Major Plant Organs Research Articles

Effects of Limiting Nitrate Concentration on Morphological and Differential Expression of High- and Low-Affinity Nitrate Transporter Genes of Diverse Wheat Genotypes

Nitrate uptake in wheat is an essential and complex process that involves multiple proteins. Roots are the major plant organs that take nitrate molecules from the soil and transport them to the above-ground parts. They involve several high- and low-affinity nitrate transporters located in the plasma membrane of different root and shoot tissues. In the present study, we investigated the responses of different selected wheat genotypes, released in India for different agroclimatic regions in a different year, for their biomass, root traits, and expression of high- and low-affinity nitrate transporters genes under optimal and limiting nitrate conditions at the 14 days seedling stage. The maximum number of genotypes showed increased biomass and total root size (TRS) under nitrate starvation, which indicates that the variation of TRS traits in different genotypes responds differently to nitrate starvation. Gene expression of TaNRT2.1-4 showed up-regulated expression under low external N-concentration in all genotypes. Among the four TaNRT1.1 orthologs, TaNPF6.1 and TaNPF6.4 showed up-regulated expression, whereas TaNPF6.2 and TaNPF6.3 expressed down-regulated in root at higher nitrate concentrations. TaNRT1.1 (TaNPF7.1 and TaNPF7.2) showed up-regulated in root at optimum nitrate concentrations. TaNPF6.1, TaNPF6.4, TaNPF7.1, and TaNPF7.2 showed a typical expression of low-affinity nitrate transporter genes under optimum external N-concentrations. Our findings indicate that HD2967, NP890, and VL804 showed the highest expression of TaNRT2.1-4, TaNRT1.1, and TaNRT1.5, respectively, possibly involved in nitrate uptake and translocation (from root to shoot).

Diversity of fungal endophytes associated with the Philippine endemic ginger Vanoverberghia sepulchrei Merr. (Zingiberaceae)

The pantropical Zingiberaceae family is among the most threatened monocotyledonous plants, with numerous species endemic to the Philippines, and thus, is an ideal host to study fungal endophytes. In this study, we determined the occurrence and diversity of fungal endophytes associated with the Philippine endemic ginger Vanoverberghia sepulchrei Merr. All major plant organs, i.e., leaf, pseudostem, rhizome, and root, were surface-sterilized to isolate Vanoverberghia-associated fungal endophytes (VFE). A combination of morphological characteristics and phylogenetic analyses of DNA sequences derived from ITS were used to identify selected isolated fungi. Our results showed the isolation of twenty fungal morphospecies, and these were identified as belonging to the genera Bjerkandera, Cladosporium, Colletotrichum, Cosmospora, Diaporthe, Fusarium, Leptographium, Mucor, Nigrospora, Perenniporia, Phomopsis, Phyllosticta, Phytophthora, Pseudopithomyces, Rhizopus, and Trichoderma. The results of this study paved the way for the first report of fungal endophytes from the Philippine endemic ginger.

Spatial variation and allocation of sulfur among major plant organs in China

Sulfur (S) is a functional element that plays an important role in abiotic stress resistance and environmental adaptation in plants. However, knowledge of the biogeographic patterns of S among major plant organs remains limited. We conducted a methodologically consistent field survey of 2745 plant species across 78 typical communities throughout China. From this, we constructed a new matched database of S content in leaves, twigs, trunks, and roots to explore S allocation strategies in plants to better understand the regulatory mechanisms on a large scale. The average S content in leaves, twigs, trunks, and roots of plants in China was 2.32 ± 0.04, 1.13 ± 0.02, 0.15 ± 0.01, and 1.23 ± 0.02 g kg−1, respectively. S content was significantly higher in leaves than in other organs, and S content of plants in deserts was higher than that of plants in forests and grasslands. S content changed faster in roots and showed divergent allocation relationships among organs across communities at different scales. Climate and soil properties jointly regulated the spatial variation and allocation relationships of S among different organs. This study further broadens our understanding of the biological functions of S and their role in the interactions between plants and the environment.

