Cell wall structural changes lead to separation and shedding of biofouled epidermal cell wall layers by the brown alga Ascophyllum nodosum

Marine plants control the accumulation of biofouling organisms (epibionts) on their surfaces by various chemical and physical means. Ascophyllum nodosum is a perennial multicellular brown alga known to shed patches of epidermal material, thus removing epibionts and exposing unfouled surfaces to another cycle of colonization. While surface shedding is documented in multiple marine macroalgae, the cell and developmental biology of the phenomenon is almost unexplored. A previous investigation of Ascophyllum not only revealed regular cycles of epibiont accumulation and epidermal shedding but also stimulated the development of methods to detect the corresponding changes in epidermal (meristoderm) cells that are reported here. Confocal laser scanning microscopy of cell walls and cytoplasm fluorescently stained with Solophenyl Flavine 7GFE (Direct Yellow 96) and the lipophilic dye Rhodamine B (respectively) was combined with light and electron microscopy of chemically fixed or freeze-substituted tissues. As epibionts accumulated, epidermal cells generated thick, apical cell walls in which differentially stained central layers subsequently developed, marking the site of future cell wall separation. During cell wall separation, the outermost part of the cell wall and its epibionts plus the upper parts of the anticlinal walls between neighboring cells detached in a layer from multiple epidermal cells, exposing the remaining inner part of the cell wall to new colonizing organisms. These findings highlight the dynamic nature of apical cell wall structure and composition in response to colonizing organisms and lay a foundation for further investigations on the periodic removal of biofouling epibionts from the surface of Ascophyllum fronds.

The anisotropy1 D604N mutation in the Arabidopsis cellulose synthase1 catalytic domain reduces cell wall crystallinity and the velocity of cellulose synthase complexes

Multiple cellulose synthase (CesA) subunits assemble into plasma membrane complexes responsible for cellulose production. In the Arabidopsis (Arabidopsis thaliana) model system, we identified a novel D604N missense mutation, designated anisotropy1 (any1), in the essential primary cell wall CesA1. Most previously identified CesA1 mutants show severe constitutive or conditional phenotypes such as embryo lethality or arrest of cellulose production but any1 plants are viable and produce seeds, thus permitting the study of CesA1 function. The dwarf mutants have reduced anisotropic growth of roots, aerial organs, and trichomes. Interestingly, cellulose microfibrils were disordered only in the epidermal cells of the any1 inflorescence stem, whereas they were transverse to the growth axis in other tissues of the stem and in all elongated cell types of roots and dark-grown hypocotyls. Overall cellulose content was not altered but both cell wall crystallinity and the velocity of cellulose synthase complexes were reduced in any1. We crossed any1 with the temperature-sensitive radial swelling1-1 (rsw1-1) CesA1 mutant and observed partial complementation of the any1 phenotype in the transheterozygotes at rsw1-1's permissive temperature (21°C) and full complementation by any1 of the conditional rsw1-1 root swelling phenotype at the restrictive temperature (29°C). In rsw1-1 homozygotes at restrictive temperature, a striking dissociation of cellulose synthase complexes from the plasma membrane was accompanied by greatly diminished motility of intracellular cellulose synthase-containing compartments. Neither phenomenon was observed in the any1 rsw1-1 transheterozygotes, suggesting that the proteins encoded by the any1 allele replace those encoded by rsw1-1 at restrictive temperature.

