Significance The endodermis acts as a “second skin” in plant roots by providing the cellular control necessary for the selective entry of water and mineral nutrients into the vascular system. To enable such control, Casparian strips span the cell wall of adjacent endodermal cells to form a tight junction that blocks diffusion across the endodermis in the cell wall. This junction is composed of a fine band of lignin, the polymer that gives wood its strength. Here, we characterize a dirigent protein (from Latin, dirigere : to guide or align) as playing a vital role in the patterning of lignin in the Casparian strip, identifying a new component of the molecular machinery that builds Casparian strips.

The endodermis in roots acts as a selectivity filter for nutrient and water transport essential for growth and development. This selectivity is enabled by the formation of lignin-based Casparian strips. Casparian strip formation is initiated by the localization of the Casparian strip domain proteins (CASPs) in the plasma membrane, at the site where the Casparian strip will form. Localized CASPs recruit Peroxidase 64 (PER64), a Respiratory Burst Oxidase Homolog F, and Enhanced Suberin 1 (ESB1), a dirigent-like protein, to assemble the lignin polymerization machinery. However, the factors that control both expression of the genes encoding this biosynthetic machinery and its localization to the Casparian strip formation site remain unknown. Here, we identify the transcription factor, MYB36, essential for Casparian strip formation. MYB36 directly and positively regulates the expression of the Casparian strip genes CASP1, PER64, and ESB1. Casparian strips are absent in plants lacking a functional MYB36 and are replaced by ectopic lignin-like material in the corners of endodermal cells. The barrier function of Casparian strips in these plants is also disrupted. Significantly, ectopic expression of MYB36 in the cortex is sufficient to reprogram these cells to start expressing CASP1–GFP, correctly localize the CASP1–GFP protein to form a Casparian strip domain, and deposit a Casparian strip-like structure in the cell wall at this location. These results demonstrate that MYB36 is controlling expression of the machinery required to locally polymerize lignin in a fine band in the cell wall for the formation of the Casparian strip.

The endodermis represents the main barrier to extracellular diffusion in plant roots, and it is central to current models of plant nutrient uptake. Despite this, little is known about the genes setting up this endodermal barrier. In this study, we report the identification and characterization of a strong barrier mutant, schengen3 (sgn3). We observe a surprising ability of the mutant to maintain nutrient homeostasis, but demonstrate a major defect in maintaining sufficient levels of the macronutrient potassium. We show that SGN3/GASSHO1 is a receptor-like kinase that is necessary for localizing CASPARIAN STRIP DOMAIN PROTEINS (CASPs)—major players of endodermal differentiation—into an uninterrupted, ring-like domain. SGN3 appears to localize into a broader band, embedding growing CASP microdomains. The discovery of SGN3 strongly advances our ability to interrogate mechanisms of plant nutrient homeostasis and provides a novel actor for localized microdomain formation at the endodermal plasma membrane.DOI: http://dx.doi.org/10.7554/eLife.03115.001

Accumulation of cadmium (Cd) in rice (Oryza sativa L.) grains poses a potential health problem, especially in Asia. Most Cd in rice grains accumulates through phloem transport, but the molecular mechanism of this transport has not been revealed. In this study, we identified a rice Cd transporter, OsLCT1, involved in Cd transport to the grains. OsLCT1-GFP was localized at the plasma membrane in plant cells, and OsLCT1 showed Cd efflux activity in yeast. In rice plants, strong OsLCT1 expression was observed in leaf blades and nodes during the reproductive stage. In the uppermost node, OsLCT1 transcripts were detected around large vascular bundles and in diffuse vascular bundles. RNAi-mediated knockdown of OsLCT1 did not affect xylem-mediated Cd transport but reduced phloem-mediated Cd transport. The knockdown plants of OsLCT1 accumulated approximately half as much Cd in the grains as did the control plants. The content of other metals in rice grains and plant growth were not negatively affected by OsLCT1 suppression. These results suggest that OsLCT1 functions at the nodes in Cd transport into grains and that in a standard japonica cultivar, the regulation of OsLCT1 enables the generation of "low-Cd rice" without negative effects on agronomical traits. These findings identify a transporter gene for phloem Cd transport in plants.heavy metals | food safety

We present paleoclimate evidence for rapid ( < 100 years) shifts of f 2-4 jC in Chesapeake Bay (CB) temperature f2100, 1600, 950, 650, 400 and 150 years before present (years BP) reconstructed from magnesium/calcium (Mg/Ca) paleothermometry. These include large temperature excursions during the Little Ice Age (f1400 -1900 AD) and the Medieval Warm Period (f800 -1300 AD) possibly related to changes in the strength of North Atlantic thermohaline circulation (THC). Evidence is presented for a long period of sustained regional and North Atlantic-wide warmth with low-amplitude temperature variability between f450 and 1000 AD. In addition to centennial-scale temperature shifts, the existence of numerous temperature maxima between 2200 and 250 years BP (average f70 years) suggests that multi-decadal processes typical of the North Atlantic Oscillation (NAO) are an inherent feature of late Holocene climate. However, late 19th and 20th century temperature extremes in Chesapeake Bay associated with NAO climate variability exceeded those of the prior 2000 years, including the interval 450 -1000 AD, by 2 -3 jC, suggesting anomalous recent behavior of the climate system. D

