Actin depolymerization affects stress-induced translational activity of potato tuber tissue

Comparative Proteomics of Potato Cultivars with a Variable Dormancy Period

The control of the duration of the dormancy phase is a significant challenge in the potato industry and for seed producers. However, the proteome landscape involved in the regulation of the length of the dormancy period over potato cultivars remains largely unexplored. In this study, we performed for the first time a comparative proteome profiling of potato cultivars with differential duration of tuber dormancy. More specifically, the proteome profiling of Agata, Kennebec and Agria commercial potato varieties with short, medium and medium-long dormancy, respectively, was assessed at the endodormancy stage using high-resolution two-dimensional electrophoresis (2-DE) coupled to reversed-phase liquid chromatography–tandem mass spectrometry (LC-TripleTOF MS/MS). A total of 11 proteins/isoforms with statistically significant differential abundance among cultivars were detected on 2-DE gels and confidently identified by LC-TripleTOF MS/MS. Identified proteins have known functions related to tuber development, sprouting and the oxylipins biosynthesis pathway. Fructokinase, a mitochondrial ADP/ATP carrier, catalase isozyme 2 and heat shock 70 kDa were the proteins with the strongest response to dormancy variations. To the best of our knowledge, this study reports the first candidate proteins underlying variable dormancy length in potato cultivars.

Potato Periderm Development and Tuber Skin Quality

The periderm is a corky tissue that replaces the epidermis when the latter is damaged, and is critical for preventing pathogen invasion and water loss. The periderm is formed through the meristematic activity of phellogen cells (cork cambium). The potato skin (phellem cells) composes the outer layers of the tuber periderm and is a model for studying cork development. Early in tuber development and following tuber expansion, the phellogen becomes active and produces the skin. New skin layers are continuously added by division of the phellogen cells until tuber maturation. Some physiological disorders of the potato tuber are related to abnormal development of the skin, including skinning injuries and russeting of smooth-skinned potatoes. Thus, characterizing the potato periderm contributes to modeling cork development in plants and helps to resolve critical agricultural problems. Here, we summarize the data available on potato periderm formation, highlighting tissue characteristics rather than the suberization processes.

The fasciola cinereum subregion of the hippocampus is important for the acquisition of visual contextual memory

The fasciola cinereum (FC) is a subregion of the hippocampus that has received relatively little attention compared with other hippocampal subregions with respect to anatomical characteristics and functional significance. Here, we show that the FC exhibits clear anatomical borders with the distalmost region of the CA1. Principal neurons in the FC resemble the granule cells in the dentate gyrus (DG). However, adult neurogenesis was not found unlike in the DG. The FC receives inputs mostly from the lateral entorhinal cortex and perirhinal cortex while projecting exclusively to the crest of the DG within the hippocampus. Neurotoxic lesions in the FC using colchicine impaired the acquisition, but not retrieval, of visual contextual memory in rats. FC lesions also impaired place recognition and object-in-place memory. As the rat performed the contextual memory task on the T-maze, place cells in the FC exhibited robust place fields and were indiscriminable from those in CA1 with respect to the basic firing properties. However, place cells in the FC fired only transiently in their place fields on the maze compared with those in CA1. Our findings suggest that the episodic firing pattern of the place cells in the FC may play critical roles in learning a novel contextual environment by facilitating temoporally structured contextual pattern separation in the DG of the hippocampus.

Abiotic Stress-Induced Actin-Depolymerizing Factor 3 From Deschampsia antarctica Enhanced Cold Tolerance When Constitutively Expressed in Rice

The Antarctic flowering plant Deschampsia antarctica is highly sensitive to climate change and has shown rapid population increases during regional warming of the Antarctic Peninsula. Several studies have examined the physiological and biochemical changes related to environmental stress tolerance that allow D. antarctica to colonize harsh Antarctic environments; however, the molecular mechanisms of its responses to environmental changes remain poorly understood. To elucidate the survival strategies of D. antarctica in Antarctic environments, we investigated the functions of actin depolymerizing factor (ADF) in this species. We identified eight ADF genes in the transcriptome that were clustered into five subgroups by phylogenetic analysis. DaADF3, which belongs to a monocot-specific clade together with cold-responsive ADF in wheat, showed significant transcriptional induction in response to dehydration and cold, as well as under Antarctic field conditions. Multiple drought and low-temperature responsive elements were identified as possible binding sites of C-repeat-binding factors in the promoter region of DaADF3, indicating a close relationship between DaADF3 transcription control and abiotic stress responses. To investigate the functions of DaADF3 related to abiotic stresses in vivo, we generated transgenic rice plants overexpressing DaADF3. These transgenic plants showed greater tolerance to low-temperature stress than the wild-type in terms of survival rate, leaf chlorophyll content, and electrolyte leakage, accompanied by changes in actin filament organization in the root tips. Together, our results imply that DaADF3 played an important role in the enhancement of cold tolerance in transgenic rice plants and in the adaptation of D. antarctica to its extreme environment.