Root hair-specific disruption of cellulose and xyloglucan in AtCSLD3 mutants, and factors affecting the post-rupture resumption of mutant root hair growth

The glycosyl transferase encoded by the cellulose synthase-like gene CSLD3/KJK/RHD7 (At3g03050) is required for cell wall integrity during root hair formation in Arabidopsis thaliana but it remains unclear whether it contributes to the synthesis of cellulose or hemicellulose. We identified two new alleles, root hair-defective (rhd) 7-1 and rhd7-4, which affect the C-terminal end of the encoded protein. Like root hairs in the previously characterized kjk-2 putative null mutant, rhd7-1 and rhd7-4 hairs rupture before tip growth but, depending on the growth medium and temperature, hairs are able to survive rupture and initiate tip growth, indicating that these alleles retain some function. At 21°C, the rhd7 tip-growing root hairs continued to rupture but at 5ºC, rupture was inhibited, resulting in long, wild type-like root hairs. At both temperatures, the expression of another root hair-specific CSLD gene, CSLD2, was increased in the rhd7-4 mutant but reduced in the kjk-2 mutant, suggesting that CSLD2 expression is CSLD3-dependent, and that CSLD2 could partially compensate for CSLD3 defects to prevent rupture at 5°C. Using a fluorescent brightener (FB 28) to detect cell wall (1 → 4)-β-glucans (primarily cellulose) and CCRC-M1 antibody to detect fucosylated xyloglucans revealed a patchy distribution of both in the mutant root hair cell walls. Cell wall thickness varied, and immunogold electron microscopy indicated that xyloglucan distribution was altered throughout the root hair cell walls. These cell wall defects indicate that CSLD3 is required for the normal organization of both cellulose and xyloglucan in root hair cell walls.

Remodeling the cytoskeleton for growth and form: an overview with some new views

The cytoskeleton coordinates all aspects of growth in plant cells, including exocytosis of membrane and wall components during cell expansion. This review seeks to integrate current information about cytoskeletal components in plants and the role they play in generating cell form. Advances in genome analysis have fundamentally changed the nature of research strategies and generated an explosion of new information on the cytoskeleton-associated proteins, their regulation, and their role in signaling to the cytoskeleton. Some of these proteins appear novel to plants, but many have close homologues in other eukaryotic systems. It is becoming clear that the mechanisms behind cell growth are essentially similar across the growth continuum, which ranges from tip growth to diffuse expansion. Remodeling of the actin cytoskeleton at sites of exocytosis is an especially critical feature of polarized and may also contribute to axial growth. We evaluate the most recent work on the signaling mechanisms that continually remodel the actin cytoskeleton via the activation of actin-binding proteins (ABPs) and consider the role the microtubule cytoskeleton plays in this process.

Growth and ultrastructure of Arabidopsis root hairs: the rhd3 mutation alters vacuole enlargement and tip growth

The root hairs of plants are tubular projections of root epidermal cells and are suitable for investigating the control of cellular morphogenesis. In wild-type Arabidopsis thaliana (L.) Heynh, growing root hairs were found to exhibit cellular expansion limited to the apical end of the cell, a polarized distribution of organelles in the cytoplasm, and vesicles of several types located near the growing tip. The rhd3 mutant produces short and wavy root hairs with an average volume less than one-third of the wild-type hairs, indicating abnormal cell expansion. The mutant hairs display a striking reduction in vacuole size and a corresponding increase in the relative proportion of cytoplasm throughout hair development. Beadlabeling experiments and ultrastructural analyses indicate that the wavy-hair phenotype of the mutant is caused by asymmetric tip growth, possibly due to abnormally distributed vesicles in cortical areas flanking the hair tips. It is suggested that a major effect of the rhd3 mutation is to inhibit vacuole enlargement which normally accompanies root hair cell expansion.

The homeobox gene GLABRA2 is required for position-dependent cell differentiation in the root epidermis of Arabidopsis thaliana