Soil Temperature Prior to Veraison Alters Grapevine Carbon Partitioning, Xylem Sap Hormones, and Fruit Set

To gain a better understanding of environmental effects on grapevines and the physiological regulation of acclimation, we determined the effects of soil temperature (14 or 24°C) between anthesis and veraison on growth, nonstructural carbohydrates, cytokinins, abscisic acid, and leaf function of potted Vitis vinifera cv. Shiraz. Plants of each regime were selected from two groups that had been grown in a glasshouse from three weeks prior to budbreak at an average soil temperature of either 13 or 23°C. Soil temperature between anthesis and veraison affected utilization and restoration of root and trunk nonstructural carbohydrates and changes in biomass of major plant organs. Soil warming promoted shoot growth via utilization of starch reserves, while soil cooling promoted starch storage in both the root and wood and shifted overall biomass partitioning to the roots. A change in soil temperature from warm to cool through flowering was also associated with reduced fruit set. Diurnal courses of photosynthesis, transpiration, and stomatal conductance after fruit set were significantly affected by soil temperature. Phytohormones (cytokinin and abscisic acid) were measured in the xylem sap and leaves at fruit set and veraison. Differences between these two sample types during grapevine development highlight a phytohormone shift likely involved in postveraison fruit ripening. We conclude that soil temperature significantly affects grapevine growth and that the responses are mediated largely by an influence of temperature on mobilization of nonstructural carbohydrates from the roots.

Conservative allocation strategy of multiple nutrients among major plant organs: From species to community

Abstract Nutrient allocation is an important aspect of plant resource uptake and use, which is related to life‐history strategies. Although to date considerable attention has focused on plant allocation of nitrogen and phosphorus, comparatively little information is available on the allocation of various other nutrients and their up‐scaling from the species to community level. We measured 10 nutrient elements in the leaves, branches and fine roots of 551 plant species growing in eight forest ecosystems in China, ranging from cold temperate to subtropical forests. We estimated the scaling relationship of multiple nutrients among plant organs at the species level and scaled‐up the relationship to the community level by combining this information with that of community structure. Nutrient allocation among plant organs was conserved in different functional groups and biomes across broad environmental gradients. Nutrient partitioning between organs with similar function tended to be isometric, whereas partitioning between organs with distinct functions tended to be allometric. The scaling relationship between above‐ and below‐ground organs remained consistent, whereas the scaling relationship within above‐ground organs changed after scaling up from the species to the community level, with the relative change in nutrients being consistently smaller in the more active organs. Synthesis. The pattern of multiple nutrient allocation among organs showed a degree of conservatism across plant functional groups and biomes, with disproportional changes in nutrient content between functionally distinct organs and a lower relative change in more active organs. This conservative strategy implies the existence of general rules that constrain plant nutrient allocation.

Allocation of nitrogen and phosphorus within and between needles, stems and roots of Picea seedlings

The allocation of nutrients links ecosystem supply services to the functional traits and development of plants. This is particularly true for nitrogen (N) and phosphorus (P), which are important limiting resources in natural systems and are related to many aspects of plant biology. We investigated the scaling relationships associated with the N and P contents within specific organ types (needles, stems and roots) and among the different organs of Picea seedlings from nine taxa grown under greenhouse conditions. Our results showed that N and P contents were highly correlated within and between plant organs. A common isometric scaling relationship (scaling exponent ≈ 1) between N and P was observed in needles, stems, and roots. Howver, the N and P contents had different scaling exponents in different plant organs. The scaling relationships of the N content across different organ types tended to be allometric (scaling exponent < 1) between stems and non‐stem organs, and isometric between needles and roots. For P contents, similar scaling relationships were found among the three organs. These results improve our understanding of nutrient allocation strategies within and between major plant organs.

