An ethylene-induced cDNA encoding a lipase expressed at the onset of senescence

Exploration of the pearl millet phospholipase gene family to identify potential candidates for grain quality traits

BACKGROUND

Phospholipases constitute a diverse category of enzymes responsible for the breakdown of phospholipids. Their involvement in signal transduction with a pivotal role in plant development and stress responses is well documented.

RESULTS

In the present investigation, a thorough genome-wide analysis revealed that the pearl millet genome contains at least 44 phospholipase genes distributed across its 7 chromosomes, with chromosome one harbouring the highest number of these genes. The synteny analysis suggested a close genetic relationship of pearl millet phospholipases with that of foxtail millet and sorghum. All identified genes were examined to unravel their gene structures, protein attributes, cis-regulatory elements, and expression patterns in two pearl millet genotypes contrasting for rancidity. All the phospholipases have a high alpha-helix content and distorted regions within the predicted secondary structures. Moreover, many of these enzymes possess binding sites for both metal and non-metal ligands. Additionally, the putative promoter regions associated with these genes exhibit multiple copies of cis-elements specifically responsive to biotic and abiotic stress factors and signaling molecules. The transcriptional profiling of 44 phospholipase genes in two genotypes contrasting for rancidity across six key tissues during pearl millet growth revealed a predominant expression in grains, followed by seed coat and endosperm. Specifically, the genes PgPLD-alpha1-1, PgPLD-alpha1-5, PgPLD-delta1-7a, PgPLA1-II-1a, and PgPLD-delta1-2a exhibited notable expression in grains of both the genotypes while showing negligible expression in the other five tissues. The sequence alignment of putative promoters revealed several variations including SNPs and InDels. These variations resulted in modifications to the corresponding cis-acting elements, forming distinct transcription factor binding sites suggesting the transcriptional-level regulation for these five genes in pearl millet.

CONCLUSIONS

The current study utilized a genome-wide computational analysis to characterize the phospholipase gene family in pearl millet. A comprehensive expression profile of 44 phospholipases led to the identification of five grain-specific candidates. This underscores a potential role for at least these five genes in grain quality traits including the regulation of rancidity in pearl millet. Therefore, this study marks the first exploration highlighting the possible impact of phospholipases towards enhancing agronomic traits in pearl millet.

The acceleration of yellow lupine flower abscission by jasmonates is accompanied by lipid-related events in abscission zone cells

Yellow lupine is an economically important crop. This species has been used as a great model for abscission processes for several years due to extreme flower abortion, which takes place in the abscission zone (AZ). AZ activation involves modifications of cell walls, membranes, and cellular structure. In this paper, we applied physiological, molecular, biochemical, and instrumental methods to explore lipid-associated changes and the possible involvement of lipid-derived phytohormones - jasmonates (JAs) - in flower AZ activation. Our comprehensive analyses revealed that natural abscission is accompanied by the upregulation of peroxidase, which reflects a disruption of redox balance and/or lipids peroxidation in AZ cell membranes. Redox imbalance was confirmed by appearance of malondialdehyde. Lipid-related processes involved the specific localization and increased level and activity of lipase and LOX, enzymes associated with cell membrane rupture, and JA biosynthesis. Lipid-hydrolyzing phospholipase D, implicated previously in abscission, is also found in naturally active AZs. Observed changes are accompanied by the accumulation of jasmonates, both free jasmonic acid and its methyl ester. The JA derivative exhibited higher biological activity than the nonconjugated form. Overall, our study shed new light on the lipid and phytohormonal regulation of AZ functioning supporting a role of JAs during abscission-associated events.

Rose (Rosa hybrida) Ethylene Responsive Factor 3 Promotes Rose Flower Senescence via Direct Activation of the Abscisic Acid Synthesis-Related 9-CIS-EPOXYCAROTENOID DIOXYGENASE Gene

During plant senescence, energy and nutrients are transferred to young leaves, fruits or seeds. However, senescence reduces flower quality, which leads to huge economic losses in flower production. Ethylene is an important factor affecting the quality of cut roses during transportation and storage. Ethylene-responsive factors (ERFs) are key nodes in ethylene signaling, but the molecular mechanism underlying ERFs regulated flower senescence is not well understood. We addressed this issue in the present study by focusing on RhERF3 from Rosa hybrida, an ERF identified in a previous transcriptome analysis of ethylene-treated rose flowers. Expression of RhERF3 was strongly induced by ethylene during rose flower senescence. Transient silencing of RhERF3 delayed flower senescence, whereas overexpression (OE) accelerated the process. RNA sequencing analysis of RhERF3 OE and pSuper vector control samples identified 13,214 differentially expressed genes that were mostly related to metabolic process and plant hormone signal transduction. Transient activation and yeast one-hybrid assays demonstrated that RhERF3 directly bound the promoter of the 9-cis-epoxycarotenoid dioxygenase (RhNCED1) gene and activated gene expression. Thus, a RhERF3/RhNCED1 axis accelerates rose flower senescence.

Effects of active and passive modified atmosphere packaging on biochemical properties of cut Dendrobium orchid flowers

Optimal storage conditions are essential to preserving the quality of postharvest produce and cut flowers during transportation, especially to overseas destinations. As a result, this research investigated the effects of active and passive modified atmosphere packaging (MAP) on the biochemical properties of cut Dendrobium Sonia orchid flowers. In active MAP, the orchid flowers were wrapped in MAP plastic film filled with 5 % carbon dioxide (CO2) and 2 % oxygen (O2). Meanwhile, in passive MAP the flowers were stored inside MAP plastic film without filling with 5 % CO2 and 2 % O2. The experimental MAP plastic films were polyethylene, low density polyethylene, high density polyethylene, polypropylene, and polyvinyl chloride films. The biochemical parameters included storage life, respiration rate, ethylene production, internal O2 and CO2, anthocyanin content, protein degradation, and electrolyte leakage. The results showed that the average storage life of orchid flowers under passive MAP condition was 9-15 days, depending on the plastic film types. The longest storage life of 15.66 days was achieved with polypropylene film. The storage life of orchid flowers in active MAP was 9.33 days on average. Without MAP (control), the storage life was 7 days under normal atmosphere condition (0.03 % CO2 and 21 % O2). The experiments also demonstrated that MAP efficiently reduced respiration rate, ethylene production, anthocyanin degradation, protein degradation, and electrolyte leakage. Unlike existing research on MAP which focused primarily on extending the shelf life of fresh produce or cut flowers, this study comparatively investigated the biochemical properties of cut orchid flowers stored in MAP environment, in addition to the storage life.

