Morphological Characteristics of Maize Canopy Development as Affected by Increased Plant Density

Improving crop productivity through higher plant density requires a detailed understanding of organ development in response to increased interplant competition. The objective of this paper is thus to investigate the characteristics of organ development under increased interplant competition. A field experiment was conducted to investigate organ development across 4 maize plant densities i.e. 2, 6, 12 and 20 plants m^–2 (referred to PD2, PD6, PD12 and PD20 respectively). In response to increased interplant competition, lengths of both laminae and sheaths increased in lower phytomers, but decreased in upper phytomers. Sheath extension appeared to be less sensitive to increased interplant competition than lamina extension. Extension of laminae and internodes responded to increased plant density as soon as onset of mild interplant competition, but did not respond any further to severe competition. Both lamina width and internode diameter were reduced due to a smaller growth rate in response to increased plant density. Overall, this study identified that organ expansion rate can be taken as the key morphological factor to determine the degree of interplant competition.     

Plasmodesmata during development: re-examination of the importance of primary,secondary, and branched plasmodesmata structure versus function

Plasmodesmata (PD) structure and function vary temporally and spatially during all stages of plant development. PD that originate during, or post, cell division are designated as primary or secondary according to classical terminology. PD structure may be simple, twinned, or branched. Studies of PD during leaf, root, and embryo development have lead to the generalization that cells in less mature tissues contain predominantly simple PD. New quantitative analyses reveal that twinned and branched PD also occur in immature tissues. New data also highlight the versatility of viral movement proteins as tags for labeling PD in immature tissues as well as PD in mature tissues. A summary of the formation and function of primary, secondary, and branched PD during leaf, trichome, embryo, apical meristem, vascular cambium, and root development underscores the remarkable and indispensible plant-specific intercellular communication system that is mediated by PD.     

Effects of MAPKK inhibitor PD98059 on the gravitropism of primary roots of maize

Gravity plays a fundamental role in plant growth and development, yet the molecular details of gravitropism is not fully understood. Here, we report the effects of PD98059, a specific inhibitor of mitogen-activated protein (MAP) kinase kinase, on the gravitropism of primary roots of maize. Unilateral application of PD98059 to horizontal roots led to different gravitropic growth. Placing PD98059-containing agar on the upper side of the root tips accelerated gravitropic curvature, whereas placing the agar on the lower side inhibited gravitropic curvature. However, no effect was detected when asymmetric application of PD98059 to vertical roots. Global application of maize primary root with PD98059 suppressed root gravitropism. Furthermore, the effects of H₂O₂ on horizontal root gravitropism and vertical root bending were compromised by pretreatment with PD98059. These results suggest an involvement of MAP kinase pathway(s) in gravitropism of maize roots.     

Choline transporter‐like1 (CHER1) is crucial for plasmodesmata maturation in Arabidopsis thaliana

Plasmodesmata (PD) are microscopic pores connecting plant cells and enable cell‐to‐cell transport. Currently, little information is known about the molecular mechanisms regulating PD formation and development. To uncover components of PD development we made use of the 17 kDa movement protein (MP17) encoded by the Potato leafroll virus (PLRV). The protein is required for cell‐to‐cell movement of the virus and localises to complex PD. Forward genetic screening for Arabidopsis mutants with altered PD binding of MP17 revealed several mutant lines, while molecular genetics, biochemical and microscopic studies allowed further characterisation. Map‐based cloning of one mutant revealed a point mutation in the choline transporter‐like 1 (CHER1) protein, changing glycine²⁴⁷ into glutamate. Mutation in CHER1 resulted in a starch excess phenotype and stunted growth. Ultrastructure analysis of shoot apical meristems, developing and fully developed leaves showed reduced PD numbers and the absence of complex PD in fully developed leaves. This indicates that cher1 mutants are impaired in PD formation and development. Global lipid profiling revealed only slight modifications in the overall lipid composition, however, altered composition of PD‐associated lipids cannot be ruled out. Thus, cher1 is devoid of complex PD in developed leaves and provides insights into the formation of complex PD at the molecular level.     

Pathogen effectors: What do they do at plasmodesmata?

