Inhibition of plant viral systemic infection by non-toxic concentrations of cadmium

Growth and nutritional responses of wild and domesticated cacao genotypes to soil Cd stress

Cadmium (Cd), a toxic non-essential metal, is easily accumulated in cacao tissues. This represents a risk for cacao exportation, and consequently it affects the economic well-being of the resource-poor-small-producers in Latin America. A greenhouse experiment was conducted with 53 wild and domesticated cacao genotypes to determine their response to Cd in terms of growth and Cd and essential nutrients accumulation. Cacao seedlings were grown for 6 months in an acidic soil with or without added Cd. The total concentration of macro (Ca, K, Mg, N and P) and micronutrients (B, Cu, Fe, Mn and Zn) as well as Cd were measured in shoots along with growth (biometric) parameters after harvest. The results revealed that even if there was a wide range of Cd concentrations among genotypes, there was a reduction in the concentration of essential nutrients in genotypes grown in Cd spiked soils, however these concentrations were not significantly different from the control. In the case of growth parameters, the effects of Cd were diverse across all genotypes some of them being more tolerant to Cd stress than others. Thus, different growth responses to Cd stress are related to a genotype effect. Based on their lower Cd concentration, a total of 11 cacao genotypes (AYP-22, PAS-105, UGU-126, ICT-1026, ICT-1087, ICT-1189, ICT-1292, PH-17, CCN-51, ICS-39 and TSH-565) are proposed here as low Cd-accumulating genotypes. Therefore, these genotypes are potentially useful as rootstock to reduce uptake and transport of Cd, especially in economically important cacao cultivars.

Movement of small RNAs in and between plants and fungi

RNA interference is a biological process whereby small RNAs inhibit gene expression through neutralizing targeted mRNA molecules. This process is conserved in eukaryotes. Here, recent work regarding the mechanisms of how small RNAs move within and between organisms is examined. Small RNAs can move locally and systemically in plants through plasmodesmata and phloem, respectively. In fungi, transportation of small RNAs may also be achieved by septal pores and vesicles. Recent evidence also supports bidirectional cross-kingdom communication of small RNAs between host plants and adapted fungal pathogens to affect the outcome of infection. We discuss several mechanisms for small RNA trafficking and describe evidence for transport through naked form, combined with RNA-binding proteins or enclosed by vesicles.

Hexanoic Acid Treatment Prevents Systemic MNSV Movement in Cucumis melo Plants by Priming Callose Deposition Correlating SA and OPDA Accumulation

Unlike fungal and bacterial diseases, no direct method is available to control viral diseases. The use of resistance-inducing compounds can be an alternative strategy for plant viruses. Here we studied the basal response of melon to Melon necrotic spot virus (MNSV) and demonstrated the efficacy of hexanoic acid (Hx) priming, which prevents the virus from systemically spreading. We analysed callose deposition and the hormonal profile and gene expression at the whole plant level. This allowed us to determine hormonal homeostasis in the melon roots, cotyledons, hypocotyls, stems and leaves involved in basal and hexanoic acid-induced resistance (Hx-IR) to MNSV. Our data indicate important roles of salicylic acid (SA), 12-oxo-phytodienoic acid (OPDA), jasmonic-isoleucine, and ferulic acid in both responses to MNSV. The hormonal and metabolites balance, depending on the time and location associated with basal and Hx-IR, demonstrated the reprogramming of plant metabolism in MNSV-inoculated plants. The treatment with both SA and OPDA prior to virus infection significantly reduced MNSV systemic movement by inducing callose deposition. This demonstrates their relevance in Hx-IR against MNSV and a high correlation with callose deposition. Our data also provide valuable evidence to unravel priming mechanisms by natural compounds.