SlLAX1 is Required for Normal Leaf Development Mediated by Balanced Adaxial and Abaxial Pavement Cell Growth in Tomato.

Leaves are the major plant organs with a primary function for photosynthesis. Auxin controls various aspects of plant growth and development, including leaf initiation, expansion and differentiation. Unique and intriguing auxin features include its polar transport, which is mainly controlled by the AUX1/LAX and PIN gene families as influx and efflux carriers, respectively. The role of AUX1/LAX genes in root development is well documented, but the role of these genes in leaf morphogenesis remains unclear. Moreover, most studies have been conducted in the plant model Arabidopsis thaliana, while studies in tomato are still scarce. In this study, we isolated six lines of the allelic curly leaf phenotype 'curl' mutants from a γ-ray and EMS (ethyl methanesulfonate) mutagenized population. Using a map-based cloning strategy combined with exome sequencing, we observed that a mutation occurred in the SlLAX1 gene (Solyc09g014380), which is homologous to an Arabidopsis auxin influx carrier gene, AUX1 (AtAUX1). Characterization of six alleles of single curl mutants revealed the pivotal role of SlLAX1 in controlling tomato leaf flatness by balancing adaxial and abaxial pavement cell growth, which has not been reported in tomato. Using TILLING (Targeting Induced Local Lesions IN Genome) technology, we isolated an additional mutant allele of the SlLAX1 gene and this mutant showed a curled leaf phenotype similar to other curl mutants, suggesting that Solyc09g014380 is responsible for the curl phenotype. These results showed that SlLAX1 is required for normal leaf development mediated by balanced adaxial and abaxial pavement cell growth in tomato.

The Sorghum bicolor reference genome: improved assembly, gene annotations, a transcriptome atlas, and signatures of genome organization.

Sorghum bicolor is a drought tolerant C4 grass used for the production of grain, forage, sugar, and lignocellulosic biomass and a genetic model for C4 grasses due to its relatively small genome (approximately 800Mbp), diploid genetics, diverse germplasm, and colinearity with other C4 grass genomes. In this study, deep sequencing, genetic linkage analysis, and transcriptome data were used to produce and annotate a high-quality reference genome sequence. Reference genome sequence order was improved, 29.6Mbp of additional sequence was incorporated, the number of genes annotated increased 24% to 34211, average gene length and N50 increased, and error frequency was reduced 10-fold to 1 per 100kbp. Subtelomeric repeats with characteristics of Tandem Repeats in Miniature (TRIM) elements were identified at the termini of most chromosomes. Nucleosome occupancy predictions identified nucleosomes positioned immediately downstream of transcription start sites and at different densities across chromosomes. Alignment of more than 50 resequenced genomes from diverse sorghum genotypes to the reference genome identified approximately 7.4M single nucleotide polymorphisms (SNPs) and 1.9M indels. Large-scale variant features in euchromatin were identified with periodicities of approximately 25kbp. A transcriptome atlas of gene expression was constructed from 47 RNA-seq profiles of growing and developed tissues of the major plant organs (roots, leaves, stems, panicles, and seed) collected during the juvenile, vegetative and reproductive phases. Analysis of the transcriptome data indicated that tissue type and protein kinase expression had large influences on transcriptional profile clustering. The updated assembly, annotation, and transcriptome data represent a resource for C4 grass research and crop improvement.

High resolution mass spectrometry imaging of plant tissues: towards a plant metabolite atlas.