The transcriptome of potato tuber phellogen reveals cellular functions of cork cambium and genes involved in periderm formation and maturation

The periderm is a protective corky tissue that is formed through the cambial activity of phellogen cells, when the outer epidermis is damaged. Timely periderm formation is critical to prevent pathogen invasion and water loss. The outer layers of the potato periderm, the tuber skin, serves as a model to study cork development. Early in tuber development the phellogen becomes active and produces the skin. During tuber maturation it becomes inactive and the skin adheres to the tuber flesh. The characterization of potato phellogen may contribute to the management of costly agricultural problems related to incomplete skin-set and the resulting skinning injuries, and provide us with new knowledge regarding cork development in planta. A transcriptome of potato tuber phellogen isolated by laser capture microdissection indicated similarity to vascular cambium and the cork from trees. Highly expressed genes and transcription factors indicated that phellogen activation involves cytokinesis and gene reprograming for the establishment of a dedifferentiation state; whereas inactivation is characterized by activity of genes that direct organ identity in meristem and cell-wall modifications. The expression of selected genes was analyzed using qPCR in native and wound periderm at distinct developmental stages. This allowed the identification of genes involved in periderm formation and maturation.

Interactions between the Translation Machinery and Microtubules

Microtubules are components of eukaryotic cytoskeleton that are involved in the transport of various components from the nucleus to the cell periphery and back. They also act as a platform for assembly of complex molecular ensembles. Ribonucleoprotein (RNP) complexes, such as ribosomes and mRNPs, are transported over significant distances (e.g. to neuronal processes) along microtubules. The association of RNPs with microtubules and their transport along these structures are essential for compartmentalization of protein biosynthesis in cells. Microtubules greatly facilitate assembly of stress RNP granules formed by accumulation of translation machinery components during cell stress response. Microtubules are necessary for the cytoplasm-to-nucleus transport of proteins, including ribosomal proteins. At the same time, ribosomal proteins and RNA-binding proteins can influence cell mobility and cytoplasm organization by regulating microtubule dynamics. The molecular mechanisms underlying the association between the translation machinery components and microtubules have not been studied systematically; the results of such studies are mostly fragmentary. In this review, we attempt to fill this gap by summarizing and discussing the data on protein and RNA components of the translation machinery that directly interact with microtubules or microtubule motor proteins.

Biocontrol agents promote growth of potato pathogens, depending on environmental conditions

There is a pressing need to understand and optimize biological control so as to avoid over‐reliance on the synthetic chemical pesticides that can damage environmental and human health. This study focused on interactions between a novel biocontrol‐strain, Bacillus sp. JC12GB43, and potato‐pathogenic Phytophthora and Fusarium species. In assays carried out in vitro and on the potato tuber, the bacterium was capable of near‐complete inhibition of pathogens. This Bacillus was sufficiently xerotolerant (water activity limit for growth = 0.928) to out‐perform Phytophthora infestans (~0.960) and challenge Fusarium coeruleum (~0.847) and Fusarium sambucinum (~0.860) towards the lower limits of their growth windows. Under some conditions, however, strain JC12GB43 stimulated proliferation of the pathogens: for instance, Fusarium coeruleum growth‐rate was increased under chaotropic conditions in vitro (132 mM urea) by >100% and on tubers (2‐M glycerol) by up to 570%. Culture‐based assays involving macromolecule‐stabilizing (kosmotropic) compatible solutes provided proof‐of‐principle that the Bacillus may provide kosmotropic metabolites to the plant pathogen under conditions that destabilize macromolecular systems of the fungal cell. Whilst unprecedented, this finding is consistent with earlier reports that fungi can utilize metabolites derived from bacterial cells. Unless the antimicrobial activities of candidate biocontrol strains are assayed over a full range of field‐relevant parameters, biocontrol agents may promote plant pathogen infections and thereby reduce crop yields. These findings indicate that biocontrol activity, therefore, ought to be regarded as a mode‐of‐behaviour (dependent on prevailing conditions) rather than an inherent property of a bacterial strain.