Investigation of the interaction of DAD1-LIKE LIPASE 3 (DALL3) with Selenium Binding Protein 1 (SBP1) in Arabidopsis thaliana

Phospholipase PLA₁-Iγ2 or otherwise DAD1-LIKE LIPASE 3 (DALL3) is a member of class I phospholipases and has a role in JA biosynthesis. AtDALL3 was previously identified in a yeast two-hybrid screening as an interacting protein of the Arabidopsis Selenium Binding Protein 1 (SBP1). In this work, we have studied AtDALL3 as an interacting partner of the Arabidopsis Selenium Binding Protein 1 (SBP1). Phylogenetic analysis showed that DALL3 appears in the PLA1-Igamma1, 2 group, paired with PLA1-Igammma1. The highest level of expression of AtDALL3 was observed in 10-day-old roots and in flowers, while constitutive levels were maintained in seedlings, cotyledons, shoots and leaves. In response to abiotic stress, DALL3 was shown to participate in the network of genes regulated by cadmium, selenite and selenate compounds. DALL3 promoter driven GUS assays revealed that the expression patterns defined were overlapping with the patterns reported for AtSBP1 gene, indicating that DALL3 and SBP1 transcripts co-localize. Furthermore, quantitative GUS assays showed that these compounds elicited changes in activity in specific cells files, indicating the differential response of DALL3 promoter. GFP::DALL3 studies by confocal microscopy demonstrated the localization of DALL3 in the plastids of the root apex, the plastids of the central root and the apex of emerging lateral root primordia. Additionally, we confirmed by yeast two hybrid assays the physical interaction of DALL3 with SBP1 and defined a minimal SBP1 fragment that DALL3 binds to. Finally, by employing bimolecular fluorescent complementation we demonstrated the in planta interaction of the two proteins.

Functional diversity of glycerolipid acylhydrolases in plant metabolism and physiology

Most current knowledge about plant lipid metabolism has focused on the biosynthesis of lipids and their transport between different organelles. However, lipid composition changes during development and in response to environmental cues often go beyond adjustments of lipid biosynthesis. When lipids have to be removed to adjust the extent of membranes during down regulation of photosynthesis, or lipid composition has to be adjusted to alter the biophysical properties of membranes, or lipid derived chemical signals have to be produced, lipid-degrading enzymes come into play. This review focuses on glycerolipid acylhydrolases that remove acyl groups from glycerolipids and will highlight their roles in lipid remodeling and lipid-derived signal generation. One emerging theme is that these enzymes are involved in the dynamic movement of acyl groups through different lipid pools, for example from polar membrane lipids to neutral lipids sequestered in lipid droplets during de novo triacylglycerol synthesis. Another example of acyl group sequestration in the form of triacylglycerols in lipid droplets is membrane lipid remodeling in response to abiotic stresses. Fatty acids released for membrane lipids can also give rise to potent signaling molecules and acylhydrolases are therefore often the first step in initiating the formation of these lipid signals.

Thermodynamic insights into an interaction between ACYL-CoA-BINDING PROTEIN2 and LYSOPHOSPHOLIPASE2 in Arabidopsis

Lysophospholipids (LPLs) are important lipid-signaling molecules in plants, of which lysophosphatidylcholine (lysoPC) is one of the most well-characterized LPLs, having important roles in plant stress responses. It is broken down by lysophospholipases, but the molecular mechanism involved in lysoPC degradation is unclear. Recombinant Arabidopsis thaliana ACYL-CoA-BINDING PROTEIN2 (AtACBP2) has been reported to bind lysoPC via its acyl-CoA-binding domain and also LYSOPHOSPHOLIPASE 2 (AtLYSOPL2) via its ankyrin repeats in vitro To investigate the interactions of AtACBP2 with AtLYSOPL2 and lysoPC in more detail, we conducted isothermal titration calorimetry with AtACBP2_70-354, an AtACBP2 derivative consisting of amino acids 70-354, containing both the acyl-CoA-binding domain and ankyrin repeats. We observed that the interactions of AtACBP2_70-354 with AtLYSOPL2 and lysoPC were both endothermic, favored by solvation entropy and opposed by enthalpy, with dissociation constants in the micromolar range. Of note, three AtLYSOPL2 catalytic triad mutant proteins (S147A, D268A, and H298A) bound lysoPC only weakly, with an exothermic burst and dissociation constants in the millimolar range. Furthermore, the binding affinity of lysoPC-premixed AtACBP2_70-354 to AtLYSOPL2 was 10-fold higher than that of AtACBP2_70-354 alone to AtLYSOPL2. We conclude that AtACBP2 may play a role in facilitating a direct interaction between AtLYSOPL2 and lysoPC. Our results suggest that AtACBP2_70-354 probably binds to lysoPC through a hydrophobic interface that enhances a hydrotropic interaction of AtACBP2_70-354 with AtLYSOPL2 and thereby facilitates AtLYSOPL2's lysophospholipase function.

Arabidopsis thaliana responds to colonisation of Piriformospora indica by secretion of symbiosis-specific proteins

Plants interact with a wide variety of fungi in a mutualistic, parasitic or neutral way. The associations formed depend on the exchange of nutrients and signalling molecules between the partners. This includes a diverse set of protein classes involved in defence, nutrient uptake or establishing a symbiotic relationship. Here, we have analysed the secretomes of the mutualistic, root-endophytic fungus Piriformospora indica and Arabidopsis thaliana when cultivated alone or in a co-culture. More than one hundred proteins were identified as differentially secreted, including proteins associated with growth, development, abiotic and biotic stress response and mucilage. While some of the proteins have been associated before to be involved in plant-microbial interaction, other proteins are newly described in this context. One plant protein found in the co-culture is PLAT1 (Polycystin, Lipoxygenase, Alpha-toxin and Triacylglycerol lipase). PLAT1 has not been associated with plant-fungal-interaction and is known to play a role in abiotic stress responses. In colonised roots PLAT1 shows an altered gene expression in a stage specific manner and plat1 knock-out plants are colonised stronger. It co-localises with Brassicaceae-specific endoplasmic reticulum bodies (ER-bodies) which are involved in the formation of the defence compound scopolin. We observed degraded ER-bodies in infected Arabidopsis roots and a change in the scopolin level in response to the presence of the fungus.

HEAT INDUCIBLE LIPASE1 Remodels Chloroplastic Monogalactosyldiacylglycerol by Liberating α-Linolenic Acid in Arabidopsis Leaves under Heat Stress

Under heat stress, polyunsaturated acyl groups, such as α-linolenate (18:3) and hexadecatrienoate (16:3), are removed from chloroplastic glycerolipids in various plant species. Here, we showed that a lipase designated HEAT INDUCIBLE LIPASE1 (HIL1) induces the catabolism of monogalactosyldiacylglycerol (MGDG) under heat stress in Arabidopsis thaliana leaves. Using thermotolerance tests, a T-DNA insertion mutant with disrupted HIL1 was shown to have a heat stress-sensitive phenotype. Lipidomic analysis indicated that the decrease of 34:6-MGDG under heat stress was partially impaired in the hil1 mutant. Concomitantly, the heat-induced increment of 54:9-triacylglycerol in the hil1 mutant was 18% lower than that in the wild-type plants. Recombinant HIL1 protein digested MGDG to produce 18:3-free fatty acid (18:3-FFA), but not 18:0- and 16:0-FFAs. A transient assay using fluorescent fusion proteins confirmed chloroplastic localization of HIL1. Transcriptome coexpression network analysis using public databases demonstrated that the HIL1 homolog expression levels in various terrestrial plants are tightly associated with chloroplastic heat stress responses. Thus, HIL1 encodes a chloroplastic MGDG lipase that releases 18:3-FFA in the first committed step of 34:6 (18:3/16:3)-containing galactolipid turnover, suggesting that HIL1 has an important role in the lipid remodeling process induced by heat stress in plants.