Plants perceive an assortment of external cues during their life cycle, including abiotic and biotic stressors. Biotic stress from a variety of pathogens, including viruses, oomycetes, fungi, and bacteria, is considered to be a substantial factor hindering plant growth and development. To hijack the host cell's defence machinery, plant pathogens have evolved sophisticated attack strategies mediated by numerous effector proteins. Several studies have indicated that plasmodesmata (PD), symplasmic pores that facilitate cell-to-cell communication between a cell and neighbouring cells, are one of the targets of pathogen effectors. However, in contrast to plant-pathogenic viruses, reports of fungal- and bacterial-encoded effectors that localize to and exploit PD are limited. Surprisingly, a recent study of PD-associated bacterial effectors has shown that a number of bacterial effectors undergo cell-to-cell movement via PD. Here we summarize and highlight recent advances in the study of PD-associated fungal/oomycete/bacterial effectors. We also discuss how pathogen effectors interfere with host defence mechanisms in the context of PD regulation.     

Regulation of short-distance transport of RNA and protein

The intercellular trafficking of proteins and RNAs has emerged as a novel mechanism of cell-cell communication in plant development. Plasmodesmata (PD), intercellular cytoplasmic channels, have a central role in cell-cell trafficking of regulatory proteins and RNAs. Recent studies have demonstrated that plants use either a selective or a non-selective PD trafficking pathway for regulatory proteins. Moreover, plants have developed strategies to regulate both selective and non-selective movement. Recent work has focused especially on integrating the recent understanding of the function and mechanisms of intercellular macromolecule movement through PD.     

Plasmodesmal receptor-like kinases identified through analysis of rice cell wall extracted proteins

In plants, plasmodesmata (PD) are intercellular channels that function in both metabolite exchange and the transport of proteins and RNAs. Currently, many of the PD structural and regulatory components remain to be elucidated. Receptor-like kinases (RLKs) belonging to a notably expanded protein family in plants compared to the animal kingdom have been shown to play important roles in plant growth, development, pathogen resistance, and cell death. In this study, cell biological approaches were used to identify potential PD-associated RLK proteins among proteins contained within cell walls isolated from rice callus cultured cells. A total of 15 rice RLKs were investigated to determine their subcellular localization, using an Agrobacterium-mediated transient expression system. Of these six PD-associated RLKs were identified based on their co-localization with a viral movement protein that served as a PD marker, plasmolysis experiments, and subcellular localization at points of wall contact between spongy mesophyll cells. These findings suggest potential PD functions in apoplasmic signaling in response to environmental stimuli and developmental inputs.     

High resolution scanning electron microscopy of plasmodesmata

Symplastic transport occurs between neighbouring plant cells through functionally and structurally dynamic channels called plasmodesmata (PD). Relatively little is known about the composition of PD or the mechanisms that facilitate molecular transport into neighbouring cells. While transmission electron microscopy (TEM) provides 2-dimensional information about the structural components of PD, 3-dimensional information is difficult to extract from ultrathin sections. This study has exploited high-resolution scanning electron microscopy (HRSEM) to reveal the 3-dimensional morphology of PD in the cell walls of algae, ferns and higher plants. Varied patterns of PD were observed in the walls, ranging from uniformly distributed individual PD to discrete clusters. Occasionally the thick walls of the giant alga Chara were fractured, revealing the surface morphology of PD within. External structures such as spokes, spirals and mesh were observed surrounding the PD. Enzymatic digestions of cell wall components indicate that cellulose or pectin either compose or stabilise the extracellular spokes. Occasionally, the PD were fractured open and desmotubule-like structures and other particles were observed in their central regions. Our observations add weight to the argument that Chara PD contain desmotubules and are morphologically similar to higher plant PD.     

NbEXPA1, an α‐expansin,is plasmodesmata‐specific and a novel host factor for potyviral infection