Anthocyanin synthesis in native and wound periderms of potato

Skin color of red potatoes is due to accumulation of anthocyanins in the tuber periderm, a protective tissue that replaces the epidermis at an early stage of tuber development. The periderm consists of external layers of suberized phellem cells making up the skin, and internal layers of parenchyma-like phelloderm cells. Red pigmentation is an important marketing factor for red-skinned potatoes. However, injuries to the tuber surface, which are common in the potato industry, result in the development of a wound periderm that is devoid of the characteristic red coloration. To study the reason for these differences in anthocyanin accumulation, the expression level of anthocyanin biosynthesis genes and regulators was monitored in native and wound periderm using microarray analysis and quantitative polymerase chain reaction. We found significantly higher expression of the anthocyanin pathway in the phelloderm cells compared with the skin and tuber-flesh samples. However, in wound periderm, the anthocyanin pathway was strongly downregulated relative to the native periderm. This was true for two developmental stages of the native periderm--'immature', when the skin is prone to skinning injuries, and 'mature', following skin set--suggesting that anthocyanin synthesis continues postharvest. Wound-induced expression of steroidal glycoalkaloid glycosyltransferases, suberin-related 3-ketoacyl-CoA synthase and actin indicated that downregulation of the anthocyanin-specific pathway does not reflect global repression of the wound-periderm transcriptome. Loss of pigmentation may result from reduced expression of the Myb-bHLH-WD40 anthocyanin regulatory complex--a possible candidate might be the bHLH transcription factor JAF13.

Starch granule-associated proteins of hull-less barley (Hordeum vulgare L.) from the Qinghai-Tibet Plateau in China

BACKGROUND

The starch granule-associated proteins (SGAPs) are the minor components of the starch granules and a majority of them are believed to be starch biosynthetic enzymes. The Qinghai-Tibet Plateau in China, one of the centres of origin of cultivated barley, is abundant in hull-less barley resources which exhibit high polymorphism in SGAPs.

RESULTS

The SGAPs of hull-less barley from Qinghai-Tibet Plateau were analysed by one-dimensional (1-D) SDS-PAGE, 2-D PAGE and ESI-Q-TOF MS/MS. In the 1-D SDS-PAGE gel, four proteins including a 80 kDa starch synthase, actin, actin 4 and ATP synthase β-subunit were identified as novel SGAPs. A total of six different bands were identified as starch granule-bound starch synthase I (GBSSI) and the segregation of the novel GBSSI bands in F(1) and F(2) seeds derived from yf127 × yf70 was in accordance with Mendel's law. In the 2-D PAGE gel, 92 spots were identified as 42 protein species which could be classified into 15 functional groups. Thirteen protein species were identified as SGAPs for the first time and multiple spots were identified as GBSSI.

CONCLUSION

This study revealed novel SGAPs in hull-less barley from the Qinghai-Tibet Plateau in China and these will be significant in further studies of starch biosynthesis in barley.

Emerging role for the cytoskeleton as an organizer and regulator of translation

The cytoskeleton is an intricate and dynamic fibrous network that has an essential role in the generation and regulation of cell architecture and cellular mechanical properties. The cytoskeleton also evolved as a scaffold that supports diverse biochemical pathways. Recent evidence favours the hypothesis that the cytoskeleton participates in the spatial organization and regulation of translation, at both the global and local level, in a manner that is crucial for cellular growth, proliferation and function.

Potato skin proteome is enriched with plant defence components

Periderm is a tissue of secondary origin that replaces damaged epidermis. It can be found in underground plant organs, as an above-ground tissue of woody species (cork), and as a wound-healing tissue. Its outer layers are composed of phellem cells with suberized walls that constitute a protective barrier, preventing pathogen invasion and fluid loss. In potato, a model for periderm studies, periderm tissue replaces the epidermis early in tuber development and the suberized phellems constitute the tuber's skin. To identify factors involved in phellem/skin development and that play a role in its defensive characteristics, two-dimensional gel electrophoresis was used to compare the skin and parenchymatic flesh proteomes of young developing tubers. Proteins exhibiting differentially high signal intensity in the skin were sorted by functional categories. As expected, the differential skin proteome was enriched in proteins whose activity is characteristic of actively dividing tissues such as cell proliferation, C(1) metabolism, and the oxidative respiratory chain. Interestingly, the major functional category consisted of proteins (63%) involved in plant defence responses to biotic and abiotic stresses. This group included three isozymes of caffeoyl-CoA O-methyltransferase and five isozymes of peroxidase that may play a role in suberization processes. The differential expression of these proteins in the skin was further verified by RT-PCR of their corresponding transcripts in skin and tuber flesh samples. The results presented here shed light on the early events in skin development and further expand the concept of the periderm as a protective tissue containing an array of plant defence components.