The transcriptomics of an experimentally evolved plant-virus interaction

Models of plant-virus interaction assume that the ability of a virus to infect a host genotype depends on the matching between virulence and resistance genes. Recently, we evolved tobacco etch potyvirus (TEV) lineages on different ecotypes of Arabidopsis thaliana, and found that some ecotypes selected for specialist viruses whereas others selected for generalists. Here we sought to evaluate the transcriptomic basis of such relationships. We have characterized the transcriptomic responses of five ecotypes infected with the ancestral and evolved viruses. Genes and functional categories differentially expressed by plants infected with local TEV isolates were identified, showing heterogeneous responses among ecotypes, although significant parallelism existed among lineages evolved in the same ecotype. Although genes involved in immune responses were altered upon infection, other functional groups were also pervasively over-represented, suggesting that plant resistance genes were not the only drivers of viral adaptation. Finally, the transcriptomic consequences of infection with the generalist and specialist lineages were compared. Whilst the generalist induced very similar perturbations in the transcriptomes of the different ecotypes, the perturbations induced by the specialist were divergent. Plant defense mechanisms were activated when the infecting virus was specialist but they were down-regulated when infecting with generalist.

Going mobile: non-cell-autonomous small RNAs shape the genetic landscape of plants

RNA silencing is a form of genetic regulation, which is conserved across eukaryotes and has wide ranging biological functions. Recently, there has been a growing appreciation for the importance of mobility in RNA silencing pathways, particularly in plants. Moreover, in addition to the importance for mobile RNA silencing in an evolutionary context, the potential for utilizing mobile short silencing RNAs in biotechnological applications is becoming apparent. This review aims to set current knowledge of this topic in a historical context and provides examples to illustrate the importance of mobile RNA silencing in both natural and artificially engineered systems in plants.

The current status of the elemental defense hypothesis in relation to pathogens

Metal hyperaccumulating plants are able to accumulate exceptionally high concentrations of metals, such as zinc, nickel, or cadmium, in their aerial tissues. These metals reach concentrations that would be toxic to most other plant species. This trait has evolved multiple times independently in the plant kingdom. Recent studies have provided new insight into the ecological and evolutionary significance of this trait, by showing that some metal hyperaccumulating plants can use high concentrations of accumulated metals to defend themselves against attack by pathogenic microorganisms and herbivores. Here, we review the evidence that metal hyperaccumulation acts as a defensive trait in plants, with particular emphasis on plant-pathogen interactions. We discuss the mechanisms by which defense against pathogens might have driven the evolution of metal hyperaccumulation, including the interaction of this trait with other forms of defense. In particular, we consider how physiological adaptations and fitness costs associated with metal hyperaccumulation could have resulted in trade-offs between metal hyperaccumulation and other defenses. Drawing on current understanding of the population ecology of metal hyperaccumulator plants, we consider the conditions that might have been necessary for metal hyperaccumulation to be selected as a defensive trait, and discuss the likelihood that these were fulfilled. Based on these conditions, we propose a possible scenario for the evolution of metal hyperaccumulation, in which selective pressure for resistance to pathogens or herbivores, combined with gene flow from non-metallicolous populations, increases the likelihood that the metal hyperaccumulating trait becomes established in plant populations.

To gate, or not to gate: regulatory mechanisms for intercellular protein transport and virus movement in plants

Cell-to-cell signal transduction is vital for orchestrating the whole-body physiology of multi-cellular organisms, and many endogenous macromolecules, proteins, and nucleic acids function as such transported signals. In plants, many of these molecules are transported through plasmodesmata (Pd), the cell wall-spanning channel structures that interconnect plant cells. Furthermore, Pd also act as conduits for cell-to-cell movement of most plant viruses that have evolved to pirate these channels to spread the infection. Pd transport is presumed to be highly selective, and only a limited repertoire of molecules is transported through these channels. Recent studies have begun to unravel mechanisms that actively regulate the opening of the Pd channel to allow traffic. This macromolecular transport between cells comprises two consecutive steps: intracellular targeting to Pd and translocation through the channel to the adjacent cell. Here, we review the current knowledge of molecular species that are transported though Pd and the mechanisms that control this traffic. Generally, Pd traffic can occur by passive diffusion through the trans-Pd cytoplasm or through the membrane/lumen of the trans-Pd ER, or by active transport that includes protein-protein interactions. It is this latter mode of Pd transport that is involved in intercellular traffic of most signal molecules and is regulated by distinct and sometimes interdependent mechanisms, which represent the focus of this article.