Mass spectrometry (MS) imaging provides spatial and molecular information for a wide range of compounds. This tool can be used to investigate metabolic changes in plant physiology and environmental interactions. A major challenge in our study was to prepare tissue sections that were compatible with high spatial resolution analysis and therefore dedicated sample preparation protocols were established and optimized for the physicochemical properties of all major plant organs. We combined high spatial resolution (5 μm), in order to detect cellular features, and high mass accuracy (<2 ppm root mean square error), for molecular specificity. Mass spectrometry imaging experiments were performed in positive and negative ion mode. Changes in metabolite patterns during plant development were investigated for germination of oilseed rape. The detailed localization of more than 90 compounds allowed assignment to metabolic processes and indicated possible functions in plant tissues. The 'untargeted' nature of MS imaging allows the detection of marker compounds for the physiological status, as demonstrated for plant-pathogen interactions. Our images show excellent correlation with optical/histological examination. In contrast to previous MS imaging studies of plants, we present a complete workflow that covers multiple species, such as oilseed rape, wheat seed and rice. In addition, different major plant organs and a wide variety of compound classes were analyzed. Thus, our method could be used to develop a plant metabolite atlas as a reference to investigate systemic and local effects of pathogen infection or environmental stress.

Why functional ecology should consider all plant organs: An allocation-based perspective

Functional ecology often analyses a few selected traits and relates them either to environmental conditions or ecosystem properties. However, not the individual trait, but the whole plant with a set of coordinated traits responds to the environment or affects ecosystem properties. Here we argue that the correlation among traits of all major plant organs should be an integral part of response or effect studies. Plants allocate elements and biomass among roots, perennial clonal organs, stems, leaves and seeds to ensure growth and reproduction. Assessment of trait responses to the environment and effects on ecosystems is hardly possible without simultaneously considering all plant organs and the biological functions they perform, namely resource uptake, vegetative regeneration, support and hydraulic pathways, photosynthesis and generative reproduction. Suitable traits to indicate these functions include those of mass, density, size, volume, and element contents of the main plant organs. In principle, we do not propose to collect many traits, but those of similar significance across organs. For instance, specific leaf area should be complemented by specific root length and specific stem length. We present some thoughts on how coordinated allocation to biological functions sets boundaries to the range of trait expressions in successional series and consequently also to species responses to the environment and effects on ecosystems. Considering the coordination of traits amongst all major plant organs will improve our understanding of plant strategies ensuring survival in patterned landscapes.

Uptake of Di(2-ethylhexyl) Phthalate (DEHP) by the Plant Benincasa hispida and Its Use for Lowering DEHP Content of Intercropped Vegetables

Uptake of di(2-ethylhexyl) phthalate (DEHP) by the plant Benincasa hispida and its use for topical phytoremediation were investigated by cultivation of plants in DEHP-contaminated environments. The results showed that major plant organs of B. hispida , including leaves, stems, and fruits, readily absorbed DEHP from the air. The amount of DEHP that accumulated in leaves, stems, and fruits was mainly dependent upon exposure time, and most DEHP accumulated in their inner tissues. A single plant of B. hispida with a gourd was able to absorb more than 700 mg of DEHP when it was exposed to DEHP-contaminated air for 6 week. B. hispida reduced air DEHP concentration by 65-76% as the air DEHP concentration ranged from 2351 to 3955 μg/m³ (high DEHP level) and 85-92% as the air DEHP concentration ranged from 35.1 to 65.3 μg/m³ (low DEHP level) in greenhouse experiments. When intercropping of B. hispida and Brassica chinensis or Brassica campestris , B. hispida reduced more than 87% of DEHP accumulation in the latter, which indicates that B. hispida has excellent use potential for lowering the DEHP content of intercropped vegetables.

