Asymmetric wall ingrowth deposition in Arabidopsis phloem parenchyma transfer cells is tightly associated with sieve elements

In Arabidopsis, polarized deposition of wall ingrowths in phloem parenchyma (PP) transfer cells (TCs) occurs adjacent to cells of the sieve element/companion cell (SE/CC) complex. However, the spatial relationships between these different cell types in minor veins, where phloem loading occurs, are poorly understood. PP TC development and wall ingrowth localization were compared to other phloem cells in leaves of Col-0 and the transgenic lines AtSUC2::AtSTP9-GFP and AtSWEET11::AtSWEET11-GFP that identify CCs and PP respectively. The development of PP TCs in minor veins, indicated by deposition of wall ingrowths, proceeded basipetally in leaves. However, not all PP develop ingrowths and higher levels of wall ingrowth deposition occur in abaxial- compared to adaxial-positioned PP TCs. Furthermore, the deposition of wall ingrowths was exclusively initiated on and preferentially covered the PP TC/SE interface, rather than the PP TC/CC interface, and only occurred in PP cells that were adjacent to SEs. Collectively, these results demonstrate the dominant impact of SEs on wall ingrowth deposition in PP TCs and suggest the existence of two sub-types of PP cells in leaf minor veins. Compared to PP cells, PP TCs showed more abundant accumulation of AtSWEET11-GFP, indicating functional differences in phloem loading between PP and PP TCs.

OPT3 plays a central role in both copper and iron homeostasis and signaling due to its ability to deliver copper as well as iron to the phloem in Arabidopsis

Copper and iron are micronutrients but are toxic when they accumulate in cells in excess. Crosstalk between copper and iron homeostasis in Arabidopsis thaliana has been documented and includes iron accumulation under copper deficiency and vice versa. However, molecular components of this crosstalk are not well understood. Iron concentration in the phloem has been suggested to act systemically, negatively regulating iron uptake to the root. Consistently, systemic iron signaling is disrupted in A. thaliana mutants lacking the phloem companion cell-localized iron transporter, AtOPT3, and opt3 mutants hyperaccumulate iron. Here, we report that in addition to iron, AtOPT3 transports copper and mediates copper loading to the phloem for delivery from sources to sinks. As a result of this function, the opt3-3 mutant accumulates less copper in the phloem, roots, developing leaves and embryos compared to wild type, is sensitive to copper deficiency, and mounts transcriptional copper deficiency response. Because copper deficiency has been shown to stimulate iron accumulation, we propose that reduced copper concentration in the phloem of the opt3-3 mutant and its constitutive copper deficiency contribute to iron overaccumulation in its tissues. Our data assign new transport capabilities to AtOPT3 and increase understanding of copper - iron interactions and signaling.

Ultrastructural and Histochemical Changes in Glyphosate-Tolerant Soybean Leaves Exposed to Glyphosate

Is it transgenic soy, resistant to glyphosate, does not suffer any injury or stress in contact with this herbicide? Anatomic studies of plant tissue are necessary to answer this question. This study investigated the influence of glyphosate in glyphosate-resistant soybean plants by analysis of leaf ultrastructure and histochemistry in a morphophysiological context. The experiment was carried out in a greenhouse, using RR soybean seeds (Glycine max (L.) Merrill, cultivar BRS Valiosa) in pots containing vermiculite and washed sand (1:1). Between the phenological stages V2 and V4, two treatments with glyphosate [N-(fosfonometil) glicina] were sprayed once a week: recommended dose (5.0 mg ae plant-1) and control (0.0 mg ae plant-1), with four repetitions each. Samples of midrib and internervural area of the leaves were fixed, dehydrated in ethyl series and blocks were sectioned at a 5-10 μm thickness. The material was stained with toluidine blue 0.05% and blades mounted on “Entellan”. Glyphosate decreased the thickness of the adaxial epidermis, palisade parenchyma, spongy parenchyma and total thickness of the leaf. Although, the diameter of companion cell was decreased with herbicide treatment, the diameter of the vase element increased, also increasing the size of the vascular bundle. Ultrastructural and histochemical changes caused by glyphosate can extend dysfunctions in the metabolic apparatus and plant relationship with the environment, given the inter-relation between tissue structure and its functions.

Natural depletion of histone H1 in sex cells causes DNA demethylation, heterochromatin decondensation and transposon activation

Transposable elements (TEs), the movement of which can damage the genome, are epigenetically silenced in eukaryotes. Intriguingly, TEs are activated in the sperm companion cell – vegetative cell (VC) – of the flowering plant Arabidopsis thaliana. However, the extent and mechanism of this activation are unknown. Here we show that about 100 heterochromatic TEs are activated in VCs, mostly by DEMETER-catalyzed DNA demethylation. We further demonstrate that DEMETER access to some of these TEs is permitted by the natural depletion of linker histone H1 in VCs. Ectopically expressed H1 suppresses TEs in VCs by reducing DNA demethylation and via a methylation-independent mechanism. We demonstrate that H1 is required for heterochromatin condensation in plant cells and show that H1 overexpression creates heterochromatic foci in the VC progenitor cell. Taken together, our results demonstrate that the natural depletion of H1 during male gametogenesis facilitates DEMETER-directed DNA demethylation, heterochromatin relaxation, and TE activation.

Sieve element biology provides leads for research on phytoplasma lifestyle in plant hosts

Phytoplasmas reside exclusively in sieve tubes, tubular arrays of sieve element–companion cell complexes. Hence, the cell biology of sieve elements may reveal (ultra)structural and functional conditions that are of significance for survival, propagation, colonization, and effector spread of phytoplasmas. Electron microscopic images suggest that sieve elements offer facilities for mobile and stationary stages in phytoplasma movement. Stationary stages may enable phytoplasmas to interact closely with diverse sieve element compartments. The unique, reduced sieve element outfit requires permanent support by companion cells. This notion implies a future focus on the molecular biology of companion cells to understand the sieve element–phytoplasma inter-relationship. Supply of macromolecules by companion cells is channelled via specialized symplasmic connections. Ca2+-mediated gating of symplasmic corridors is decisive for the communication within and beyond the sieve element–companion cell complex and for the dissemination of phytoplasma effectors. Thus, Ca2+ homeostasis, which affects sieve element Ca2+ signatures and induces a range of modifications, is a key issue during phytoplasma infection. The exceptional physical and chemical environment in sieve elements seems an essential, though not the only factor for phytoplasma survival.

Natural depletion of H1 in sex cells causes DNA demethylation, heterochromatin decondensation and transposon activation

Transposable elements (TEs), the movement of which can damage the genome, are epigenetically silenced in eukaryotes. Intriguingly, TEs are activated in the sperm companion cell – vegetative cell (VC) – of the flowering plant Arabidopsis thaliana. However, the extent and mechanism of this activation are unknown. Here we show that about 100 heterochromatic TEs are activated in VCs, mostly by DEMETER-catalyzed DNA demethylation. We further demonstrate that DEMETER access to some of these TEs is permitted by the natural depletion of linker histone H1 in VCs. Ectopically expressed H1 suppresses TEs in VCs by reducing DNA demethylation and via a methylation-independent mechanism. We demonstrate that H1 is required for heterochromatin condensation in plant cells and show that H1 overexpression creates heterochromatic foci in the VC progenitor cell. Taken together, our results demonstrate that the natural depletion of H1 during male gametogenesis facilitates DEMETER-directed DNA demethylation, heterochromatin relaxation, and TE activation.