In higher plants, [2Fe-2S] ferredoxin (Fd) proteins are the unique electron acceptors from photosystem I (PSI).. Whereas FdC1 was capable of electron transfer with FNR, redox potentiometry showed that it had a more positive redox potential than photosynthetic Fds by around 220 mV. These results indicate that FdC1 electron donation to FNR is prevented because it is thermodynamically unfavorable. Based on our data, we speculate that FdC1 has a specific function in conditions of acceptor limitation at PSI, and channels electrons away from NADP ؉ photoreduction.Ferredoxins (Fds) 3 are small soluble electron carrier proteins. In the final reaction of photosynthetic electron transfer (PET), photosystem I (PSI) donates electrons to Fd (1), which acts as the soluble electron donor to various acceptors in the chloroplast stroma and can also return electrons to the thylakoid in cyclic electron flow (CET) (2). The electron cascade to supply carbon fixation requires photoreduction of NADP ϩ by Fd, catalyzed by Fd-NADP(H) oxidoreductase (FNR) (3). Many other plastid enzymes accept electrons directly from Fd for metabolic processes. These include, but are not limited to, nitrite reductase and sulfite reductase, which are essential for assimilation of inorganic nitrogen and sulfur, respectively, and Fd-dependent glutamine oxoglutarate aminotransferase and fatty acid desaturase, which catalyze key steps in amino acid and fatty acid metabolism, respectively (4). In addition, Fd donation to thioredoxin via the Fd:thioredoxin reductase translates the redox state of the electron transfer chain into a regulatory signal controlling the activity of many enzymes (5). Fds are also capable of accepting electrons from NADPH via FNR, in a reversal of the photosynthetic reaction (6), allowing electron donation from reduced Fd to different acceptors under non-photosynthetic conditions. Most higher plants studied possess genes for several different Fd isoproteins (7-9). There is always an isoprotein that is more abundant in non-photosynthetic tissues and has higher affinity than photosynthetic and PetF-type Fds for FNR in the non-photosynthetic (often called "root") cascade (9, 10), where electrons are transferred from NADPH to Fd. In all plants for which we possess significant EST and cDNA information at least 2 separate photosynthetic isoproteins have been identified (7,8). In the C4-plant maize, different functions have been identified for two of the leaf-type Fds (11). There is a higher demand for ATP (which is disproportionately produced in CET) in the bundle sheath cells of NADP ϩ malic enzyme type C4 plants, and maize FdI and FdII are differentially expressed in mesophyll and bundle sheath cells, respectively (12). FdII has decreased affinity for FNR (13) and demonstrates a higher activity in CET around the photosystems, whereas FdI drives linear electron flow (11). In C3 plants, this spatial distribution is not observed, but duplicate photosynthetic Fds are still present, and there is some evi-* This work was supported by Deutsche For...

The plant NAC transcription factors depict one of the largest plant transcription factor families. They regulate a wide range of different developmental processes and most probably played an important role in the evolutionary diversification of plants. This makes comparative studies of the NAC transcription factor family between individual species and genera highly relevant and such studies have in recent years been greatly facilitated by the increasing number of fully sequenced complex plant genomes. This study combines the characterization of the NAC transcription factors in the recently sequenced genome of the cereal crop barley with expression analysis and a comprehensive phylogenetic characterization of the NAC transcription factors in other monocotyledonous plant species. Our results provide evidence for the emergence of a NAC transcription factor subclade that is exclusively expressed in the grains of the Poaceae family of grasses. These notably comprise a number of cereal crops other than barley, such as wheat, rice, maize or millet, which are all cultivated for their starchy edible grains. Apparently, the grain specific subclade emerged from a well described subgroup of NAC transcription factors associated with the senescence process. A promoter exchange subsequently resulted in grain specific expression. We propose to designate this transcription factor subclade Grain-NACs and we discuss their involvement in programmed cell death as well as their potential role in the evolution of the Poaceae grain, which doubtlessly is of central importance for human nutrition.