Molecular Analysis of a Family of Arabidopsis Genes Related to Galacturonosyltransferases

We are studying a Galacturonosyltransferase-Like (GATL) gene family in Arabidopsis (Arabidopsis thaliana) that was identified bioinformatically as being closely related to a group of 15 genes (Galacturonosyltransferase1 [GAUT1] to -15), one of which (GAUT1) has been shown to encode a functional galacturonosyltransferase. Here, we describe the phylogeny, gene structure, evolutionary history, genomic organization, protein topology, and expression pattern of this gene family in Arabidopsis. Expression studies (reverse transcription-polymerase chain reaction) demonstrate that all 10 AtGATL genes are transcribed, albeit to varying degrees, in Arabidopsis tissues. Promoter::β-glucuronidase expression studies show that individual AtGATL gene family members have both overlapping and unique expression patterns. Nine of the 10 AtGATL genes are expressed in all major plant organs, although not always in all cell types of those organs. AtGATL4 expression appears to be confined to pollen grains. Most of the AtGATL genes are expressed strongly in vascular tissue in both the stem and hypocotyl. Subcellular localization studies of several GATL proteins using yellow fluorescent protein tagging provide evidence supporting the Golgi localization of these proteins. Plants carrying T-DNA insertions in three AtGATL genes (atgatl3, atgatl6, and atgatl9) have reduced amounts of GalA in their stem cell walls. The xylose content increased in atgatl3 and atgatl6 stem walls. Glycome profiling of cell wall fractions from these mutants using a toolkit of diverse plant glycan-directed monoclonal antibodies showed that the mutations affect both pectins and hemicelluloses and alter overall wall structure, as indicated by altered epitope extractability patterns. The data presented suggest that the AtGATL genes encode proteins involved in cell wall biosynthesis, but their precise roles in wall biosynthesis remain to be substantiated.

Cloning and functional characterization ofLjPLT4, a plasma membrane xylitol H+- symporter fromLotus japonicus

Polyols are compounds that play various physiological roles in plants. Here we present the identification of four cDNA clones of the model legume Lotus japonicus, encoding proteins of the monosaccharide transporter-like (MST) superfamily that share significant homology with previously characterized polyol transporters (PLTs). One of the transporters, named LjPLT4, was characterized functionally after expression in yeast. Transport assays revealed that LjPLT4 is a xylitol-specific H(+)-symporter (K (m), 0.34 mM). In contrast to the previously characterized homologues, LjPLT4 was unable to transport other polyols, including mannitol, sorbitol, myo-inositol and galactitol, or any of the monosaccharides tested. Interestingly, some monosaccharides, including fructose and xylose, inhibited xylitol uptake, although no significant uptake of these compounds was detected in the LjPLT4 transformed yeast cells, suggesting interactions with the xylitol binding site. Subcellular localization of LjPLT4-eYFP fusions expressed in Arabidopsis leaf epidermal cells indicated that LjPLT4 is localized in the plasma membrane. Real-time RT-PCR revealed that LjPLT4 is expressed in all major plant organs, with maximum transcript accumulation in leaves correlating with maximum xylitol levels there, as determined by GC-MS. Thus, LjPLT4 is the first plasma membrane xylitol-specific H(+)-symporter to be characterized in plants.