Molecular and biochemical characterizations of the monoacylglycerol lipase gene family of Arabidopsis thaliana

Monoacylglycerol lipase (MAGL) catalyzes the last step of triacylglycerol breakdown, which is the hydrolysis of monoacylglycerol (MAG) to fatty acid and glycerol. Arabidopsis harbors over 270 genes annotated as 'lipase', the largest class of acyl lipid metabolism genes that have not been characterized experimentally. In this study, computational modeling suggested that 16 Arabidopsis putative MAGLs (AtMAGLs) have a three-dimensional structure that is similar to a human MAGL. Heterologous expression and enzyme assays indicated that 11 of the 16 encoded proteins indeed possess MAG lipase activity. Additionally, AtMAGL4 displayed hydrolase activity with lysophosphatidylcholine and lysophosphatidylethanolamine (LPE) substrates and AtMAGL1 and 2 utilized LPE as a substrate. All recombinant AtMAGLs preferred MAG substrates with unsaturated fatty acids over saturated fatty acids and AtMAGL8 exhibited the highest hydrolase activities with MAG containing 20:1 fatty acids. Except for AtMAGL4, -14 and -16, all AtMAGLs showed similar activity with both sn-1 and sn-2 MAG isomers. Spatial, temporal and stress-induced expression of the 16 AtMAGL genes was analyzed by transcriptome analyses. AtMAGL:eYFP fusion proteins provided initial evidence that AtMAGL1, -3, -6, -7, -8, -11, -13, -14 and -16 are targeted to the endoplasmic reticulum and/or Golgi network, AtMAGL10, -12 and -15 to the cytosol and AtMAGL2, -4 and -5 to the chloroplasts. Furthermore, AtMAGL8 was associated with the surface of oil bodies in germinating seeds and leaves accumulating oil bodies. This study provides the broad characterization of one of the least well-understood groups of Arabidopsis lipid-related enzymes and will be useful for better understanding their roles in planta.

Constitutive expression of CaPLA1 conferred enhanced growth and grain yield in transgenic rice plants

Phospholipids are not only important components of cell membranes, but participate in diverse processes in higher plants. In this study, we generated Capsicum annuum phospholipiase A1 (CaPLA1) overexpressing transgenic rice (Oryza sativa L.) plants under the control of the maize ubiquitin promoter. The T4 CaPLA1-overexpressing rice plants (Ubi:CaPLA1) had a higher root:shoot mass ratio than the wild-type plants in the vegetative stage. Leaf epidermal cells from transgenic plants had more cells than wild-type plants. Genes that code for cyclin and lipid metabolic enzymes were up-regulated in the transgenic lines. When grown under typical paddy field conditions, the transgenic plants produced more tillers, longer panicles and more branches per panicle than the wild-type plants, all of which resulted in greater grain yield. Microarray analysis suggests that gene expressions that are related with cell proliferation, lipid metabolism, and redox state were widely altered in CaPLA1-overexpressing transgenic rice plants. Ubi:CaPLA1 plants had a reduced membrane peroxidation state, as determined by malondialdehyde and conjugated diene levels and higher peroxidase activity than wild-type rice plants. Furthermore, three isoprenoid synthetic genes encoding terpenoid synthase, hydroxysteroid dehydrogenase and 3-hydroxy-3-methyl-glutaryl-CoA reductase were up-regulated in CaPLA1-overexpressing plants. We suggest that constitutive expression of CaPLA1 conferred increased grain yield with enhanced growth in transgenic rice plants by alteration of gene activities related with cell proliferation, lipid metabolism, membrane peroxidation state and isoprenoid biosynthesis.

Molecular Mechanisms of Phosphorus Metabolism and Transport during Leaf Senescence

Leaf senescence, being the final developmental stage of the leaf, signifies the transition from a mature, photosynthetically active organ to the attenuation of said function and eventual death of the leaf. During senescence, essential nutrients sequestered in the leaf, such as phosphorus (P), are mobilized and transported to sink tissues, particularly expanding leaves and developing seeds. Phosphorus recycling is crucial, as it helps to ensure that previously acquired P is not lost to the environment, particularly under the naturally occurring condition where most unfertilized soils contain low levels of soluble orthophosphate (Pi), the only form of P that roots can directly assimilate from the soil. Piecing together the molecular mechanisms that underpin the highly variable efficiencies of P remobilization from senescing leaves by different plant species may be critical for devising effective strategies for improving overall crop P-use efficiency. Maximizing Pi remobilization from senescing leaves using selective breeding and/or biotechnological strategies will help to generate P-efficient crops that would minimize the use of unsustainable and polluting Pi-containing fertilizers in agriculture. This review focuses on the molecular mechanisms whereby P is remobilized from senescing leaves and transported to sink tissues, which encompasses the action of hormones, transcription factors, Pi-scavenging enzymes, and Pi transporters.

The Arabidopsis PLAT domain protein1 promotes abiotic stress tolerance and growth in tobacco

Plant growth and consequently crop yield can be severely compromised by abiotic and biotic stress conditions. Transgenic approaches that resulted in increased tolerance against abiotic stresses often were typically accompanied by adverse effects on plant growth and fitness under optimal growing conditions. Proteins that belong to the PLAT-plant-stress protein family harbour a single PLAT (Polycystin, Lipoxygenase, Alpha-toxin and Triacylglycerol lipase) domain and are ubiquitously present in monocot and dicot plant species. Until now, only limited data is available for PLAT-plant-stress family members, which suggested that these proteins in general could promote tolerance towards stress responses. We studied the function of the Arabidopsis PLAT-plant-stress protein AtPLAT1 employing heterologous gain-of-function analysis in tobacco. AtPLAT1 conferred increased abiotic stress tolerance in tobacco, evident by improved tolerance towards cold, drought and salt stresses, and promoted growth, reflected by a faster development under non-stressed conditions. However, the overexpression of AtPLAT1 in tobacco reduced the tolerance towards biotic stress conditions and, therefore, could be involved in regulating the crosstalk between abiotic and biotic stress responses. Thus, we showed that heterologously expressed AtPLAT1 functions as positive regulator of abiotic stress tolerance and plant growth, which could be an important new asset for strategies to develop plants with improved abiotic stress tolerance, without growth and subsequent yield penalties under optimal growth conditions.

Plant phospholipase A: advances in molecular biology, biochemistry, and cellular function

Plant phospholipase As (PLAs) are a complex group of enzymes that catalyze the release of free fatty acids from phospholipids. Plant PLAs can be grouped into three families, PLA1, PLA2, and patatin-like PLA, that catalyze the hydrolysis of acyl groups from the sn-1 and/or sn-2 position. Each family is composed of multiple isoforms of phospholipases that differ in structural, catalytic, and physiological characteristics. In this review, recently acquired information on molecular, biochemical, and functional aspects of plant PLAs will be discussed.