Effect of lead (Pb) on the systemic movement of RNA viruses in tobacco (Nicotiana tabacum var. Turkish)

Effect of various lead (Pb) concentrations on the systemic movement of RNA viruses was examined in tobacco plants. Prior to inoculation, plants were grown hydroponically for 6 days in Hoagland's solution supplemented with five concentrations of lead nitrate [Pb(NO(3))(2)]: 0.0 (control), 10, 15, 50, and 100 μM. Four different RNA viruses with different cell-to-cell movement mechanisms were used. Two weeks after inoculation lower and upper leaves of each treatment were harvested and examined for the presence of viral coat protein. In plants inoculated with Tobacco mosaic virus, Potato virus X, and Tobacco etch virus, TEM images and western blot assays confirmed the presence of viral coat proteins in the upper leaves of all lead treatments. However, in plants inoculated with Turnip vein-clearing virus (TVCV), no signs of viral particles were detected in the upper leaves of plants treated with 10 μM or 15 μM lead nitrate. In contrast, plants treated with high concentrations of lead nitrate (50 μM or 100 μM) showed viral particles in their upper leaves. In plants treated with 10 μM or 15 μM lead nitrate, callose accumulation was the same as in control plants. This suggests that non-toxic concentrations of lead nitrate may trigger the production of putative cellular factors in addition to callose that interfere with the TVCV systemic movement. In contrast, plants treated with 100 μM lead nitrate showed less callose as compared to control plants, facilitating the systemic movement of TVCV.

ANK, a host cytoplasmic receptor for the Tobacco mosaic virus cell-to-cell movement protein, facilitates intercellular transport through plasmodesmata

Plasmodesma (PD) is a channel structure that spans the cell wall and provides symplastic connection between adjacent cells. Various macromolecules are known to be transported through PD in a highly regulated manner, and plant viruses utilize their movement proteins (MPs) to gate the PD to spread cell-to-cell. The mechanism by which MP modifies PD to enable intercelluar traffic remains obscure, due to the lack of knowledge about the host factors that mediate the process. Here, we describe the functional interaction between Tobacco mosaic virus (TMV) MP and a plant factor, an ankyrin repeat containing protein (ANK), during the viral cell-to-cell movement. We utilized a reverse genetics approach to gain insight into the possible involvement of ANK in viral movement. To this end, ANK overexpressor and suppressor lines were generated, and the movement of MP was tested. MP movement was facilitated in the ANK-overexpressing plants, and reduced in the ANK-suppressing plants, demonstrating that ANK is a host factor that facilitates MP cell-to-cell movement. Also, the TMV local infection was largely delayed in the ANK-suppressing lines, while enhanced in the ANK-overexpressing lines, showing that ANK is crucially involved in the infection process. Importantly, MP interacted with ANK at PD. Finally, simultaneous expression of MP and ANK markedly decreased the PD levels of callose, β-1,3-glucan, which is known to act as a molecular sphincter for PD. Thus, the MP-ANK interaction results in the downregulation of callose and increased cell-to-cell movement of the viral protein. These findings suggest that ANK represents a host cellular receptor exploited by MP to aid viral movement by gating PD through relaxation of their callose sphincters.

Long-distance transport of macromolecules through the phloem

Long-distance phloem transport of small metabolites has long been the subject of many different studies concentrating on resource allocation and signalling between plant organs. Also, phloem movement of viruses has long been examined as the route for systemic infection of the plant. Only recently, the transport of macromolecules, such as proteins and nucleic acids, has received increasing attention because they are regarded as being a new class of potential information-transmitter. A set of recent publications allows the first insights into the important roles that phloem-mobile macromolecules might play in the regulation of development and the responses to stress. Furthermore, they start to shed light on the mechanisms involved in systemic macromolecule transport.

Intercellular trafficking of macromolecules during embryogenesis

Plasmodesmata provide routes for communication and nutrient transfer between plant cells by interconnecting the cytoplasm of adjacent cells. A simple fluorescent tracer-loading assay was developed to monitor patterns of cell to cell transport via plasmodesmata specifically during embryogenesis. A developmental transition in plasmodesmatal size exclusion limit was found to occur at the torpedo stage of embryogenesis in Arabidopsis; at this time, plasmodesmata are downregulated, allowing transport of small (approximately 0.5 kDa) but not large (approximately 10 kDa) tracers. This assay system was used to screen for embryo defective mutants, designated increased size exclusion limit of plasmodesmata that maintain dilated plasmodesmata at the torpedo stage.

