Manganese (Mn) is an important micronutrient for plant growth and development and sustains metabolic roles within different plant cell compartments. The metal is an essential cofactor for the oxygen-evolving complex (OEC) of the photosynthetic machinery, catalyzing the water-splitting reaction in photosystem II (PSII). Despite the importance of Mn for photosynthesis and other processes, the physiological relevance of Mn uptake and compartmentation in plants has been underrated. The subcellular Mn homeostasis to maintain compartmented Mn-dependent metabolic processes like glycosylation, ROS scavenging, and photosynthesis is mediated by a multitude of transport proteins from diverse gene families. However, Mn homeostasis may be disturbed under suboptimal or excessive Mn availability. Mn deficiency is a serious, widespread plant nutritional disorder in dry, well-aerated and calcareous soils, as well as in soils containing high amounts of organic matter, where bio-availability of Mn can decrease far below the level that is required for normal plant growth. By contrast, Mn toxicity occurs on poorly drained and acidic soils in which high amounts of Mn are rendered available. Consequently, plants have evolved mechanisms to tightly regulate Mn uptake, trafficking, and storage. This review provides a comprehensive overview, with a focus on recent advances, on the multiple functions of transporters involved in Mn homeostasis, as well as their regulatory mechanisms in the plant's response to different conditions of Mn availability.

Lignin is the defining constituent of wood and the second most abundant natural polymer on earth. Lignin is produced by the oxidative coupling of three monolignols: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. Monolignols are synthesized via the phenylpropanoid pathway and eventually polymerized in the cell wall by peroxidases and laccases. However, the mechanism whereby monolignols are transported from the cytosol to the cell wall has remained elusive. Here we report the discovery that AtABCG29, an ATP-binding cassette transporter, acts as a p-coumaryl alcohol transporter. Expression of AtABCG29 promoter-driven reporter genes and a Citrine-AtABCG29 fusion construct revealed that AtABCG29 is targeted to the plasma membrane of the root endodermis and vascular tissue. Moreover, yeasts expressing AtABCG29 exhibited an increased tolerance to p-coumaryl alcohol by excreting this monolignol. Vesicles isolated from yeasts expressing AtABCG29 exhibited a p-coumaryl alcohol transport activity. Loss-of-function Arabidopsis mutants contained less lignin subunits and were more sensitive to p-coumaryl alcohol. Changes in secondary metabolite profiles in abcg29 underline the importance of regulating p-coumaryl alcohol levels in the cytosol. This is the first identification of a monolignol transporter, closing a crucial gap in our understanding of lignin biosynthesis, which could open new directions for lignin engineering.

HghlightWe provide the first evidence for a mechanism of growth inhibition by ABA during germination and seedling establishment based on inhibition of PM H+-ATPase and altered pH, K+, and anion homeostasis.

Plants require trace levels of manganese (Mn) for survival, as it is an essential cofactor in oxygen metabolism, especially O production via photosynthesis and the disposal of superoxide radicals. These processes occur in specialized organelles, requiring membrane-bound intracellular transporters to partition Mn between cell compartments. We identified an member of the NRAMP family of divalent metal transporters, NRAMP2, which functions in the intracellular distribution of Mn. Two knockdown alleles of showed decreased activity of photosystem II and increased oxidative stress under Mn-deficient conditions, yet total Mn content remained unchanged. At the subcellular level, these phenotypes were associated with a loss of Mn content in vacuoles and chloroplasts. NRAMP2 was able to rescue the mitochondrial yeast mutant In plants, NRAMP2 is a resident protein of the-Golgi network. NRAMP2 may act indirectly on downstream organelles by building up a cytosolic pool that is used to feed target compartments. Moreover, not only does the mutant accumulate superoxide ions, but NRAMP2 can functionally replace cytosolic superoxide dismutase in yeast, indicating that the pool of Mn displaced by NRAMP2 is required for the detoxification of reactive oxygen species.

SummaryIn order to investigate the effects of a permanent increase in cellular H 2 O 2 on cation homeostasis we have studied a T-DNA insertion mutant of the Arabidopsis CATALASE 2 gene. This mutant (cat2-1) exhibits 20% of wild-type leaf catalase activity and accumulates more H 2 O 2 than the wild type under normal growth conditions. In addition to reduced size, a pale green color and great reduction in secondary roots, the cat2-1 mutant exhibited increased sensitivity to H 2 O 2 , NaCl, norspermidine, high light and cold stress. On the other hand, the germination of the cat2-1 mutant is more tolerant to lithium than the wild type. This novel phenotype cannot be explained by changes in lithium transport. Actually, the uptake of lithium (and of other toxic cations such as sodium and norspermidine) is increased in the cat2-1 mutant while K + levels were decreased. The lithium tolerance of this mutant seems to result both from insensitivity to the inhibitory ethylene induced by this cation and a reduced capability for ethylene production. Accordingly, induction by ethylene of responsive genes such as PR4 and EBP/ERF72 is decreased in cat2-1. Mutants insensitive to ethylene such as etr1-1 and ein3-3 are lithium tolerant, and inhibition of ethylene biosynthesis with 2-aminoisobutyrate protects against lithium toxicity. Microarray analysis of gene expression indicates that the expression of genes related to cation transport and ethylene synthesis and perception was not altered in the cat2-1 mutant, suggesting that H 2 O 2 modulates these processes at the protein level. These results uncover a cross-talk between oxidative stress, cation homeostasis and ethylene.