The Arabidopsis PLAT domain protein1 is critically involved in abiotic stress tolerance

Despite the completion of the Arabidopsis genome sequence, for only a relatively low percentage of the encoded proteins experimental evidence concerning their function is available. Plant proteins that harbour a single PLAT (Polycystin, Lipoxygenase, Alpha-toxin and Triacylglycerol lipase) domain and belong to the PLAT-plant-stress protein family are ubiquitously present in monocot and dicots. However, the function of PLAT-plant-stress proteins is still poorly understood. Therefore, we have assessed the function of the uncharacterised Arabidopsis PLAT-plant-stress family members through a combination of functional genetic and physiological approaches. PLAT1 overexpression conferred increased abiotic stress tolerance, including cold, drought and salt stress, while loss-of-function resulted in opposite effects on abiotic stress tolerance. Strikingly, PLAT1 promoted growth under non-stressed conditions. Abiotic stress treatments induced PLAT1 expression and caused expansion of its expression domain. The ABF/ABRE transcription factors, which are positive mediators of abscisic acid signalling, activate PLAT1 promoter activity in transactivation assays and directly bind to the ABRE elements located in this promoter in electrophoretic mobility shift assays. This suggests that PLAT1 represents a novel downstream target of the abscisic acid signalling pathway. Thus, we showed that PLAT1 critically functions as positive regulator of abiotic stress tolerance, but also is involved in regulating plant growth, and thereby assigned a function to this previously uncharacterised PLAT domain protein. The functional data obtained for PLAT1 support that PLAT-plant-stress proteins in general could be promising targets for improving abiotic stress tolerance without yield penalty.

Lipid turnover during senescence

Rapid turnover of stored triacylglycerol occurs after seed germination, releasing fatty acids that provide carbon and energy for seedling establishment. Glycerolipid and fatty acid turnover that occurs at other times in the plant life cycle, including senescence is less studied. Although the entire pathway of β-oxidation is induced during senescence, Arabidopsis leaf fatty acids turnover at rates 50 fold lower than in seedlings. Major unknowns in lipid turnover include the identity of lipases responsible for degradation of the wide diversity of galactolipid, phospholipid, and other lipid class structures. Also unknown is the relative flux of the acetyl-CoA product of β-oxidation into alternative metabolic pathways. We present an overview of senescence-related glycerolipid turnover and discuss its function(s) and speculate about how it might be controlled to increase the energy density and nutritional content of crops. To better understand regulation of lipid turnover, we developed a database that compiles and plots transcript expression of lipid-related genes during natural leaf senescence of Arabidopsis. The database allowed identification of coordinated patterns of down-regulation of lipid biosynthesis genes and the contrasting groups of genes that increase, including 68 putative lipases.

Structural and functional studies of casein kinase I-like protein from rice

Casein kinase I (CKI) is a protein serine/threonine kinase that is highly conserved from plants to animals. It performs various functions in both the cytoplasm and nucleus, such as DNA repair, cell cycle, cytokinesis, vesicular trafficking, morphogenesis and circadian rhythm. CKI proteins contain a highly conserved kinase domain responsible for catalytic activity at the N-terminus and a highly diverse regulatory domain responsible for determining substrate specificity at the C-terminus. CKI-like protein has been identified in plants, including in rice, but its function and structure have not been reported. Here, we report the 2.0 Å crystal structure of the kinase domain of CKI-like protein from rice. Although the structure adopts the typical bi-lobal kinase architecture, the length and orientation of the glycine-rich ATP-binding motif are dynamic within the CKI family. A loop between α5 and α6 (the α5-α6 loop), which was previously not detected in the CKI family because of flexibility, was clearly detected in our structure. In addition, we identified a lipase as a substrate of CKI-like protein from rice. Phosphorylation of the lipase dramatically reduced its catalytic activity, suggesting that CKI may play a role in the regulation of lipase activity.

Plant phospholipases: an overview

Plant phospholipases can be grouped into four major types, phospholipase D, phospholipase C, phospholipase A1 (PLA(1)), and phospholipase A2 (PLA(2)), that hydrolyze glycerophospholipids at different ester bonds. Within each type, there are different families or subfamilies of enzymes that can differ in substrate specificity, cofactor requirement, and/or reaction conditions. These differences provide insights into determining the cellular function of specific phospholipases in plants, and they can be explored for different industrial applications.

A Brassica napus lipase locates at the membrane contact sites involved in chloroplast development

Background

Fatty acids synthesized in chloroplast are transported to endoplasmic reticulum (ER) for triacylglycerols (TAGs) resembling. The development of chloroplast also requires lipids trafficking from ER to chloroplast. The membrane contact sites (MCSs) between ER and chloroplast has been demonstrated to be involved for the trafficking of lipids and proteins. Lipids trafficking between ER and chloroplast is often accompanied by lipids interconversion. However, it is rarely known how lipids interconversion happens during their trafficking.

Methodology/Principal Findings

We cloned a lipase gene from Brassica napus L., designated as BnCLIP1. Green fluorescence protein (GFP)-tagged BnCLIP1 was shown to locate at the MCSs between ER and chloroplasts in tobacco leaves. Heterogeneous expression of BnCLIP1 in Saccharomyces cerevisiae (pep4) reduced the total amount of fatty acid. Gas chromatography-mass spectrometry (GC-MS) analysis revealed that the truncated BnCLIP1 had a substrate preference for C16:0 lipids in Saccharomyces cerevisiae (pep4). To probe the physiological function of BnCLIP1, two Brassica napus lines with different oil-content were introduced to investigate the transcript patterns of BnCLIP1 during seed development. Intriguingly, the transcript level of BnCLIP1 was found to be immediately up-regulated during the natural seed senescence of both lines; the transcription response of BnCLIP1 in the high oil-content seeds was faster than the lower ones, suggesting a potential role of BnCLIP1 in affecting seed oil synthesis via regulating chloroplast integrity. Further researches showed that chemical disruption of leaf chloroplast also activated the transcription of BnCLIP1.

Conclusions/Significance

The findings of this study show that BnCLIP1 encodes a lipase, localizes at the MCSs and involves in chloroplast development.

A specific group of genes respond to cold dehydration stress in cut Alstroemeria flowers whereas ambient dehydration stress accelerates developmental senescence expression patterns

Petal development and senescence entails a normally irreversible process. It starts with petal expansion and pigment production, and ends with nutrient remobilization and ultimately cell death. In many species this is accompanied by petal abscission. Post-harvest stress is an important factor in limiting petal longevity in cut flowers and accelerates some of the processes of senescence such as petal wilting and abscission. However, some of the effects of moderate stress in young flowers are reversible with appropriate treatments. Transcriptomic studies have shown that distinct gene sets are expressed during petal development and senescence. Despite this, the overlap in gene expression between developmental and stress-induced senescence in petals has not been fully investigated in any species. Here a custom-made cDNA microarray from Alstroemeria petals was used to investigate the overlap in gene expression between developmental changes (bud to first sign of senescence) and typical post-harvest stress treatments. Young flowers were stressed by cold or ambient temperatures without water followed by a recovery and rehydration period. Stressed flowers were still at the bud stage after stress treatments. Microarray analysis showed that ambient dehydration stress accelerates many of the changes in gene expression patterns that would normally occur during developmental senescence. However, a higher proportion of gene expression changes in response to cold stress were specific to this stimulus and not senescence related. The expression of 21 transcription factors was characterized, showing that overlapping sets of regulatory genes are activated during developmental senescence and by different stresses.