Physiological effects of constitutive expression of Oilseed Rape Mosaic Tobamovirus (ORMV) movement protein in Arabidopsis thaliana

Movement proteins (MPs) are non-cell autonomous viral-encoded proteins that assist viruses in their cell-to-cell movement. The MP encoded by Tobamoviruses is the best characterized example among MPs of non-tubule-inducing plant RNA viruses. The MP of Oilseed Rape Mosaic Tobamovirus (ORMV) was transgenically expressed in Arabidopsis thaliana, ecotype RLD, under the expression of the 35S promoter from Cauliflower Mosaic Virus. Transgenic lines were obtained in sense and antisense orientations. One of the sense transgenic lines was further characterized turning out to carry one copy of the transgene inserted in the terminal region of the right arm of chromosome 1. The constitutive expression of ORMV-MP induced mild physiological effects in Arabidopsis. Plants of the transgenic line allowed a faster systemic movement of the phloem tracer carboxyfluorescein. The tracer was unloaded differentially in different flower parts, revealing differential effects of ORMV-MP on phloem unloading in sink organs. On the other hand, transgenic Arabidopsis did not show any effect on biomass partitioning or sugar availability, effects reported for equivalent transgenic solanaceous plants expressing the MP of Tobacco Mosaic Virus, another Tobamovirus. Finally, the transgenic Arabidopsis plants were susceptible to ORMV infection, although showing milder overall symptoms than non-transgenic controls. The results highlight the relevance of the specific host-virus system, in the physiological outcome of the molecular interactions established by MPs.

Can metals defend plants against biotic stress?

Farmers have used metal compounds in phytosanitary treatments for more than a century; however, it has recently been suggested that plants absorb high concentrations of metals from the substrate as a self-defense mechanism against pathogens and herbivores. This metal defense hypothesis is among the most attractive proposals for the 'reason to be' of metal hyperaccumulator species. On a molecular basis, metal defense against biotic stress seems to imply common and/or complementary pathways of signal perception, signal transduction and metabolism. This does not imply a broad band of co-resistance to different stress types but reflects a continuous cross talk during the coevolution of plants, pathogens and herbivores competing in an environment where efficient metal ion acquisition and ion homeostasis are essential for survival.

Identification of an interactor of cadmium ion-induced glycine-rich protein involved in regulation of callose levels in plant vasculature

Cadmium-induced glycine-rich protein (cdiGRP) is a cell wall-associated factor that increases callose levels in plant vasculature. To better understand the cdiGRP/callose regulation system, we identified a tobacco protein, GrIP (cdiGRP-interacting protein, GrIP), that associates with cdiGRP and localizes at the plant cell wall. Constitutive overexpression of GrIP enhanced the accumulation of the cdiGRP protein and callose in vasculature-associated cells with or without treatment with cadmium ions. That GrIP gene expression was not affected by cadmium ions indicated that GrIP does not directly modulate the callose levels induced by the treatment. Instead, GrIP most likely functions by further elevating the accumulated amount of cdiGRP, the expression of which is up-regulated by the cadmium ions. Interestingly, the levels of cdiGRP mRNA were not affected by constitutive expression of GrIP, demonstrating that the enhancement in cdiGRP protein accumulation by GrIP overexpression occurs posttranslationally. Collectively, these observations suggest that GrIP interacts with cdiGRP and increases its level of accumulation; in turn, the elevated amounts of cdiGRP induce callose deposits in the plant cell walls. Therefore, GrIP and cdiGRP represent sequentially acting factors in a biochemical pathway that regulates callose accumulation in the plant vasculature.

Effects of calreticulin on viral cell-to-cell movement

Cell-to-cell tobacco mosaic virus movement protein (TMV MP) mediates viral spread between the host cells through plasmodesmata. Although several host factors have been shown to interact with TMV MP, none of them coresides with TMV MP within plasmodesmata. We used affinity purification to isolate a tobacco protein that binds TMV MP and identified it as calreticulin. The interaction between TMV MP and calreticulin was confirmed in vivo and in vitro, and both proteins were shown to share a similar pattern of subcellular localization to plasmodesmata. Elevation of the intracellular levels of calreticulin severely interfered with plasmodesmal targeting of TMV MP, which, instead, was redirected to the microtubular network. Furthermore, in TMV-infected plant tissues overexpressing calreticulin, the inability of TMV MP to reach plasmodesmata substantially impaired cell-to-cell movement of the virus. Collectively, these observations suggest a functional relationship between calreticulin, TMV MP, and viral cell-to-cell movement.