The role of the Arabidopsis homeobox gene, GLABRA 2 (GL2), in the development of the root epidermis has been investigated. The wild-type epidermis is composed of two cell types, root-hair cells and hairless cells, which are located at distinct positions within the root, implying that positional cues control cell-type differentiation. During the development of the root epidermis, the differentiating root-hair cells (trichoblasts) and the differentiating hairless cells (atrichoblasts) can be distinguished by their cytoplasmic density, vacuole formation, and extent of elongation. We have determined that mutations in the GL2 gene specifically alter the differentiation of the hairless epidermal cells, causing them to produce root hairs, which indicates that GL2 affects epidermal cell identity. Detailed analyses of these differentiating cells showed that, despite forming root hairs, they are similar to atrichoblasts of the wild type in their cytoplasmic characteristics, timing of vacuolation, and extent of cell elongation. The results of in situ nucleic acid hybridization and GUS reporter gene fusion studies show that the GL2 gene is preferentially expressed in the differentiating hairless cells of the wild type, during a period in which epidermal cell identity is believed to be established. These results indicate that the GL2 homeodomain protein normally regulates a subset of the processes that occur during the differentiation of hairless epidermal cells of the Arabidopsis root. Specifically, GL2 appears to act in a cell-position-dependent manner to suppress hair formation in differentiating hairless cells.

The TTG gene is required to specify epidermal cell fate and cell patterning in the Arabidopsis root

The control of cell fate was investigated in the root epidermis of Arabidopsis thaliana. Two distinct types of differentiated epidermal cells are normally present: root-hair-bearing cells and hairless cells. In wild-type Arabidopsis roots, epidermal cell fate was found to be correlated with cell position, with root-hair cells located over radial walls between cortical cells, and with hairless cells located directly over cortical cells. This normal positional relationship was absent in ttg (transparent testa glabrous) mutants (lacking trichomes, anthocyanins, and seed coat mucilage); epidermal cells in all positions differentiate into root-hair cells. The opposite condition was generated in roots of transgenic Arabidopsis expressing the maize R (R-Lc) gene product (a putative TTG homologue) under the control of a strong promoter (CaMV35S), which produced hairless epidermal cells in all positions. In both the ttg and R-expressing roots, epidermal cell differentiation was affected at an early stage, prior to the onset of cell elongation or root-hair formation. The ttg mutations were also associated with abnormalities in the morphology and organization of cells within and surrounding the root apical meristem. The results indicate that alterations in TTG activity cause developing epidermal cells to misinterpret their position and differentiate into inappropriate cell types. This suggests that, in wild-type roots, TTG provides, or responds to, positional signals to cause differentiating epidermal cells that lie over cortical cells to adopt a hairless cell fate.

Ultrastructure of the endocytotic pathway in glutaraldehyde-fixed and high-pressure frozen/freeze-substituted protoplasts of white spruce (Picea glauca)

An ultrastructural study of endocytosis has been made for the first time in protoplasts of a gymnosperm, white spruce (Picea glauca), fixed by high-pressure freezing and freeze substitution. Protoplasts derived from the WS1 line of suspension-cultured embryogenic white spruce were labelled with cationized ferritin, a non-specific marker of the plasma membrane. The timing of cationized ferritin uptake and its subcellular distribution were determined by fixing protoplasts at various intervals after labelling. To address concerns about using chemical fixation to study the membrane-bound transport of cationized ferritin, protoplasts were fixed both by conventional glutaraldehyde fixation and by rapid freezing in a Balzers high-pressure freezing apparatus (followed by freeze substitution). Cationized ferritin appeared rapidly in coated pits and coated vesicles after labelling. Later it was present in uncoated vesicles, and in Golgi bodies, trans-Golgi membranes and partially coated reticula, then subsequently in multivesicular bodies, which may ultimately fuse with and deliver their contents to lytic vacuoles. The results show that the time course and pathway of cationized ferritin uptake in the gymnosperm white spruce is very similar to the time course and pathway elucidated for cationized ferritin uptake in the angiosperm soybean. High-pressure freezing yielded much better preservation of intracellular membranes and organelles, although plasma membranes appeared ruffled. Protoplasts fixed by both methods possessed numerous smooth vesicles in the cortex and smooth invaginations of the plasma membrane. These became labelled with cationized ferritin, but apparently did not contribute directly to the internalization of cationized ferritin, except via the formation of coated pits and vesicles from their surfaces.