Differential expression of genes identified by suppression subtractive hybridization in petals of opening carnation flowers

Flower opening is an event accompanied by morphological changes in petals which include elongation, expansion, and outward-curving. Petal cell growth is a fundamental process that underlies such phenomena, but its molecular mechanism remains largely unknown. Suppression subtractive hybridization was performed between petals during the early elongation period (stage 1) and during the opening period (stage 5) in carnation flowers and a pair of subtraction libraries abundant in differentially expressed genes was constructed at each stage. 393 cDNA clones picked up by differential screening out of 1728 clones were sequenced and 235 different cDNA fragments were identified, among which 211 did not match any known nucleotide sequence of carnation genes in the databases. BLASTX search of nucleotide sequences revealed that putative functions of the translational products can be classified into several categories including transcription, signalling, cell wall modification, lipid metabolism, and transport. Open reading frames of 15 selected genes were successfully determined by rapid amplification of cDNA ends (RACE). Time-course analysis of these genes by real-time RT-PCR showed that transcript levels of several genes correlatively fluctuate in petals of opening carnation flowers, suggesting an association with the morphological changes by elongation or curving. Based on the results, it is suggested that the growth of carnation petals is controlled by co-ordinated gene expression during the progress of flower opening. In addition, the possible roles of some key genes in the initiation of cell growth, the construction of the cell wall and cuticle, and transport across membranes were discussed.

Overexpression of Arabidopsis acyl-CoA binding protein ACBP3 promotes starvation-induced and age-dependent leaf senescence

In Arabidopsis thaliana, a family of six genes (ACBP1 to ACBP6) encodes acyl-CoA binding proteins (ACBPs). Investigations on ACBP3 reported here show its upregulation upon dark treatment and in senescing rosettes. Transgenic Arabidopsis overexpressing ACBP3 (ACBP3-OEs) displayed accelerated leaf senescence, whereas an acbp3 T-DNA insertional mutant and ACBP3 RNA interference transgenic Arabidopsis lines were delayed in dark-induced leaf senescence. Acyl-CoA and lipid profiling revealed that the overexpression of ACBP3 led to an increase in acyl-CoA and phosphatidylethanolamine (PE) levels, whereas ACBP3 downregulation reduced PE content. Moreover, significant losses in phosphatidylcholine (PC) and phosphatidylinositol, and gains in phosphatidic acid (PA), lysophospholipids, and oxylipin-containing galactolipids (arabidopsides) were evident in 3-week-old dark-treated and 6-week-old premature senescing ACBP3-OEs. Such accumulation of PA and arabidopsides (A, B, D, E, and G) resulting from lipid peroxidation in ACBP3-OEs likely promoted leaf senescence. The N-terminal signal sequence/transmembrane domain in ACBP3 was shown to be essential in ACBP3-green fluorescent protein targeting and in promoting senescence. Observations that recombinant ACBP3 binds PC, PE, and unsaturated acyl-CoAs in vitro and that ACBP3 overexpression enhances degradation of the autophagy (ATG)-related protein ATG8 and disrupts autophagosome formation suggest a role for ACBP3 as a phospholipid binding protein involved in the regulation of leaf senescence by modulating membrane phospholipid metabolism and ATG8 stability in Arabidopsis. Accelerated senescence in ACBP3-OEs is dependent on salicylic acid but not jasmonic acid signaling.

Lipid deacylating enzymes in plants: old activities, new genes

Because lipids are major components of cellular membranes, their degradation under stress conditions compromises compartmentalization. However, in addition to having structural roles, membrane lipids are also implicated in signalling processes involving the activity of lipolytic enzymes. Phospholipases D and C, acting on the polar heads of phospholipids, have been relatively well characterized in plants. In contrast, knowledge of lipid deacylating enzymes remains limited. Lipid acyl hydrolases (LAH) are able to hydrolyse both fatty acid moieties of polar lipids. They differ from phospholipases A(1) or A(2) (PLA) acting on sn-1 or sn-2 positions of phospholipids, respectively, as well as from lipases which de-esterify triacylglycerols. The free polyunsaturated fatty acids generated by deacylating enzymes can be used in the biosynthesis of oxylipins and the lysophospholipids, provided by PLAs, are also bioactive molecules. In the four decades that have passed since the first description of LAH activities in plants some enzymes have been purified. In recent years, the widespread use of molecular approaches together with the attention paid to lipid signalling has contributed to a renewed interest in LAH and has led to the identification of different gene families and the characterization of new enzymes. Additionally, several proteins with putative lipase/esterase signatures have been identified. In the present paper we review currently available data on LAHs, PLAs, triacylglycerol lipases and other putative deacylating enzymes. The roles of lipid deacylating enzymes in plant growth, development and stress responses are discussed in the context of their involvement in membrane deterioration, lipid turnover and cellular signalling.

Comparative genomic analysis of light-regulated transcripts in the Solanaceae

Background

Plants use different light signals to adjust their growth and development to the prevailing environmental conditions. Studies in the model species Arabidopsis thaliana and rice indicate that these adjustments are mediated by large changes in the transcriptome. Here we compared transcriptional responses to light in different species of the Solanaceae to investigate common as well as species-specific changes in gene expression.

Results

cDNA microarrays were used to identify genes regulated by a transition from long days (LD) to short days (SD) in the leaves of potato and tobacco plants, and by phytochrome B (phyB), the photoreceptor that represses tuberization under LD in potato. We also compared transcriptional responses to photoperiod in Nicotiana tabacum Maryland Mammoth (MM), which flowers only under SD, with those of Nicotiana sylvestris, which flowers only under LD conditions. Finally, we identified genes regulated by red compared to far-red light treatments that promote germination in tomato.

Conclusion

Most of the genes up-regulated in LD were associated with photosynthesis, the synthesis of protective pigments and the maintenance of redox homeostasis, probably contributing to the acclimatization to seasonal changes in irradiance. Some of the photoperiodically regulated genes were the same in potato and tobacco. Others were different but belonged to similar functional categories, suggesting that conserved as well as convergent evolutionary processes are responsible for physiological adjustments to seasonal changes in the Solanaceae. A β-ZIP transcription factor whose expression correlated with the floral transition in Nicotiana species with contrasting photoperiodic responses was also regulated by photoperiod and phyB in potato, and is a candidate gene to act as a general regulator of photoperiodic responses. Finally, GIGANTEA, a gene that controls flowering time in Arabidopsis thaliana and rice, was regulated by photoperiod in the leaves of potato and tobacco and by red compared to far-light treatments that promote germination in tomato seeds, suggesting that a conserved light signaling cascade acts across developmental contexts and species.

GDSL-lipase1 (CaGL1) contributes to wound stress resistance by modulation of CaPR-4 expression in hot pepper

A full length cDNA clone encoding Capsicum annuum GDSL-lipase 1 (CaGL1) was isolated by microarray analysis. The expression of CaGL1 was triggered by methyl jasmonic acid (MeJA), an important signal in abiotic/biotic stress response. However, the expression of this gene was not increased by the application of salicylic acid (SA) or ethylene treatment. And, local/systemic wounding stimuli resulted in rapid accumulation of CaGL1 mRNA. However, CaGL1 was not specifically induced during the hypersensitive response upon Tobacco mosaic virus (TMV) inoculation. By using a virus-induced gene silencing (VIGS)-based reverse genetic approach, it was observed that the suppression of CaGL1 attenuates the expression of Capsicum annuumpathogenesis-related protein 4 (CaPR-4) during wound stress. However, the CaPR-4 transcript level induced by TMV was not regulated by CaGL1 expression. These results indicate that CaGL1 may be involved in signaling pathway of MeJA and/or the wound responses through CaPR-4 expression modulation.