A role for F-actin in hexokinase-mediated glucose signaling

HEXOKINASE1 (HXK1) from Arabidopsis (Arabidopsis thaliana) has dual roles in glucose (Glc) signaling and in Glc phosphorylation. The cellular context, though, for HXK1 function in either process is not well understood. Here we have shown that within normal experimental detection limits, AtHXK1 is localized continuously to mitochondria. Two mitochondrial porin proteins were identified as capable of binding to overexpressed HXK1 protein, both in vivo and in vitro. We also found that AtHXK1 can be associated with its structural homolog, F-actin, based on their coimmunoprecipitation from transgenic plants that overexpress HXK1-FLAG or from transient expression assays, and based on their localization in leaf cells after cryofixation. This association might be functionally important because Glc signaling in protoplast transient expression assays is compromised by disruption of F-actin. We also demonstrate that Glc treatment of Arabidopsis seedlings rapidly and reversibly disrupts fine mesh actin filaments. The possible roles of actin in HXK-dependent Glc signaling are discussed.

Messages on the move: the role of the cytoskeleton in mRNA localization and translation in plant cellsThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

The cytoskeleton plays an important role in numerous cellular processes, including subcellular mRNA localization and translation. Several examples of mRNA localization have emerged in plant cells, and these appear to function in protein targeting, the establishment of polarity, and cell-to-cell trafficking. The identification of several cytoskeleton-associated RNA-binding proteins in plant cells has made available candidate proteins that mediate the interaction between mRNA and the cytoskeleton, and possibly play a role in mRNA localization and translational control. We propose a model that links mRNA–microtubule interactions to translational autoregulation, a process that may assist in the efficient and regulated binding of proteins to microtubules.

Towards the proteome of Brassica napus phloem sap

The soluble proteins in sieve tube exudate from Brassica napus plants were systematically analyzed by 1-DE and high-resolution 2-DE, partial amino acid sequence determination by MS/MS, followed by database searches. 140 proteins could be identified by their high similarity to database sequences (135 from 2-DE, 5 additional from 1-DE). Most analyzed spots led to successful protein identifications, demonstrating that Brassica napus, a close relative of Arabidopsis thaliana, is a highly suitable model plant for phloem research. None of the identified proteins was formerly known to be present in Brassica napus phloem, but several proteins have been described in phloem sap of other species. The data, which is discussed with respect to possible physiological importance of the proteins in the phloem, further confirms and substantially extends earlier findings and uncovers the presence of new protein functions in the vascular system. For example, we found several formerly unknown phloem proteins that are potentially involved in signal generation and transport, e.g., proteins mediating calcium and G-protein signaling, a set of RNA-binding proteins, and FLOWERING LOCUS T (FT) and its twin sister that might be key components for the regulation of flowering time.

Transcriptional profiling of sunflower plants growing under low temperatures reveals an extensive down-regulation of gene expression associated with chilling sensitivity

Being able to sow early to maximize the growing season and to escape drought stress has increased the importance of low-temperature tolerance in sunflower. Yet knowledge about the molecular basis of sunflower response to low temperature is still lacking. To address this issue, nylon microarrays containing >8000 putative unigenes were developed and used. Early- and late-flowering genotypes were sown at 15 degrees C and grown until the two-leaf stage when they were subjected to 7 degrees C until the four-leaf stage. The transcriptional profiles of low temperature-grown plants (15 degrees C and 7 degrees C) were compared with those grown under standard conditions (25 degrees C). Two-step ANOVA normalization and analysis models were used to identify the differentially expressed genes. A total of 108 cDNA clones having a P-value <10(-3) were found to be differentially expressed between the low temperature-grown plants (15 degrees C and 7 degrees C) and their corresponding two-leaf- and four-leaf-stage controls across the two genotypes. About 90% of these genes were down-regulated. This includes genes potentially involved in the metabolism of carbohydrate and energy, protein synthesis, signal transduction, and transport function. Comparing gene expression profiles at 15 degrees C and 7 degrees C revealed that only four genes can be considered as differentially expressed, in both genotypes, suggesting that similar genetic programmes underlie the response of sunflower plants to these temperature regimes. The analysis also revealed that early- and late-flowering genotypes respond similarly to low-temperature tolerance as justified by the low number of genes showing a significant genotype x treatment interaction effect. It seems likely that the down-regulation and/or non-induction of genes having a critical role in low-temperature tolerance may be responsible for the sensitivity of sunflower plants to low-temperature tolerance. The results reported provide an initial characterization of the transcriptome activity of sunflower, as a chilling-sensitive plant under suboptimal temperatures, and could be of importance to reveal the potential differences between chilling-sensitive and chilling-tolerant species.