Direct role of a viroid RNA motif in mediating directional RNA trafficking across a specific cellular boundary

The plasmodesmata and phloem form a symplasmic network that mediates direct cell-cell communication and transport throughout a plant. Selected endogenous RNAs, viral RNAs, and viroids traffic between specific cells or organs via this network. Whether an RNA itself has structural motifs to potentiate trafficking is not well understood. We have used mutational analysis to identify a motif that the noncoding Potato spindle tuber viroid RNA evolved to potentiate its efficient trafficking from the bundle sheath into mesophyll that is vital to establishing systemic infection in tobacco (Nicotiana tabacum). Surprisingly, this motif is not necessary for trafficking in the reverse direction (i.e., from the mesophyll to bundle sheath). It is not required for trafficking between other cell types either. We also found that the requirement for this motif to mediate bundle sheath-to-mesophyll trafficking is dependent on leaf developmental stages. Our results provide genetic evidence that (1) RNA structural motifs can play a direct role in mediating trafficking across a cellular boundary in a defined direction, (2) the bundle sheath-mesophyll boundary serves as a novel regulatory point for RNA trafficking between the phloem and nonvascular tissues, and (3) the symplasmic network remodels its capacity to traffic RNAs during plant development. These findings may help further studies to elucidate the interactions between RNA motifs and cellular factors that potentiate directional trafficking across specific cellular boundaries.

Synthesis of phytochelatins and modulation of antioxidants in response to cadmium stress in Cuscuta reflexa--an angiospermic parasite

Effect of cadmium on growth, antioxidative enzymes namely catalase, peroxidase, glutathione reductase, level of glutathione and phytochelatin synthesis was investigated in callus and seedlings of Cuscuta reflexa. A time, concentration and tissue dependent response of Cd was observed. Cd inhibited the growth of callus and seedlings by 50% at 300 and 500 micromol/L concentrations, respectively. Shorter exposure of low concentration of Cd led to augmentation of antioxidant activity, both in callus and seedlings, while longer exposure and high concentration of Cd led to a concentration dependent decrease in callus. Analysis of phytochelatin (PC) synthesis in callus and seedlings of C. reflexa revealed both quantitative and qualitative changes. Cd at low concentrations led to synthesis of predominantly PC4, while at higher concentrations, PC3 was the major form being synthesized. Amelioration of antioxidative systems of C. reflexa in response to Cd stress might be playing a protective role, alleviating the damaging effects of ROS, generated during Cd stress. Concomitantly, chelation and sequestering of toxic Cd ions in this parasite was mediated by synthesis of PC. The response to Cd stress shown by this holoparasitic plant was found to be similar to those of non-parasitic plants (hosts).

Macromolecular Transport and Signaling Through Plasmodesmata

Plasmodesmata (Pd) are channels in the plant cell wall that in conjunction with associated phloem form an intercellular communication network that supports the cell-to-cell and long-distance trafficking of a wide spectrum of endogenous proteins and ribonucleoprotein complexes. The trafficking of such macromolecules is of importance in the orchestration of non-cell autonomous developmental and physiological processes. Plant viruses encode movement proteins (MPs) that subvert this communication network to facilitate the spread of infection. These viral proteins thus represent excellent experimental keys for exploring the mechanisms involved in intercellular trafficking and communication via Pd.

Systemic movement of a tobamovirus requires host cell pectin methylesterase

Systemic movement of plant viruses through the host vasculature, one of the central events of the infection process, is essential for maximal viral accumulation and development of disease symptoms. The host plant proteins involved in this transport, however, remain unknown. Here, we examined whether or not pectin methylesterase (PME), one of the few cellular proteins known to be involved in local, cell-to-cell movement of tobacco mosaic virus (TMV), is also required for the systemic spread of viral infection through the plant vascular system. In a reverse genetics approach, PME levels were reduced in tobacco plants using antisense suppression. The resulting PME antisense plants displayed a significant degree of PME suppression in their vascular tissues but retained the wild-type pattern of phloem loading and unloading of a fluorescent solute. Systemic transport of TMV in these plants, however, was substantially delayed as compared to the wild-type tobacco, suggesting a role for PME in TMV systemic infection. Our analysis of virus distribution in the PME antisense plants suggested that TMV systemic movement may be a polar process in which the virions enter and exit the vascular system by two different mechanisms, and it is the viral exit out of the vascular system that involves PME.