Drought and salt are major abiotic stresses that adversely affect crop productivity. Thus identification of factors that confer resistance to these stresses would pave a way to increasing agricultural productivity. When grown on soil in greenhouse longer than 5 weeks, transgenic Arabidopsis plants that over-express an ABC (ATP-binding cassette) transporter, AtABCG36/AtPDR8, produced higher shoot biomass and less chlorotic leaves than the wild type. We investigated whether the improved growth of AtABCG36-overexpressing plants was due to their improved resistance to abiotic stresses, and found that AtABCG36-over-expressing plants were more resistant to drought and salt stress and grew to higher shoot fresh weight than the wild type. In contrast, T-DNA insertional knockout lines were more sensitive to drought stress than the wild type and were reduced in shoot fresh weight. To understand the mechanism of enhanced salt and drought resistance of the AtABCG36-over-expressing plants, we measured sodium contents, and found that AtABCG36-over-expressing plants were lower in sodium content than the wild type. Our data suggest that AtABCG36 contributes to drought and salt resistance in Arabidopsis by a mechanism that includes reduction of sodium content in plants. Overexpression of AtABCG36 improves drought and salt stress resistance in Arabidopsis AbstractDrought and salt are major abiotic stresses that adversely affect crop productivity. Thus identification of factors that confer resistance to these stresses would pave a way to increasing agricultural productivity. When grown on soil in greenhouse longer than 5 weeks, transgenic Arabidopsis plants that over-express AtABCG36/AtPDR8 produced higher shoot biomass and less chlorotic leaves than the wild type. We investigated whether the improved growth of AtABCG36-overexpressing plants was due to their improved resistance to abiotic stresses, and found that AtABCG36-over-expressing plants were more resistant to drought and salt stress and grew to higher shoot fresh weight than the wild type. In contrast, T-DNA insertional knockout lines were more sensitive to drought stress than the wild type and were reduced in shoot fresh weight. To understand the mechanism of enhanced salt and drought resistance of the AtABCG36-over-expressing plants, we measured sodium contents, and found that AtABCG36-over-expressing plants were lower in sodium content than the wild type. Our data suggest that AtABCG36 contributes to drought and salt resistance in Arabidopsis by a mechanism that includes reduction of sodium content in plant.

Calcium (Ca2+) and manganese (Mn2+) are essential elements for plants and have similar ionic radii and binding coordination. They are assigned specific functions within organelles, but share many transport mechanisms to cross organellar membranes. Despite their points of interaction, those elements are usually investigated and reviewed separately. This review takes them out of this isolation. It highlights our current mechanistic understanding and points to open questions of their functions, their transport, and their interplay in the endoplasmic reticulum (ER), vesicular compartments [Golgi apparatus, trans-Golgi Network (TGN), prevacuolar compartment (PVC)], vacuoles, chloroplasts, mitochondria, and peroxisomes. Complex processes demanding these cations, such as Mn2+-dependent glycosylation or systemic Ca2+ signaling, are covered in some detail if they have not been reviewed recently or if recent findings add to current models. The function of Ca2+ as signaling agent released from organelles into the cytosol and within the organelles themselves is a recurrent theme of this review, again keeping the interference by Mn2+ in mind. The involvement of organellar channels [e.g., Glutamate-Receptor-Likes (GLRs), Cyclic-Nucleotide-Gated Channels (CNGCs), Mitochondrial Conductivity Units (MCUs), Two-Pore Channel1 (TPC1)], transporters [e.g., Natural-Resistance-Associated Macrophage Proteins (NRAMPs), Calcium Exchangers (CAXs), Metal-Tolerance Proteins (MTPs), Bivalent-Cation Transporters (BICATs)] and pumps [Autoinhibited Ca2+-ATPases (ACAs), ER Ca2+-ATPases (ECAs)] in the import and export of organellar Ca2+ and Mn2+ is scrutinized, whereby current controversial issues are pointed out. Mechanisms in animals and yeast are taken into account where they may provide a blueprint for processes in plants, in particular with respect to tunable molecular mechanisms of Ca2+-versus-Mn2+ selectivity.

SUMMARYIntracellular pH (pH i ) is a crucial parameter in cellular physiology but its mechanisms of homeostasis are only partially understood. To uncover novel roles and participants of the pH i regulatory system, we have screened an Arabidopsis mutant collection for resistance of seed germination to intracellular acidification induced by weak organic acids (acetic, propionic, sorbic). The phenotypes of one identified mutant, weak acid-tolerant 1-1D (wat1-1D) are due to the expression of a truncated form of AP-3 b-adaptin (encoded by the PAT2 gene) that behaves as a as dominant-negative. During acetic acid treatment the root epidermal cells of the mutant maintain a higher pH i and a more depolarized plasma membrane electrical potential than wild-type cells. Additional phenotypes of wat1-1D roots include increased rates of acetate efflux, K + uptake and H + efflux, the latter reflecting the in vivo activity of the plasma membrane H + -ATPase. The in vitro activity of the enzyme was not increased but, as the H + -ATPase is electrogenic, the increased ion permeability would allow a higher rate of H + efflux. The AP-3 adaptor complex is involved in traffic from Golgi to vacuoles but its function in plants is not much known. The phenotypes of the wat1-1D mutant can be explained if loss of function of the AP-3 b-adaptin causes activation of channels or transporters for organic anions (acetate) and for K + at the plasma membrane, perhaps through miss-localization of tonoplast proteins. This suggests a role of this adaptin in trafficking of ion channels or transporters to the tonoplast.