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Silicon (Si) has many beneficial effects in plants, especially for the survival from biotic and abiotic stresses. However, Si may negatively affect the quality of lignocellulosic biomass for bioenergy purposes. Despite many studies, the regulation of Si distribution and deposition in plants remains to be fully understood. Here, we have identified the Brachypodium distachyon mutant low-silicon 1 (Bdlsi1-1), with impaired channeling function of the Si influx transporter BdLSI1, resulting in a substantial reduction of Si in shoots. Bioimaging by laser ablation-inductively coupled plasma-mass spectrometry showed that the wild-type plants deposited Si mainly in the bracts, awns and leaf macrohairs. The Bdlsi1-1 mutants showed substantial (>90%) reduction of Si in the mature shoots. The Bdlsi1-1 leaves had fewer, shorter macrohairs, but the overall pattern of Si distribution in bracts and leaf tissues was similar to that in the wild-type. The Bdlsi1-1 plants supplied with Si had significantly lower seed weights, compared to the wild-type. In low-Si media, the seed weight of wild-type plants was similar to that of Bdlsi1-1 mutants supplied with Si, while the Bdlsi1-1 seed weight decreased further. We conclude that Si deficiency results in widespread alterations in leaf surface morphology and seed formation in Brachypodium, showing the importance of Si for successful development in grasses.

GenotypeSilicon Wheat (Triticum aestivum L.) straw a b s t r a c t Crop residues are utilized as lignocellulosic biomass for production of energy via biochemical or thermochemical degradation. The conversion efficiency depends on the content of major organic components, but also other elements play a role and are thus

Improved agricultural and industrial production organisms are required to meet the future global food demands and minimize the effects of climate change. A new resource for crop and microbe improvement, designated FIND-IT (Fast Identification of Nucleotide variants by droplet DigITal PCR), provides ultrafast identification and isolation of predetermined, targeted genetic variants in a screening cycle of less than 10 days. Using large-scale sample pooling in combination with droplet digital PCR (ddPCR) greatly increases the size of low–mutation density and screenable variant libraries and the probability of identifying the variant of interest. The method is validated by screening variant libraries totaling 500,000 barley ( Hordeum vulgare ) individuals and isolating more than 125 targeted barley gene knockout lines and miRNA or promoter variants enabling functional gene analysis. FIND-IT variants are directly applicable to elite breeding pipelines and minimize time-consuming technical steps to accelerate the evolution of germplasm.

Tonoplast intrinsic proteins (TIPs) and plasma membrane intrinsic proteins (PIPs) form subgroups of plant major intrinsic proteins (MIPs) that channel water as well as various small neutral molecules across the tonoplast and plasma membrane. Most MIPs are believed to form homotetramers, while some plant PIPs have been shown to form heterotetramers composed of different isoforms. This study investigated in vivo molecular interactions between different Arabidopsis TIP isoforms and between TIPs and a PIP member. The interactions were assayed by bimolecular fluorescence complementation optimized for use in Saccharomyces cerevisiae as a heterologous expression system. Fluorescence of re-assembled Venus yellow fluorescent protein was monitored by fluorescence microscopy and flow cytometry. The results showed strong interactions between TIP1;2, TIP2;1 and TIP3;1. Surprisingly, the three TIP isoforms also interacted with PIP2;1. The potassium channel AKT1 was used as a negative control and exhibited no interaction with any of the MIPs. The observed interactions may play a role in targeting and regulation of MIPs in plants.

Vascular plants reinforce the cell walls of the different xylem cell types with lignin phenolic polymers. Distinct lignin chemistries differ between each cell wall layer and each cell type to support their specific functions. Yet the mechanisms controlling the tight spatial localization of specific lignin chemistries remain unclear. Current hypotheses focus on control by monomer biosynthesis and/or export, while cell wall polymerization is viewed as random and non-limiting. Here we show that combinations of multiple individual laccases (LACs) are nonredundantly and specifically required to set the lignin chemistry in different cell types and their distinct cell wall layers. We dissected the roles of Arabidopsis thaliana LAC4, 5, 10, 12 and 17 by generating quadruple and quintuple loss-of-function mutants. Loss of these LACs in different combinations led to specific changes in lignin chemistry affecting both residue ring structures and/or aliphatic tails in specific cell types and cell wall layers. Moreover, we showed that LAC-mediated lignification has distinct functions in specific cell types, waterproofing fibers and strengthening vessels. Altogether, we propose that the spatial control of lignin chemistry depends on different combinations of LACs with nonredundant activities immobilized in specific cell types and cell wall layers.