Heterologous expression, and biochemical and cellular characterization of CaPLA1 encoding a hot pepper phospholipase A1 homolog

Phospholipid signaling has been recently implicated in diverse cellular processes in higher plants. We identified a cDNA encoding the phospholipase A1 homolog (CaPLA1) from 5-day-old early roots of hot pepper. The deduced amino acid sequence showed that the lipase-specific catalytic triad is well conserved in CaPLA1. In vitro lipase assays and site-directed mutagenesis revealed that CaPLA1 possesses PLA1 activity, which catalyzes the hydrolysis of phospholipids at the sn-1 position. CaPLA1 was selectively expressed in young roots, at days 4-5 after germination, and rapidly declined thereafter, suggesting that the expression of CaPLA1 is subject to control by a development-specific mechanism in roots. Because transgenic work was extremely difficult in hot peppers, in this study we overexpressed CaPLA1 in Arabidopsis so as to provide cellular information on the function of this gene. CaPLA1 overexpressors had significantly longer roots, leaves and petioles, and grew more rapidly than the wild-type plants, leading to an early bolting phenotype with prolonged inflorescence. Microscopic analysis showed that the vegetative tissues of 35S:CaPLA1 plants contained an increased number of small-sized cells, which resulted in highly populated cell layers. In addition, mRNAs for cell cycle-controlled proteins and fatty acid catabolizing enzymes were coordinately upregulated in CaPLA1-overexpressing plants. These results suggest that CaPLA1 is functionally relevant in heterologous Arabidopsis cells, and hence might participate in a subset of positive control mechanisms of cell and tissue growth in transgenic lines. We discuss possible biochemical and cellular functions of CaPLA1 in relation to the phospholipid signaling pathway in hot pepper and transgenic Arabidopsis plants.

Function of a novel GDSL-type pepper lipase gene, CaGLIP1, in disease susceptibility and abiotic stress tolerance

GDSL-type lipase is a hydrolytic enzyme whose amino acid sequence contains a pentapeptide motif (Gly-X-Ser-X-Gly) with active serine (Ser). Pepper GDSL-type lipase (CaGLIP1) gene was isolated and functionally characterized from pepper leaf tissues infected by Xanthomonas campestris pv. vesicatoria (Xcv). The CaGLIP1 protein was located in the vascular tissues of Arabidopsis root. The CaGLIP1 gene was preferentially expressed in pepper leaves during the compatible interaction with Xcv. Treatment with salicylic acid, ethylene and methyl jasmonate induced CaGLIP1 gene expression in pepper leaves. Sodium nitroprusside, methyl viologen, high salt, mannitol-mediated dehydration and wounding also induced early and transient CaGLIP1 expression in pepper leaf tissues. Virus-induced gene silencing of CaGLIP1 in pepper conferred enhanced resistance to Xcv, accompanied by the suppressed expression of basic PR1 (CaBPR1) and defensin (CaDEF1) genes. The CaGLIP1 lipase produced in Escherichia coli hydrolyzed the substrates of short and long chain nitrophenyl esters. The CaGLIP1-overexpressing Arabidopsis exhibited enhanced hydrolytic activity toward short and long chain nitrophenyl ester, as well as enhanced susceptibility to the bacterial pathogen Pseudomonas syringae pv. tomato and the biotrophic oomycete Hyaloperonospora parasitica. SA-induced expression of AtPR1 and AtGST1, also was delayed in CaGLIP1-overexpressing plants by SA application. During seed germination and plant growth, the CaGLIP1 transgenic plants showed drought tolerance and differential expression of drought- and abscisic acid (ABA)-inducible genes AtRD29A, AtADH and AtRab18. ABA treatment differentially regulated seed germination and gene expression in wild-type and CaGLIP1 transgenic Arabidopsis. Overexpression of CaGLIP1 also regulated glucose- and oxidative stress signaling. Together, these results indicate that CaGLIP1 modulates disease susceptibility and abiotic stress tolerance.

Integrated signaling in flower senescence: an overview

Flower senescence is the terminal phase of developmental processes that lead to the death of flower, which include, flower wilting, shedding of flower parts and fading of blossoms. Since it is a rapid process as compared to the senescence of other parts of the plant it therefore provides excellent model system for the study of senescence. During flower senescence, developmental and environmental stimuli enhance the upregulation of catabolic processes causing breakdown and remobilization of cellular constituents. Ethylene is well known to play regulatory role in ethylene-sensitive flowers while in ethylene-insensitive flowers abscisic acid (ABA) is thought to be primary regulator. Subsequent to perception of flower senescence signal, death of petals is accompanied by the loss of membrane permeability, increase in oxidative and decreased level of protective enzymes. The last stages of senescence involve the loss of of nucleic acids (DNA and RNA), proteins and organelles, which is achieved by activation of several nucleases, proteases and wall modifiers. Environmental stimuli such as pollination, drought and other stresses also affect senescence by hormonal imbalance. In this article we have covered the following: perception mechanism and specificity of flower senescence, flower senescence-associated events, like degradation of cell membranes, proteins and nucleic acids, environmental/external factors affecting senescence, like pollination and abiotic stress, hormonal and non-hormonal regulation of flower/petal senescence and finally the senescence associated genes (SAGs) have also been described.

Characterization of a plastid triacylglycerol lipase from Arabidopsis

Full-length cDNA corresponding to Arabidopsis (Arabidopsis thaliana) gene At2g31690, which has been annotated in GenBank as a putative triacylglycerol (TAG) lipase, was obtained by reverse transcription-polymerase chain reaction using RNA from senescing rosette leaves of Arabidopsis as a template. The cognate protein was found to contain the lipase active site sequence, and corresponding recombinant protein proved capable of deesterifying TAG. In vitro chloroplast import assays indicated that the lipase is targeted to chloroplasts. This was confirmed by confocal microscopy of rosette leaf tissue treated with fluorescein isocyanate-labeled, lipase-specific antibody, which revealed that lipase protein colocalizes with plastoglobular neutral lipids. Western-blot analysis indicated that the lipase is expressed in roots, inflorescence stems, flowers, siliques, and leaves and that it is strongly up-regulated in senescing rosette leaf tissue. Transgenic plants with suppressed lipase protein levels were obtained by expressing At2g31690 cDNA in antisense orientation under the regulation of a constitutive promoter. Transgenic plants bolted and flowered at the same time as wild-type plants, but were severely stunted and exhibited delayed rosette senescence. Moreover, the stunted growth phenotype correlated with irregular chloroplast morphology. The chloroplasts of transgenic plants were structurally deformed, had reduced abundance of thylakoids that were abnormally stacked, and contained more plastoglobular neutral lipids than chloroplasts of wild-type plants. These observations collectively indicate that this TAG lipase plays a role in maintaining the structural integrity of chloroplasts, possibly by mobilizing the fatty acids of plastoglobular TAG.