Turnip vein-clearing virus, from pathogen to host expression profile

SUMMARY Taxonomy: Turnip vein-clearing virus (TVCV) is a member of subgroup 3 of the Tobamovirus genus and is thus a member of the alphavirus-like supergroup of positive sense RNA-containing viruses. Physical properties: Virions, typical of tobamoviruses, are rod-shaped and consist of a single species of four-helix bundle capsid proteins of 17 kDa helically arranged around a 6.3 knt RNA which accounts for 5% of the virion mass. Virions are stable for years. Hosts: Members of the crucifer family are excellent hosts. Particularly noteworthy is that hosts include the model plant for molecular genetics, Arabidopsis thaliana. No non-mechanical vectors of transmission are known.

Movement of potato spindle tuber viroid reveals regulatory points of phloem-mediated RNA traffic

Increasing evidence indicates that the phloem mediates traffic of selective RNAs within a plant. How an RNA enters, moves in, and exits the phloem is poorly understood. Potato spindle tuber viroid (PSTVd) is a pathogenic RNA that does not encode proteins and is not encapsidated, and yet it replicates autonomously and traffics systemically within an infected plant. The viroid RNA genome must interact directly with cellular factors to accomplish these functions and is, therefore, an excellent probe to study mechanisms that regulate RNA traffic. Our analyses of PSTVd traffic in Nicotiana benthamiana yielded evidence that PSTVd movement within sieve tubes does not simply follow mass flow from source to sink organs. Rather, this RNA is transported into selective sink organs. Furthermore, two PSTVd mutants can enter the phloem to spread systemically but cannot exit the phloem in systemic leaves of tobacco (Nicotiana tabacum). A viroid most likely has evolved structural motifs that mimic endogenous plant RNA motifs so that they are recognized by cellular factors for traffic. Thus, analysis of PSTVd traffic functions may provide insights about endogenous mechanisms that control phloem entry, transport, and exit of RNAs.

The systemic movement of a tobamovirus is inhibited by a cadmium-ion-induced glycine-rich protein

Systemic movement is central to plant viral infection. Exposure of tobacco plants to low levels of cadmium ions blocks the systemic spread of turnip vein-clearing tobamovirus (TVCV). We identified a tobacco glycine-rich protein, cdiGRP, specifically induced by low concentrations of cadmium and expressed in the cell walls of plant vascular tissues. Constitutive cdiGRP expression inhibited systemic transport of TVCV, whereas suppression of cdiGRP production allowed TVCV movement in the presence of cadmium. cdiGRP exerted its inhibitory effect on TVCV transport by enhancing callose deposits in the vasculature. So cdiGRP may function to control plant viral systemic movement.

RNA commutes to work: regulation of plant gene expression by systemically transported RNA molecules

Although long-distance movement of endogenous mRNAs in plants is well established, the functional contributions of these transported RNA molecules has remained unclear. In a recent report, Kim et al.2001 showed that systemically transported mRNA is capable of causing phenotypic change in developing tissue. Here, this finding and its significance are reviewed and discussed in detail. In addition, in order to give proper perspective, long-distance transport of other types of RNAs, e.g., RNA elicitors of post-transcriptional gene silencing and RNA genomes of plant viruses, and its possible regulation are discussed.

Inhibition of systemic onset of post-transcriptional gene silencing by non-toxic concentrations of cadmium

Post-transcriptional gene silencing (PTGS) is an important mechanism for regulation of plant gene expression and virus-plant interactions. To better understand this process, the heavy metal cadmium was identified as a specific inhibitor in two different PTGS systems, constitutive and inducible. The pattern of cadmium-induced inhibition of PTGS allowed several insights into PTGS development. First, cadmium treatment prevented only systemic but not local onset of PTGS, uncoupling between these two modes of PTGS. Second, non-toxic, but not toxic, levels of cadmium inhibited PTGS, suggesting induction of a pathway that interferes with PTGS. Third, cadmium effects on PTGS closely paralleled those on the movement of tobamoviruses, suggesting that both processes may share common steps in their systemic transport pathways. Interestingly, these effects of cadmium do not represent a general property of toxic metal ions because two other such elements, that is zinc and aluminum, did not interfere with PTGS and viral systemic movement.