The molecular organography of plants

In the last 20 years or so, plant molecular biology has become the dominant field of research in botany departments and institutes. The development of powerful techniques for analysis of the genome and the identification of genes has led to a rapid accumulation of data, the collection of which has absorbed much of the research funding available to botanists. There is no doubt that scientists working in more traditional fields are being side-lined as departments move their research emphasis to follow the funding. To these scientists, it can seem that molecular biologists are so involved in the pursuit of the gene that they have lost sight of the plant. This book is particularly welcome, therefore, as it provides a concise and accessible account of how knowledge about gene function and variation are related to anatomical and morphological development. The combination of molecular data with knowledge about plant structure and growth is acquiring increasing importance and has led to the discipline known as the ‘evolution of development’ or ‘evo-devo’. Cronk's excellent book is timely, scholarly and important, bringing together the evolution, anatomy and morphology of the major plant organs and how they develop. As Cronk states in his preface, the book is not a textbook of plant morphology or plant molecular genetics, but it shows what can be achieved if the two subjects are combined. It is aimed at senior undergraduates and graduate students, although I believe that many young postdoctoral scientists working in plant molecular biology would benefit from reading it. It is very readable and gives a welcome overview of evo-devo that botanists new to the area will find extremely accessible, providing an excellent introduction to plant structures and their origins while putting the molecular work into context. As one might expect in a book dealing with a subject that has attracted the attention of so many researchers, there is an extensive 33-page bibliography. The introduction provides an overview of ideas on plant evolution and their development, which sets the scene for what follows: five chapters, each dealing with a particular plant organ. There is a chapter each on stems, roots and leaves, a chapter on sporangia and seeds, and one on sporophylls and flowers. Each chapter includes descriptions of the structure of the organ in question, its evolution and the variations found. Wherever the information is available, the genes involved in the developmental process are described. As a wood anatomist, however, I have one minor criticism: I winced to read that softwoods (by which is meant conifer woods) are soft because they contain relatively more tracheids than fibres, when in fact they contain tracheids and no fibres. It is incorrect to say that this makes the wood soft. Many ‘softwoods’ (or timbers from arborescent gymnosperms) have harder wood than many ‘hardwoods’ (or timbers from arborescent angiosperms), among which may be numbered balsa. ‘Softwood’ and ‘hardwood’ are, of course, industrial terms and not biological ones. As an anatomist, I found the accounts clear and easy to follow, but some recent graduates may find they need to have a well-illustrated anatomy textbook to hand to be able to visualize what is described. I say this because experience has shown that, almost unbelievably in this age of modularized degrees and student choice, a student can graduate in botany without having attended lectures on plant anatomy. However, if this book does not fire up an interest in plant structure and development, then nothing will! I would recommend the book to all botanists who need bringing up to date in this fast-moving area of research. It is an enjoyable read and excellent value at the price.

Characterization of Arabidopsis thaliana SufE2 and SufE3: FUNCTIONS IN CHLOROPLAST IRON-SULFUR CLUSTER ASSEMBLY AND NAD SYNTHESIS

In this study we characterize two novel chloroplast SufE-like proteins from Arabidopsis thaliana. Other SufE-like proteins, including the previously described A. thaliana CpSufE, participate in sulfur mobilization for Fe-S biosynthesis through activation of cysteine desulfurization by NifS-like proteins. In addition to CpSufE, the Arabidopsis genome encodes two other proteins with SufE domains, SufE2 and SufE3. SufE2 has plastid targeting information. Purified recombinant SufE2 could activate the cysteine desulfurase activity of CpNifS 40-fold. SufE2 expression was flower-specific and high in pollen; we therefore hypothesize that SufE2 has a specific function in pollen Fe-S cluster biosynthesis. SufE3, also a plastid targeted protein, was expressed at low levels in all major plant organs. The mature SufE3 contains two domains, one SufE-like and one with similarity to the bacterial quinolinate synthase, NadA. Indeed SufE3 displayed both SufE activity (stimulating CpNifS cysteine desulfurase activity 70-fold) and quinolinate synthase activity. The full-length protein was shown to carry a highly oxygen-sensitive (4Fe-4S) cluster at its NadA domain, which could be reconstituted by its own SufE domain in the presence of CpNifS, cysteine and ferrous iron. Knock-out of SufE3 in Arabidopsis is embryolethal. We conclude that SufE3 is the NadA enzyme of A. thaliana, involved in a critical step during NAD biosynthesis.

Depletion of UDP-d-apiose/UDP-d-xylose Synthases Results in Rhamnogalacturonan-II Deficiency, Cell Wall Thickening, and Cell Death in Higher Plants

