulose Synthase (CESA), CSLD requires catalytic activity to move in the plasma membrane. We discovered that CSLD moves significantly faster, with shorter duration and less linear trajectories than CESA. In contrast to CESA, CSLD movement was insensitive to the cellulose synthesis inhibitor isoxaben, suggesting that CSLD and CESA function within different complexes possibly producing structurally distinct cellulose microfibrils.

Coat Protein complex II (COPII), a coat protein complex that forms vesicles on the endoplasmic reticulum (ER), mediates trafficking to the Golgi. While metazoans have few genes encoding each COPII component, plants have expanded these gene families, leading to the hypothesis that plant COPII has functionally diversified. In the moss Physcomitrium (Physcomitrella) patens, the Sec23/24 gene families are each composed of seven genes. Silencing Sec23/24 revealed isoform-specific contributions to polarized growth, with the closely related Sec23D/E and Sec24C/D essential for protonemal development. Focusing on Sec23, we discovered that Sec23D/E mediate ER-to Golgi transport and are essential for tip growth, with Sec23D localizing to presumptive ER exit sites. In contrast, Sec23A, B, C, F, and G are dispensable and do not quantitatively affect ER-to-Golgi trafficking. However, Δsec23abcfg plants exhibited reduced secretion of plasma membrane cargo. Of the four highly expressed protonemal Sec23 genes, Sec23F/G are members of a divergent Sec23 clade specifically retained in land plants. Notably, Sec23G accumulates on ER-associated foci that are significantly larger, do not overlap with, and are independent of Sec23D. While Sec23D/E form ER exit sites and function as bona fide COPII components essential for tip-growing protonemata, Sec23G and the closely related Sec23F have likely functionally diversified, forming separate and independent ER exit sites and participating in Golgi-independent trafficking pathways.

COPII, a coat of proteins that form vesicles on the ER, mediates vesicle traffic from the ER to the Golgi. In contrast to metazoans that have few genes encoding each COPII component, plants have expanded these gene families leading to the hypothesis that plant COPII has functionally diversified. Here, we analyzed the gene families encoding for the Sec23/24 heterodimer in the moss Physcomitrium (Physcomitrella) patens . In P. patens, Sec23 and Sec24 gene families are each comprised of seven genes. Silencing the Sec23/24 genes revealed isoform specific contributions to polarized growth, with the closely related Sec23D/E and Sec24C/D essential for protonemal development. Focusing on the Sec23 gene family, we discovered that loss of Sec23D alters ER morphology, increases ER stress, inhibits trafficking to the Golgi and to the plasma membrane in tip growing protonemata. In contrast, the remaining five Sec23 genes are dispensable for tip growth. While Sec23A/B/C/F/G do not quantitatively affect ER to Golgi trafficking in protonemata, they do contribute to secretion to the plasma membrane. Of the three highly expressed Sec23 isoforms in protonemata, Sec23G forms ER exit sites that are larger than Sec23B and Sec23D and do not overlap with Sec23D. Furthermore, ER exit sites labeled by Sec23B or Sec23G form in the absence of Sec23D. These data suggest that Sec23D/E form unique ER exit sites contributing to secretion that is essential for tip growing protonemata.