Phosphorylation down-regulates the RNA binding function of the coat protein of potato virus A

Plant viruses encode movement proteins (MPs) to facilitate transport of their genomes from infected into neighboring healthy cells through plasmodesmata. Growing evidence suggests that specific phosphorylation events can regulate MP functions. The coat protein (CP) of potato virus A (PVA; genus Potyvirus) is a multifunctional protein involved both in virion assembly and virus movement. Labeling of PVA-infected tobacco leaves with [(33)P]orthophosphate demonstrated that PVA CP is phosphorylated in vivo. Competition assays established that PVA CP and the well characterized 30-kDa MP of tobacco mosaic virus (genus Tobamovirus) are phosphorylated in vitro by the same Ser/Thr kinase activity from tobacco leaves. This activity exhibits a strong preference for Mn(2+) over Mg(2+), can be inhibited by micromolar concentrations of Zn(2+) and Cd(2+), and is not Ca(2+)-dependent. Tryptic phosphopeptide mapping revealed that PVA CP was phosphorylated by this protein kinase activity on multiple sites. In contrast, PVA CP was not phosphorylated when packaged into virions, suggesting that the phosphorylation sites are located within the RNA binding domain and not exposed on the surface of the virion. Furthermore, two independent experimental approaches demonstrated that the RNA binding function of PVA CP is strongly inhibited by phosphorylation. From these findings, we suggest that protein phosphorylation represents a possible mechanism regulating formation and/or stability of viral ribonucleoproteins in planta.

Nucleic acid transport in plant-microbe interactions: the molecules that walk through the walls

Many microbes "genetically invade" plants by introducing DNA or RNA molecules into the host cells. For example, plant viruses transport their genomes between host cells, whereas Agrobacterium spp. transfer T-DNA to the cell nucleus and integrate it into the plant DNA. During these events, the transported nucleic acids must negotiate several barriers, such as plant cell walls, plasma membranes, and nuclear envelopes. This review describes the microbial and host proteins that participate in cell-to-cell transport and nuclear import of nucleic acids during infection by plant viruses and Agrobacterium spp. Possible molecular mechanisms by which these transport processes occur are discussed.

Nickel increases susceptibility of a nickel hyperaccumulator to Turnip mosaic virus

Hyperaccumulated Ni can defend plant tissues against herbivores and pathogens. The effectiveness of this defense, however, has not been tested with a viral pathogen. Turnip mosaic virus (TuMV) accumulation was studied in two serpentine species of Streptanthus with different Ni uptake abilities. Plants of a Ni hyperaccumulator, milk-wort jewelflower (S. polygaloides Gray), and a non-hyperaccumulator, plumed jewelflower (S. insignis Jepson), were grown on Ni-amended and unamended soils. Plants were inoculated with TuMV at three different phenological stages: basal rosette, bolting, and flowering. Susceptibility of experimental plants to TuMV was determined either by the magnitude of TuMV accumulation (measured by indirect enzyme-linked immunosorbent assay [ELISA]) or by plant survival. Streptanthus polygaloides plants grown on high-Ni soil were more susceptible to TuMV than low-Ni S. polygaloides at all three phenological stages. All rosette and pre-bolt S. insignis plants were infected by TuMV, but survival and TuMV accumulation were not significantly affected by soil Ni. At flowering, only high-Ni S. polygaloides plants became infected. For S. polygaloides, elevated tissue Ni concentrations enhanced TuMV infection instead of defending plants from the virus. To reduce risks to nearby agricultural crops, future phytoremed. iation and phytomining operations using this species should incorporate management plans to prevent the creation of artificial reservoirs of TuMV inoculum.