Plasmodesmata (PD), unique to the plant kingdom, are structurally complex microchannels that cross the cell wall to establish symplastic communication between neighbouring cells. Viral intercellular movement occurs through PD. To better understand the involvement of PD in viral infection, we conducted a quantitative proteomic study on the PD‐enriched fraction from Nicotiana benthamiana leaves in response to infection by Turnip mosaic virus (TuMV). We report the identification of a total of 1070 PD protein candidates, of which 100 (≥2‐fold increase) and 48 (≥2‐fold reduction) are significantly differentially accumulated in the PD‐enriched fraction, when compared with protein levels in the corresponding healthy control. Among the differentially accumulated PD protein candidates, we show that an α‐expansin designated NbEXPA1, a cell wall loosening protein, is PD‐specific. TuMV infection downregulates NbEXPA1 mRNA expression and protein accumulation. We further demonstrate that NbEXPA1 is recruited to the viral replication complex via the interaction with NIb, the only RNA‐dependent RNA polymerase of TuMV. Silencing of NbEXPA1 inhibits plant growth and TuMV infection, whereas overexpression of NbEXPA1 promotes viral replication and intercellular movement. These data suggest that NbEXPA1 is a host factor for potyviral infection. This study not only generates a PD‐proteome dataset that is useful in future studies to expound PD biology and PD‐mediated virus–host interactions but also characterizes NbEXPA1 as the first PD‐specific cell wall loosening protein and its essential role in potyviral infection.     

Quantitative imaging of directional transport through plasmodesmata in moss protonemata via single-cell photoconversion of Dendra2

Cell-to-cell transport of molecules in plants must be properly regulated for plant growth and development. One specialized mechanism that plants have evolved involves transport through plasmodesmata (PD), but when and how transport of molecules via PD is regulated among individual cells remains largely unknown, particularly at the single-cell level. Here, we developed a tool for quantitatively analyzing cell-to-cell transport via PD at a single-cell level using protonemata of Physcomitrella patens and a photoconvertible fluorescent protein, Dendra2. In the filamentous protonemal tissues, one-dimensional intercellular communication can be observed easily. Using this system, we found that Dendra2 was directionally transported toward the apex of the growing protonemata. However, this directional transport could be eliminated by incubation in the dark or treatment with a metabolic inhibitor. Thus, we propose that directional transport of macromolecules can occur via PD in moss protonemata, and may be affected by the photosynthetic and metabolic activity of cells.     

Cell-to-cell transport of macromolecules during early plant development

Plant cells exchange developmental signals, distribute nutrients and ribonucleoprotein complexes through dynamic intercellular channels termed plasmodesmata (PD). Multidisciplinary investigations over the last decade have provided evidence that plasmodesmatal regulation is critical to various basic plant functions, such as development, host-pathogen interactions, and systemic RNA-silencing. This review highlights the cell-to-cell transport of micro- and macromolecules via PD during embryo and seedling growth.     

The actin cytoskeleton is involved in the regulation of the plasmodesmal size exclusion limit

Plasmodesmata (PD) are the communication channels which allow the trafficking of macromolecules between neighboring cells. Such cell-to-cell movement of macromolecules is regulated during plant growth and development; however, little is known about the regulation mechanism of PD size exclusion limit (SEL). Plant viral movement proteins (MPs) enhance the invasion of viruses from cell to cell by increasing the SEL of the PD and are therefore a powerful means for the study of the plasmodesmal regulation mechanisms. In a recent study, we reported that the actin cytoskeleton is involved in the increase of the PD SEL induced by MPs. Microinjection experiments demonstrated that actin depolymerization was required for the Cucumber mosaic virus (CMV) MP-induced increase in the PD SEL. In vitro experiments showed that CMV MP severs actin filaments (F-actin). Furthermore, through the analyses of two CMV MP mutants, we demonstrated that the F-actin severing ability of CMV MP was required to increase the PD SEL. These results are similar to what has been found in Tobacco mosaic virus MP. Thus, our data suggest that actin dynamics may participate in the regulations of the PD SEL.Key words: plasmodesmata, size exclusion limit, movement protein, actin filaments, F-actin severing     

Season-associated modifications in symplasmic organization of the cambium in Populus nigra

Background and Aims

Alterations of plasmodesma (PD) connectivity are likely to be very important for plant development. Here, the repetitive division pattern of cambial initials in Populus nigra ‘italica’ was studied to follow the development of the PD network during maturation. Furthermore, seasonal changes were investigated in order to trace indications for developmental and functional adaptations.

Methods

Cambium samples of P. nigra twigs, collected in summer, autumn and spring, were chemically fixed for transmission electron microscopy. The parameters, PD density (number of PDs per square micrometre cell-wall area) and PD frequency (total number of PDs per average cell-wall area), were determined for radial and tangential cell interfaces deposited in chronological order.