D-apiose serves as the binding site for borate cross-linking of rhamnogalacturonan II (RG-II) in the plant cell wall, and biosynthesis of D-apiose involves UDP-D-apiose/UDP-D-xylose synthase catalyzing the conversion of UDP-D-glucuronate to a mixture of UDP-D-apiose and UDP-D-xylose. In this study we have analyzed the cellular effects of depletion of UDP-D-apiose/UDP-D-xylose synthases in plants by using virus-induced gene silencing (VIGS) of NbAXS1 in Nicotiana benthamiana. The recombinant NbAXS1 protein exhibited UDP-D-apiose/UDP-D-xylose synthase activity in vitro. The NbAXS1 gene was expressed in all major plant organs, and an NbAXS1-green fluorescent protein fusion protein was mostly localized in the cytosol. VIGS of NbAXS1 resulted in growth arrest and leaf yellowing. Microscopic studies of the leaf cells of the NbAXS1 VIGS lines revealed cell death symptoms including cell lysis and disintegration of cellular organelles and compartments. The cell death was accompanied by excessive formation of reactive oxygen species and by induction of various protease genes. Furthermore, abnormal wall structure of the affected cells was evident including excessive cell wall thickening and wall gaps. The mutant cell walls contained significantly reduced levels of D-apiose as well as 2-O-methyl-L-fucose and 2-O-methyl-D-xylose, which serve as markers for the RG-II side chains B and A, respectively. These results suggest that VIGS of NbAXS1 caused a severe deficiency in the major side chains of RG-II and that the growth defect and cell death was likely caused by structural alterations in RG-II due to a D-apiose deficiency.

Accumulation and Loss of Nitrogen in White Clover (Trifolium repens L.) Plant Organs as Affected by Defoliation Regime on Two Sites in Norway

In order to improve the basis for utilising nitrogen (N) fixed by white clover (Trifolium repens L.) in northern agriculture, we studied how defoliation stress affected the N contents of major plant organs in late autumn, N losses during the winter and N accumulation in the following spring. Plants were established from stolon cuttings and transplanted to pots that were dug into the field at Apelsvoll Research Centre (60°42′ N, 10°51′ E) and at Holt Research Centre (69°40′ N, 18°56′ E) in spring 2001 and 2002. During the first growing season, the plants were totally stripped of leaves down to the stolon basis, cut at 4 cm height or left undisturbed. The plants were sampled destructively in late autumn, early spring the second year and after 6 weeks of new spring growth. The plant material was sorted into leaves, stolons and roots. Defoliation regime did not influence the total amount of leaf N harvested during and at the end of the first growing season. However, for intensively defoliated plants, the repeated leaf removal and subsequent regrowth occurred at the expense of stolon and root development and resulted in a 61–85% reduction in the total plant N present in late autumn and a 21–59% reduction in total accumulation of plant N (plant N present in autumn + previously harvested leaf N). During the winter, the net N loss from leaf tissue (N not recovered in living nor dead leaves in the spring) ranged from 57% to 74% of the N present in living leaves in the autumn, while N stored in stolons and roots was much better conserved. However, the winter loss of stolon N from severely defoliated plants (19%) was significantly larger than from leniently defoliated (12%) and non-defoliated plants (6%). Moreover, the fraction of stolon N determined as dead in the spring was 63% for severely defoliated as compared to 14% for non-defoliated plants. Accumulation in absolute terms of new leaf N during the spring was highly correlated to total plant N in early spring (R 2 = 0.86), but the growth rates relative to plant N present in early spring were not and, consequently, were similar for all treatments. The amount of inorganic N in the soil after snowmelt and the N uptake in plant root simulator probes (PRSTM) during the spring were small, suggesting that microbial immobilisation, leaching and gas emissions may have been important pathways for N lost from plant tissue.

Phytocalpain controls the proliferation and differentiation fates of cells in plant organ development.