Regulation of plasmodesmal transport by phosphorylation of tobacco mosaic virus cell-to-cell movement protein

Cell-to-cell spread of tobacco mosaic virus (TMV) through plant intercellular connections, the plasmodesmata, is mediated by a specialized viral movement protein (MP). In vivo studies using transgenic tobacco plants showed that MP is phosphorylated at its C-terminus at amino acid residues Ser258, Thr261 and Ser265. When MP phosphorylation was mimicked by negatively charged amino acid substitutions, MP lost its ability to gate plasmodesmata. This effect on MP-plasmodesmata interactions was specific because other activities of MP, such as RNA binding and interaction with pectin methylesterases, were not affected. Furthermore, TMV encoding the MP mutant mimicking phosphorylation was unable to spread from cell to cell in inoculated tobacco plants. The regulatory effect of MP phosphorylation on plasmodesmal permeability was host dependent, occurring in tobacco but not in a more promiscuous Nicotiana benthamiana host. Thus, phosphorylation may represent a regulatory mechanism for controlling the TMV MP-plasmodesmata interactions in a host-dependent fashion.

Interaction between the tobacco mosaic virus movement protein and host cell pectin methylesterases is required for viral cell-to-cell movement

Virus-encoded movement protein (MP) mediates cell-to-cell spread of tobacco mosaic virus (TMV) through plant intercellular connections, the plasmodesmata. The molecular pathway by which TMV MP interacts with the host cell is largely unknown. To understand this process better, a cell wall-associated protein that specifically binds the viral MP was purified from tobacco leaf cell walls and identified as pectin methylesterase (PME). In addition to TMV MP, PME is recognized by MPs of turnip vein clearing virus (TVCV) and cauliflower mosaic virus (CaMV). The use of amino acid deletion mutants of TMV MP showed that its domain was necessary and sufficient for association with PME. Deletion of the PME-binding region resulted in inactivation of TMV cell-to-cell movement.

Cell-to-cell movement of tobacco mosaic virus: enigmas and explanations

Abstract Tobacco mosaic virus (TMV) spreads between cells through plant intercellular connections, the plasmodesmata. This transport process is mediated by a specialized virus-encoded movement protein, TMV MP. Recent advances in two major aspects of TMV MP function highlight the limits of our current knowledge and promise exciting future developments. First, findings that TMV MP interacts with cytoskeletal elements and cell wall proteins suggest potential mechanisms for TMV MP targeting from the cell cytoplasm to plasmodesmal channels. Second, indications that TMV MP phosphorylation plays a regulatory role in several activities of TMV MP begin to unravel molecular pathways that control TMV cell-to-cell transport. TMV systemic movement that follows its initial cell-to-cell spread, on the other hand, may be controlled through two different pathways used for viral entry into and exit from the host plant vascular tissue.

Macromolecular trafficking in the phloem

Prior to the early 1990s, the functional aspects of long-distance movement in the phloem were viewed primarily in terms of the transport of sugars and other photoassimilates. The soluble proteins in phloem exudates were often considered to be artifacts arising from the cytoplasmic degeneration of the conducting elements or a confounding anomaly of the sugar transport system. Recent work indicates a much more complex system where proteins and other macromolecules play significant roles in phloem function and long-distance signaling throughout the plant.

Preservation of plant cell ultrastructure during immunolocalization of virus particles

Most immunoelectron microscopy techniques used for ultrastructural analyses of virus-infected plant tissues significantly compromise cellular membranous structures as well as overall contrast and resolution of the image. Here, we describe a protocol which avoids these flaws but retains full antigenicity of the sample. A direct comparison of the conventional and the improved electron immunostaining procedures is presented using tobacco and Arabidopsis thaliana plants infected with turnip vein clearing virus.

Non-toxic concentrations of cadmium inhibit systemic movement of turnip vein clearing virus by a salicylic acid-independent mechanism

Systemic movement of plant viruses is a central event in viral infection. To better understand this process, the heavy metal cadmium was used to inhibit systemic spread of turnip vein clearing virus (TVCV), a tobamovirus, in tobacco plants. Study of the mechanism by which cadmium exerts this inhibitory effect may provide insights into the essential steps of the TVCV systemic movement pathway. Our results demonstrated that cadmium treatment did not affect TVCV transport from the inoculated non-vascular tissue into the plant vasculature but blocked viral exit into uninoculated non-vascular tissues. Thus, TVCV virions may enter and exit the host plant vascular system by two different mechanisms. We also showed that cadmium-treated plants still supported systemic spread of an unrelated tobacco etch virus (TEV), suggesting multiple pathways for systemic infection. Finally, cadmium-induced arrest in TVCV systemic infection was shown to occur by a salicylic acid-independent mechanism.