Key Results

Data sets, presented in plasmodesmograms, show a strong variability in the PD network throughout the year. In summer, high PD numbers occur at the division wall which, after PD doubling by longitudinal fission, decline with further development both at the xylem and the phloem side. In autumn, the number of PDs at the division wall is low as they are in subsequent tangential interfaces. In spring, the first cell division coincides with a massive increase in PD numbers, in particular at the division wall. Only the radial walls between initials maintain their PD equipment throughout the year. This feature can be exploited for identification of the initial layer.

Conclusions

PD networks in the cambium go through a strict developmental programme depending on the season, which is associated with changing functional requirements. For instance, PD numbers correlate with proliferative activity and potential pathways for intercellular signalling. Increases in PD numbers are ascribed to longitudinal fission as a major mechanism, whereas the decline in older derivatives is ascribed to PD degradation.     

G-protein-coupled receptor 1, G-protein Galpha-subunit 1, and prephenate dehydratase 1 are required for blue light-induced production of phenylalanine in etiolated Arabidopsis

Different classes of plant hormones and different wavelengths of light act through specific signal transduction mechanisms to coordinate higher plant development. A specific prephenate dehydratase protein (PD1) was discovered to have a strong interaction with the sole canonical G-protein Galpha-subunit (GPA1) in Arabidopsis (Arabidopsis thaliana). PD1 is a protein located in the cytosol, present in etiolated seedlings, with a specific role in blue light-mediated synthesis of phenylpyruvate and subsequently of phenylalanine (Phe). Insertion mutagenesis confirms that GPA1 and the sole canonical G-protein-coupled receptor (GCR1) in Arabidopsis also have a role in this blue light-mediated event. In vitro analyses indicate that the increase in PD1 activity is the direct and specific consequence of its interaction with activated GPA1. Because of their shared role in the light-mediated synthesis of phenylpyruvate and Phe, because they are iteratively interactive, and because activated GPA1 is directly responsible for the activation of PD1; GCR1, GPA1, and PD1 form all of or part of a signal transduction mechanism responsible for the light-mediated synthesis of phenylpyruvate, Phe, and those metabolites that derive from that Phe. Data are also presented to confirm that abscisic acid can act through the same pathway. An additional outcome of the work is the confirmation that phenylpyruvate acts as the intermediate in the synthesis of Phe in etiolated plants, as it commonly does in bacteria and fungi.     

Isolation of mutants of Arabidopsis thaliana in which accumulation of tobacco mosaic virus coat protein is reduced to low levels.

Summary We have found that Arahidopsis thaliana is susceptible to infection with a crucifer strain of tobacco mosaic virus (TMV-Cg); the coat protein of TMV-Cg accumulated to a high level in uninoculated rosette leaves several days after inoculation. As a first step in the search for host-coded factors that are involved in virus multiplication, we isolated mutants of A. thaliana in which the accumulation of TMV-Cg coat protein was reduced to low levels. Of 6000 M₂ plants descended from ethyl methanesulfonate-treated seeds, two such lines (PD 114 and PD378) were isolated. Genetic analyses suggested that the PD 114 phenotype was caused by a single nuclear recessive mutation, and that PD114 and PD378 belonged to the same complementation group. The coat protein accumulation of a tomato strain of TMV (TMVL) was also reduced in PD 114 plants compared to that in the wild-type plants. In contrast, PD114 plants infected with turnip crinkle or turnip yellow mosaic viruses, which belong to taxonomic groups other than Tobamovirus, expressed similar levels of these coat proteins as did infected wild-type plants.In this paper, we use the term multiplication (of a virus in a plant) to mean a substantial increase in virus concentration in the uninoculated leaves of the infected plant. Therefore, the efficiency of each process of invasion of the plant by the virus, uncoating, replication and degradation of the virus genome, formation and degradation of the virus particles, and spreading of the virus in the plant will affect the degree of multiplication     

Interactive effect of nitrogen fertilizer and plant density on photosynthetic and agronomical traits of cotton at different growth stages