Lysine-containing proteins in maize endosperm: a major contribution from cytoskeleton-associated carbohydrate-metabolizing enzymes

We measured fresh weight, dry weight, total protein, and the amounts of several individual proteins during endosperm development in three varieties of maize ( Zea mays L.): W64A wild-type (WT) and opaque-2 (o2), and sweet corn (SW). By 28 days after pollination (DAP), fresh weight was much higher in WT and SW than in o2, but o2 had a higher dry weight and thus a much lower water content. By 28 DAP, protein concentration [mg (g tissue(-1))] was highest in o2 and lowest in WT, while the protein content (microg seed(-1)) was lowest in o2. The storage proteins, alpha- and gamma-zeins, were low initially, but by 28 DAP they comprised over 50% of the total protein in WT and SW, but only about 30% in o2. In all varieties, the cytoskeleton proteins, actin, tubulin and eEF1alpha, sedimented with the protein bodies at 30 g to 27,000 g in tissue homogenized in cytoskeleton-stabilizing buffer. Other cytoskeleton-associated proteins increased during development, including UDP-glucose starch glucosyltransferase (UDP-GSGT, EC 2.4.1.11), sucrose synthase 1 (SuSy-1, EC 2.4.1.13) and fructose-1,6 bisphosphate aldolase (FBA, EC 4.1.2.13). At 28 DAP, these cytoskeleton-associated proteins combined make up 27% (WT), 23% (SW) and 33% (o2) of the total protein. These proteins are all rather high (5-11%) in lysine, and so they contribute about 75% (WT), 67% (o2), and 51% (SW) of the total endosperm lysine. We conclude that efforts to elevate the levels of these proteins could make a significant contribution to the nutritional value of corn.

The systemin precursor gene regulates both defensive and developmental genes in Solanum tuberosum

Transformation of Solanum tuberosum, cv. Desiree, with the tomato prosystemin gene, regulated by the 35S cauliflower mosaic virus promoter, resulted in constitutive increase in defensive proteins in potato leaves, similar to its effects in tomato plants, but also resulted in a dramatic increase in storage protein levels in potato tubers. Tubers from selected transformed lines contained 4- to 5-fold increases in proteinase inhibitor I and II proteins, >50% more soluble and dry weight protein, and >50% more total nitrogen and total free amino acids than found in wild-type tubers. These results suggest that the prosystemin gene plays a dual role in potato plants in regulating proteinase inhibitor synthesis in leaves in response to wounding and in regulating storage protein synthesis in potato tubers in response to developmental cues. The results indicated that components of the systemin signaling pathway normally found in leaves have been recruited by potato plants to be developmentally regulated to synthesize and accumulate large quantities of storage proteins in tubers.

Early steps in cold sensing by plant cells: the role of actin cytoskeleton and membrane fluidity

Many plants acquire freezing tolerance through cold acclimatization (CA), a prolonged exposure to low but non-freezing temperatures at the onset of winter. CA is associated with gene expression that requires transient calcium influx into the cytosol. Alfalfa (Medicago sativa) cells treated with agents blocking this influx are unable to cold-acclimatize. Conversely, chemical agents causing increased calcium influx induce cold acclimatization-specific (cas) gene expression in alfalfa at 25 degrees C. How low temperature triggers calcium influx is, however, unknown. We report here that induction of a CA-specific gene (cas30), calcium influx and freezing tolerance at 4 degrees C are all prevented by cell membrane fluidization, but, conversely, are induced at 25 degrees C by membrane rigidification. cas30 expression and calcium influx at 4 degrees C are also prevented by jasplakinolide (JK), an actin microfilament stabilizer, but induced at 25 degrees C by the actin microfilament destabilizer cytochalasin D (CD). JK blocked the membrane rigidifier-induced, but not the calcium channel agonist-induced cas30 expression at 25 degrees C. These findings indicate that cytoskeleton re-organization is an integral component in low-temperature signal transduction in alfalfa cell suspension cultures, serving as a link between membrane rigidification and calcium influx in CA.