SummaryTwo genes in the rice genome were identi®ed as those encoding the g subunits, g1 and g2, of heterotrimeric G proteins. Using antibodies against the recombinant proteins for the a, b, g1, and g2 subunits of the G protein complexes, all of the subunits were proven to be localized in the plasma membrane in rice. Gel ®ltration of solubilized plasma membrane proteins showed that all of the a subunits were present in large protein complexes (about 400 kDa) containing the other subunits, b, g1, and g2, and probably also some other proteins, whereas large amounts of the b and g (g1 and g2) subunits were freed from the large complexes and took a 60-kDa form. A yeast two-hybrid assay and coimmunoprecipitation experiments showed that the b subunit interacted tightly with the g1 and g2 subunits, and so the b and g subunits appeared to form dimers in rice cells. Some dimers were associated with the a subunit, because few b, g1, and g2 subunits were present in the 400-kDa complexes in a rice mutant, d1, which was lacking in the a subunit. When a constitutively active form of the a subunit was prepared by the exchange of one amino acid residue and introduced into d1, the mutagenized subunit was localized in the plasma membrane of the transformants and took a free, and not the 400-kDa, form.

Manganese (Mn) is an essential micronutrient for plants, but is toxic when present in excess. The rice plant (Oryza sativa L.) accumulates high concentrations of Mn in the aerial parts; however, the molecular basis for Mn tolerance is poorly understood. In the present study, genes encoding Mn tolerance were screened for by expressing cDNAs of genes from rice shoots in Saccharomyces cerevisiae. A gene encoding a cation diffusion facilitator (CDF) family member, OsMTP8.1, was isolated, and its expression was found to enhance Mn accumulation and tolerance in S. cerevisiae. In plants, OsMTP8.1 and its transcript were mainly detected in shoots. High or low supply of Mn moderately induced an increase or decrease in the accumulation of OsMTP8.1, respectively. OsMTP8.1 was detected in all cells of leaf blades through immunohistochemistry. OsMTP8.1 fused to green fluorescent protein was localized to the tonoplast. Disruption of OsMTP8.1 resulted in decreased chlorophyll levels, growth inhibition in the presence of high concentrations of Mn, and decreased accumulation of Mn in shoots and roots. However, there was no difference in the accumulation of other metals, including Zn, Cu, Fe, Mg, Ca, and K. These results suggest that OsMTP8.1 is an Mn-specific transporter that sequesters Mn into vacuoles in rice and is required for Mn tolerance in shoots.

In this study, three independent As(III)-tolerant mutants were isolated from ethyl methanesulfonate-mutagenized M2 seeds of Arabidopsis thaliana. All three mutants carried independent mutations in Nodulin 26-like intrinsic protein 1;1 (NIP1;1), a homolog of an aquaporin. Two independent transgenic lines carrying T-DNA in NIP1;1 were highly tolerant to As(III), establishing that NIP1;1 is the causal gene of As(III) tolerance. Because an aquaglyceroporin is able to transport As(III), we measured As(III) transport activity. When expressed in Xenopus oocytes, NIP1;1 was capable of transporting As(III). As content in the mutant plants was 30% lower than in wild-type plants. Promoter ␤-glucuronidase and real-time PCR analysis showed that NIP1;1 is highly expressed in roots, and GFP-NIP1;1 is localized to the plasma membrane. These data show that NIP1;1 is involved in As(III) uptake into roots and that disruption of NIP1;1 function confers As(III) tolerance to plants. NIP1;2 and NIP5;1, closely related homologs of NIP1;1, were also permeable to As(III). Although the disruption of these genes reduced the As content in plants, As(III) tolerance was not observed in nip1;2 and nip5;1 mutants. This indicates that As(III) tolerance cannot be simply explained by decreased As contents in plants. Arsenic (As)2 is a very toxic but not essential element for most organisms except for arsenate-reducing bacteria (1).Arsenic represents a major environmental contaminant in several regions of the world. Long-term exposure to As causes skin diseases and cancers in humans. In most As-contaminated areas, As ingested by humans is derived from groundwater that is naturally contaminated with As. Both drinking water and irrigation with As-contaminated groundwater contribute to human ingestion (2, 3). This is especially serious in West Bengal, India, and Bangladesh. Bangladesh has the most serious problem in terms of both the number of people affected and the severity of health problems (2, 3). The major ingestion pathway of As is drinking As-contaminated water, followed by eating foods grown with contaminated water. Crops cultivated with As-containing groundwater accumulate As in their edible parts (4, 5). Rice is the major crop in these areas and is known to accumulate high levels of As in its grains (4). In addition to health effects, As inhibits the growth of rice, which leads to a reduction in yield (6, 7). Both reducing As uptake and generation of As-tolerant plants, especially rice, are useful methods to circumvent problems associated with As contamination.In the environment, both inorganic and organic As are present, and the inorganic form is more toxic. Two major inorganic As species in the environment are known: arsenate [As(V)] and arsenite [As(III)]. Transport processes of inorganic As have been studied in a number of organisms, and those involved in As uptake have been identified in bacteria, yeast, and animals (8). As(V) is taken up via phosphate transport systems, because As(V) (AsO 4 3Ϫ ) is an analog of phosphate. The As(III) ...