Individual effects of application of nitrogen (N) and plant densities (PD) were reported in various studies; however an interactive effect of N and PD in cotton was not studied. To explore the benefits of interactive effects of N fertilizer and PD to increase the quality of cotton. This study was carried out in randomized complete block design (RCBD) with split plot arrangement. In split plot arrangement, main plot was consisted of N application rate and in sub plots different PD were done. There were two nitrogen levels; low N level (F1) 120 kg ha⁻¹ and high N level (F2) 180 kg ha⁻¹ and three planting densities; 8 plants m⁻² as low density (LD), 10 plants m⁻² as medium density (MD) and 12 plants m⁻² as high density (HD). In this study we observed the interactive effect of N application levels and PD on cotton photosynthetic and agronomic traits of various stages of development. Results showed that cotton growth and N contents was varied among treatments on different development stages. Plant biomass production, photosynthetic rate (Pn), intercellular CO₂ (C_i), water use efficiency (WUE) and N contents were unaffected at the seedling stage by N application rate and PD, however, the highest Pn, C_i and N contents was at squaring stage followed by blooming stage. Higher seed cotton yield and lint yield were obtained F1 with HD, and F2 with MD yielded the highest N contents and cotton yield among treatments. We found that the squaring stage was more critical, followed by the blooming stage when considering N rate and PD.     

PD1, an S-like RNase gene from a self-incompatible cultivar of almond

 Many flowering plants contain stylar S-RNases that are involved in self-incompatibility and S-like RNases of which the biological function is uncertain. This paper reports the deduced amino acid sequence of an S-like RNase gene (PD1) from the self-incompatible plant Prunus dulcis (almond). The amino acid sequence of PD1, which was derived from cDNA and genomic DNA clones, showed 34–86% identity to acidic plant S-like RNases reported so far, with the highest degree of similarity being to an S-like RNase from Japanese pear (Pyrus pyrifolia). Based on RNA hybridisation experiments it appears that, like for many other S-like RNases, the expression of PD1 is not pistil-specific. Analysis of the genomic structure revealed the presence of three introns, of which one is similar in location to that of the related S-RNase gene from Solanaceae and Rosaceae. At least four bands hybridising to PD1 were found upon Southern hybridisation, suggesting the presence of a multigene family of S-like RNase genes in almond. The putative biological function of PD1 is discussed. Received: 22 November 1999 / Revision received: 18 February 2000 · Accepted: 13 March 2000     

Uptake of Phosphate by White Clover: I. EVIDENCE FOR AN ELECTROGENIC PHOSPHATE PUMP

Uptake of phosphate from flowing solution and cell membraneelectrical potential differences (PD) have been followed simultaneouslyin the roots of Trifolium repens L. Intact plants were usedand it was found that uptake and PD were strongly influencedby the shoot. They were reduced by excision of the root, defoliationof the plant, and lowering the light intensity at the leaf surface.2, 4-Dinitrophenol caused the PD to decline by approximately40 mV. A close correlation between phosphate uptake and PD wasobtained over a wide range of conditions and removal of phosphatefrom the solution resulted in a decline in PD of about 35 mV.It was concluded that there is an electrogenic phosphate pumpin T. repens roots which is closely dependent on the carbohydratesupply from the shoot.     

The cytoskeleton in plasmodesmata: a role in intercellular transport?

Actin and myosin are components of the plant cell cytoskeleton that extend from cell to cell through plasmodesmata (PD), but it is unclear how they are organized within the cytoplasmic sleeve or how they might behave as regulatory elements. Early work used antibodies to locate actin and myosin to PD, at the electron microscope level, or to pitfields (aggregations of PD in the cell wall), using immunofluorescence techniques. More recently, a green fluorescent protein (GFP)-tagged plant myosin VIII was located specifically at PD-rich pitfields in cell walls. Application of actin or myosin disrupters may modify the conformation of PD and alter rates of cell-cell transport, providing evidence for a role in regulating PD permeability. Intriguingly, there is now evidence of differentiation between types of PD, some of which open in response to both actin and myosin disrupters, and others which are unaffected by actin disrupters or which close in response to myosin inhibitors. Viruses also interact with elements of the cytoskeleton for both intracellular and intercellular transport. The precise function of the cytoskeleton in PD may change during cell development, and may not be identical in all tissue types, or even in all PD within a single cell. Nevertheless, it is likely that actin- and myosin-associated proteins play a key role in regulating cell-cell transport, by interacting with cargo and loading it into PD, and may underlie the capacity for one-way transport across particular cell and tissue boundaries.