Calpain, a calcium-dependent cysteine protease, plays an essential role in basic cellular processes in animal cells, including cell proliferation, apoptosis, and differentiation. NbDEK encodes the calpain homolog of N. benthamiana. In this study, virus-induced gene silencing (VIGS) of NbDEK resulted in arrested organ development and hyperplasia in all the major plant organs examined. The epidermal layers of the leaves and stems were covered with hyperproliferating cell masses, and stomata and trichome development was severely inhibited. During flower development, a single dome-like structure was grown from the flower meristem to generate a large cylinder-shaped flower lacking any floral organs. At the cellular level, cell division was sustained in tissues that were otherwise already differentiated, and cell differentiation was severely hampered. NbDEK is ubiquitously expressed in all the plant tissues examined. In the abnormal organs of the NbDEK VIGS lines, protein levels of D-type cyclins (CycD)2, CycD3, and proliferating cell nuclear antigen (PCNA) were greatly elevated, and transcription of E2F (E2 promoter binding factor), E2F-regulated genes, retinoblastoma (Rb), and KNOTTED1 (KN1)-type homeobox genes was also stimulated. These results suggest that phytocalpain is a key regulator of cell proliferation and differentiation during plant organogenesis, and that it acts partly by controlling the CycD/Rb pathway.

Molecular analysis of 10 coding regions from Arabidopsis that are homologous to the MUR3 xyloglucan galactosyltransferase.

Plant cell walls are composed of a large number of complex polysaccharides, which contain at least 13 different monosaccharides in a multitude of linkages. This structural complexity of cell wall components is paralleled by a large number of predicted glycosyltransferases in plant genomes, which can be grouped into several distinct families based on conserved sequence motifs (B. Henrissat, G.J. Davies [2000] Plant Physiol 124: 1515-1519). Despite the wealth of genomic information in Arabidopsis and several crop plants, the biochemical functions of these coding regions have only been established in a few cases. To lay the foundation for the genetic and biochemical characterization of putative glycosyltransferase genes, we conducted a phylogenetic and expression analysis on 10 predicted coding regions (AtGT11-20) that are closely related to the MUR3 xyloglucan galactosyltransferase of Arabidopsis. All of these proteins contain the conserved sequence motif pfam 03016 that is the hallmark of the beta-d-glucuronosyltransferase domain of exostosins, a class of animal enzymes involved in the biosynthesis of the extracellular polysaccharide heparan sulfate. Reverse transcriptase-polymerase chain reaction and promoter:beta-glucuronidase studies indicate that all AtGT genes are transcribed. Although six of the 10 AtGT genes were expressed in all major plant organs, the remaining four genes showed more restricted expression patterns that were either confined to specific organs or to highly specialized cell types such as hydathodes or pollen grains. T-DNA insertion mutants in AtGT13 and AtGT18 displayed reductions in the Gal content of total cell wall material, suggesting that the disrupted genes encode galactosyltransferases in plant cell wall synthesis.

The mur2 mutant of Arabidopsis thaliana lacks fucosylated xyloglucan because of a lesion in fucosyltransferase AtFUT1.

Cell walls of the Arabidopsis mutant mur2 contain less than 2% of the wild-type amount of fucosylated xyloglucan because of a point mutation in the fucosyltransferase AtFUT1. The mur2 mutation eliminates xyloglucan fucosylation in all major plant organs, indicating that Arabidopsis thaliana fucosyltransferase 1 (AtFUT1) accounts for all of the xyloglucan fucosyltransferase activity in Arabidopsis. Despite this alteration in structure, mur2 plants show a normal growth habit and wall strength. In contrast, Arabidopsis mur1 mutants that are defective in the de novo synthesis of l-fucose exhibit a dwarfed growth habit and decreased wall strength [Reiter, W. D., Chapple, C. & Somerville, C. R. (1993) Science 261, 1032-1035]. Because the mur1 mutation affects several cell wall polysaccharides, whereas the mur2 mutation is specific to xyloglucan, the phenotypes of mur1 plants appear to be caused by structural changes in fucosylated pectic components such as rhamnogalacturonan-II. The normal growth habit and wall strength of mur2 plants casts doubt on hypotheses regarding roles of xyloglucan fucosylation in facilitating xyloglucan-cellulose interactions or in modulating growth regulator activity.