Regulation and execution of molecular disassembly and catabolism during senescence

Senescence is a highly orchestrated developmental stage in the life cycle of plants. The onset of senescence is tightly controlled by signaling cascades that initiate changes in gene expression and the synthesis of new proteins. This complement of new proteins includes hydrolytic enzymes capable of executing catabolism of macromolecules, which in turn sets in motion disassembly of membrane molecular matrices, leading to loss of cell function and, ultimately, complete breakdown of cellular ultrastructure. A distinguishing feature of senescence that sets it apart from other types of programmed cell death is the recovery of carbon and nitrogen from the dying tissue and their translocation to growing parts of the plant such as developing seeds. For this to be accomplished, the initiation of senescence and its execution have to be meticulously regulated. For example, the initiation of membrane disassembly has to be intricately linked with the recruitment of nutrients because their ensuing translocation out of the senescing tissue requires functional membranes. Molecular mechanisms underlying this linkage and its integration with the catabolism of macromolecules in senescing tissues are addressed.

Autophagy is not a main contributor to the degradation of phospholipids in tobacco cells cultured under sucrose starvation conditions

Net degradation of cellular components occurs in plant cells cultured under starvation conditions, and autophagy contributes to the degradation of intracellular proteins. In this study, we investigated the degradation of membrane phospholipids by autophagy in cultured tobacco (Nicotiana tabacum) cells. The amounts of total phospholipids and a major phospholipid, phosphatidylcholine (PC), decreased, whereas phosphorylcholine, a degradation product of PC, increased in response to deprivation of sucrose. The addition of glycerol to the culture medium inhibited both the degradation of phospholipids and the concomitant increase of phosphorylcholine. Glycerol, however, did not block autophagy, which was assessed by the accumulation of autolysosomes in the presence of a cysteine protease inhibitor. On the other hand, 3-methyladenine, an inhibitor of autophagy, did not affect the net degradation of PC. We labeled intracellular phospholipids by loading cells with a fluorochrome-labeled fatty acid and chased it under sucrose-free conditions. Glycerol slowed down the decrease in the amount of fluorochrome-labeled PC, suggesting that it inhibits the degradation process of PC. These results show that phospholipids are degraded by mechanisms different from autophagy in tobacco cells cultured under sucrose-free conditions.

Identification and characterization of a triacylglycerol lipase in Arabidopsis homologous to mammalian acid lipases

Triacylglycerol (TAG) lipases have been thoroughly characterized in mammals and microorganisms. By contrast, very little is known on plant TAG lipases. An Arabidopsis cDNA called AtLip1 (At2g15230), which exhibits strong homology to lysosomal acid lipase, was found to drive the synthesis of an active TAG lipase when expressed in the baculovirus system. The lipase had a maximal activity at pH 6 and the specific activity was estimated to be about 45 micromol min(-1) mg(-1) protein using triolein as a substrate. Knock-out mutant analysis showed no phenotype during germination indicating that this enzyme is fully dispensable for TAG storage breakdown during germination. Northern blot analyses indicated that the transcript is present in all tissues tested.

A Colletotrichum gloeosporioides-induced esterase gene of nonclimacteric pepper (Capsicum annuum) fruit during ripening plays a role in resistance against fungal infection

Ripe fruits of pepper (Capsicum annuum) are resistant to the anthracnose fungus, Colletotrichum gloeosporioides, whereas unripe-mature fruits are susceptible. A pepper esterase gene (PepEST) that is highly expressed during an incompatible interaction between the ripe fruit of pepper and C. gloeosporioides was previously cloned. Deduced amino acid sequence of PepEST cDNA showed homology to both esterases and lipases, and contained -HGGGF- and -GXSXG- motifs and a catalytic triad. Inhibition of PepEST activity by a specific inhibitor of serine hydrolase demonstrated that a serine residue is critical for the enzyme activity. Expression of PepEST gene was fruit-specific in response to C. gloeosporioides inoculation, and up-regulated by wounding or jasmonic acid treatment during ripening. PepEST mRNA and protein was differentially accumulated in ripe vs. unripe fruit from 24 h after inoculation when C. gloeosporioides is invading into fruits. Immunochemical examination revealed that PepEST accumulation was localized in epidermal and cortical cell layers in infected ripe fruit, but rarely even in epidermal cells in infected unripe one. Over-expression of PepEST in transgenic Arabidopsis plants caused restriction of Alternaria brassicicola colonization by inhibition of spore production, resulting in enhanced resistance against A.brassicicola. These results suggest that PepEST is involved in the resistance of ripe fruit against C.gloeosporioides infection.

Cloning and characterization of the acid lipase from castor beans

Castor bean endosperm contains a well known acid lipase activity that is associated with the oil body membrane. In order to identify this enzyme, proteomic analysis was performed on purified oil bodies. A approximately 60-kDa protein was identified (RcOBL1), which shares homology with a lipase from the filamentous fungus Rhizomucor miehei. RcOBL1 contains features that are characteristic of an alpha/beta-hydrolase, such as a putative catalytic triad (SDH) and a conserved pentapeptide (GXSXG) surrounding the nucleophilic serine residue. RcOBL1 was expressed heterologously in Escherichia coli and shown to hydrolyze triolein at an acid pH (optima approximately 4.5). RcOBL1 can hydrolyze a range of triacylglycerols but is not active on phospholipids. The activity is sensitive to the serine reagent diethyl p-nitrophenyl phosphate, indicating that RcOBL1 is a serine esterase. Antibodies raised against RcOBL1 were used to show that the protein is restricted to the endosperm where it is associated with the surface of oil bodies. This is the first evidence for the molecular identity of an oil body-associated lipase from plants. Sequence comparisons reveal that families of OBL1-like proteins are present in many species, and it is likely that they play an important role in regulating lipolysis.

Transcriptome profiling in root nodules and arbuscular mycorrhiza identifies a collection of novel genes induced during Medicago truncatula root endosymbioses

Transcriptome profiling based on cDNA array hybridizations and in silico screening was used to identify Medicago truncatula genes induced in both root nodules and arbuscular mycorrhiza (AM). By array hybridizations, we detected several hundred genes that were upregulated in the root nodule and the AM symbiosis, respectively, with a total of 75 genes being induced during both interactions. The second approach based on in silico data mining yielded several hundred additional candidate genes with a predicted symbiosis-enhanced expression. A subset of the genes identified by either expression profiling tool was subjected to quantitative real-time reverse-transcription polymerase chain reaction for a verification of their symbiosis-induced expression. That way, induction in root nodules and AM was confirmed for 26 genes, most of them being reported as symbiosis-induced for the first time. In addition to delivering a number of novel symbiosis-induced genes, our approach identified several genes that were induced in only one of the two root endosymbioses. The spatial expression patterns of two symbiosis-induced genes encoding an annexin and a beta-tubulin were characterized in transgenic roots using promoter-reporter gene fusions.

A tomato lipase homologous to DAD1 (LeLID1) is induced in post-germinative growing stage and encodes a triacylglycerol lipase

A tomato lipase gene homologous to Arabidopsis DAD1 (lipase homologous to DAD1; LeLID1) was cloned and characterized. The corresponding transcript increased rapidly during germination of the seeds and reached a maximum level at four days after germination. Thereafter, it decreased rapidly. Little expression could be found in flowers or fruits. Immunoblot analyses showed that the gene products could be found in the cotyledons and hypocotyls, but not in the roots. In the cotyledons most LeLID1 could be recovered in a soluble fraction. The recombinant LeLID1 protein showed maximum lipase activity at pH 8.0. It showed high activity against triacylglycerols (TAGs) with long acyl chains, but little activity with phosphatidylcholine or monogalactosyldiacylglycerol. TAGs composed of short acyl chains could not be a substrate for the enzyme. A possible involvement of LeLID1 in fat mobilization during seed germination is discussed.

Lipid signaling

Various lipids are involved in mediating plant growth, development and responses to biotic and abiotic cues, and their production is regulated by lipid-signaling enzymes. Lipid-hydrolyzing enzymes play a pivotal role both in the production of lipid messengers and in other processes, such as cytoskeletal rearrangement, membrane trafficking, and degradation. Studies on the downstream targets and modes of action of lipid signals in plants are still in their early stages but distinguishing features of plant lipid-based signaling are being recognized. Phospholipase D enzymes and phosphatidic acid may play a broader role in lipid signaling in plants than in other systems.

Characterization of an ultraviolet B-induced lipase in Arabidopsis

An Arabidopsis expressed sequence tag clone, 221D24, encoding a lipase has been characterized using an antisense approach. The lipase gene is expressed during normal growth and development of Arabidopsis rosette leaves but is down-regulated as the leaves senesce. When plants are exposed to sublethal levels of UV-B radiation, expression of the lipase is strongly up-regulated. The lipase protein is localized in the cell cytosol and is present in all organs of Arabidopsis plants. Recombinant lipase protein produced in Escherichia coli preferentially hydrolyzed phospholipids, indicating that the gene encodes a phospholipase. Transgenic plants in which lipase expression is suppressed showed enhanced tolerance to UV-B stress but not osmotic stress and were unable to up-regulate PR-1 expression when irradiated with UV-B. The observations collectively indicate that the lipase is capable of deesterifying membrane phospholipids and is up-regulated in response to UV-B irradiation.

Capsicum annuum tobacco mosaic virus-induced clone 1 expression perturbation alters the plant's response to ethylene and interferes with the redox homeostasis

Capsicum annuum tobacco mosaic virus (TMV)-induced clone 1 (CaTin1) gene was expressed early during incompatible interaction of hot pepper (Caspsicum annuum) plants with TMV and Xanthomonas campestris. RNA-blot analysis showed that CaTin1 gene was expressed only in roots in untreated plants and induced mainly in leaf in response to ethylene, NaCl, and methyl viologen but not by salicylic acid and methyl jasmonate. The ethylene dependence of CaTin1 induction upon TMV inoculation was demonstrated by the decrease of CaTin1 expression in response to several inhibitors of ethylene biosynthesis or its action. Transgenic tobacco (Nicotiana tabacum) plants expressing CaTin1 gene in sense- or antisense-orientation showed interesting characteristics such as the accelerated growth and the enhanced resistance to biotic as well as abiotic stresses. Such characteristics appear to be caused by the elevated level of ethylene and H2O2. Moreover, in transgenic plants expressing antisense CaTin1 gene, the expression of some pathogenesis-related genes was enhanced constitutively, which may be mainly due to the increased ethylene level. The promoter of CaTin1 has four GCC-boxes, two AT-rich regions, and an elicitor-inducible W-box. The induction of the promoter activity by ethylene depends on GCC-boxes and by TMV on W-box. Taken together, we propose that the CaTin1 up-regulation or down-regulation interferes with the redox balance of plants leading to the altered response to ethylene and biotic as well as abiotic stresses.

Regulation of senescence by eukaryotic translation initiation factor 5A: implications for plant growth and development

Regulation of protein synthesis is increasingly being recognized as an important determinant of cell proliferation and senescence. In particular, recent evidence indicates that eukaryotic translation initiation factor 5A (eIF-A) plays a pivotal role in this determination. Separate isoforms of eIF-5A appear to facilitate the translation of mRNAs required for cell division and cell death. This raises the possibility that eIF-5A isoforms are elements of a biological switch that is in one position in dividing cells and in another position in dying cells. Changes in the position of this putative switch in response to physiological and environmental cues are likely to have a significant impact on plant growth and development.

Large-scale identification of leaf senescence-associated genes

Leaf senescence is a form of programmed cell death, and is believed to involve preferential expression of a specific set of "senescence-associated genes" (SAGs). To decipher the molecular mechanisms and the predicted complex network of regulatory pathways involved in the senescence program, we have carried out a large-scale gene identification study in a reference plant, Arabidopsis thaliana. Using suppression subtractive hybridization, we isolated approximately 800 cDNA clones representing SAGs expressed in senescing leaves. Differential expression was confirmed by Northern blot analysis for 130 non-redundant genes. Over 70 of the identified genes have not previously been shown to participate in the senescence process. SAG-encoded proteins are likely to participate in macromolecule degradation, detoxification of oxidative metabolites, induction of defense mechanisms, and signaling and regulatory events. Temporal expression profiles of selected genes displayed several distinct patterns, from expression at a very early stage, to the terminal phase of the senescence syndrome. Expression of some of the novel SAGs, in response to age, leaf detachment, darkness, and ethylene and cytokinin treatment was compared. The large repertoire of SAGs identified here provides global insights about regulatory, biochemical and cellular events occurring during leaf senescence.

Molecular characterization of a novel lipase-like pathogen-inducible gene family of Arabidopsis

In a differential screening between Arabidopsis plants pretreated with the resistance-inducer beta-aminobutyric acid and untreated control plants, we have identified a gene encoding a novel lipase-like protein, PRLIP1. The abundance of PRLIP1 mRNAs in Arabidopsis leaves was up-regulated by application of beta-aminobutyric acid, salicylic acid (SA), and ethylene as well as by various pathogens. Induction of PRLIP1 depended on a functioning SA and ethylene signal transduction pathway but was independent of jasmonate signaling. This novel pathogenesis-related (PR) gene of Arabidopsis belongs to a gene family consisting of six (PRLIP1, PRLIP2, PRLIP4, PRLIP5, PRLIP6, and PRLIP7) closely related members in tandem position on chromosome 5. Among these genes, PRLIP2 also was induced in leaves by SA and infections by pathogens but on a much lower level than PRLIP1. The PRLIP1 family showed a tissue-specific expression pattern. Both PRLIP1 and PRLIP2 were specifically expressed in leaves and siliques, PRLIP1 additionally in stems and flowers. The expression of PRLIP6 and PRLIP4 was root specific, whereas mRNA of PRLIP5 and PRLIP7 were not detected in any of these tissues. The more distantly related genes PRLIP3, PRLIP9, and PRLIP8 were found on chromosomes 2, 4, and 5, respectively. The expression level of PRLIP3 was checked and found constitutive during the different stress conditions tested. The PRLIP1 gene was overexpressed in Escherichia coli, and the resulting PRLIP1 protein showed esterase activity on p-nitrophenyl-butyrate and allowed the growth of the bacteria on lipidic substrates such as Tween20 or Tween80.

The molecular analysis of leaf senescence--a genomics approach

Senescence in green plants is a complex and highly regulated process that occurs as part of plant development or can be prematurely induced by stress. In the last decade, the main focus of research has been on the identification of senescence mutants, as well as on genes that show enhanced expression during senescence. Analysis of these is beginning to expand our understanding of the processes by which senescence functions. Recent rapid advances in genomics resources, especially for the model plant species Arabidopsis, are providing scientists with a dazzling array of tools for the identification and functional analysis of the genes and pathways involved in senescence. In this review, we present the current understanding of the mechanisms by which plants control senescence and the processes that are involved.