Deficiency in fatty acid synthase leads to premature cell death and dramatic alterations in plant morphology

Interactions between auxin transport and the actin cytoskeleton in developmental polarity of Fucus distichus embryos in response to light and gravity

Land plants orient their growth relative to light and gravity through complex mechanisms that require auxin redistribution. Embryos of brown algae use similar environmental stimuli to orient their developmental polarity. These studies of the brown algae Fucus distichus examined whether auxin and auxin transport are also required during polarization in early embryos and to orient growth in already developed tissues. These embryos polarize with the gravity vector in the absence of a light cue. The auxin, indole-3-acetic acid (IAA), and auxin efflux inhibitors, such as naphthylphthalamic acid (NPA), reduced environmental polarization in response to gravity and light vectors. Young rhizoids are negatively phototropic, and NPA also inhibits rhizoid phototropism. The effect of IAA and NPA on gravity and photopolarization is maximal within 2.5 to 4.5 h after fertilization (AF). Over the first 6 h AF, auxin transport is relatively constant, suggesting that developmentally controlled sensitivity to auxin determines the narrow window during which NPA and IAA reduce environmental polarization. Actin patches were formed during the first hour AF and began to photolocalize within 3 h, coinciding with the time of NPA and IAA action. Treatment with NPA reduced the polar localization of actin patches but not patch formation. Latrunculin B prevented environmental polarization in a time frame that overlaps the formation of actin patches and IAA and NPA action. Latrunculin B also altered auxin transport. Together, these results indicate a role for auxin in the orientation of developmental polarity and suggest interactions between the actin cytoskeleton and auxin transport in F. distichus embryos.

The acyl-CoA synthetase encoded by LACS2 is essential for normal cuticle development in Arabidopsis

Long-chain acyl-CoA synthetase (LACS) activities are encoded by a family of at least nine genes in Arabidopsis (Arabidopsis thaliana). These enzymes have roles in lipid synthesis, fatty acid catabolism, and the transport of fatty acids between subcellular compartments. Here, we show that the LACS2 gene (At1g49430) is expressed in young, rapidly expanding tissues, and in leaves expression is limited to cells of the adaxial and abaxial epidermal layers, suggesting that the LACS2 enzyme may act in the synthesis of cutin or cuticular waxes. A lacs2 null mutant was isolated by reverse genetics. Leaves of mutant plants supported pollen germination and released chlorophyll faster than wild-type leaves when immersed in 80% ethanol, indicating a defect in the cuticular barrier. The composition of surface waxes extracted from lacs2 leaves was similar to the wild type, and the total wax load was higher than the wild type (111.4 microg/dm(2) versus 76.4 microg/dm(2), respectively). However, the thickness of the cutin layer on the abaxial surface of lacs2 leaves was only 22.3 +/- 1.7 nm compared with 33.0 +/- 2.0 nm for the wild type. In vitro assays showed that 16-hydroxypalmitate was an excellent substrate for recombinant LACS2 enzyme. We conclude that the LACS2 isozyme catalyzes the synthesis of omega-hydroxy fatty acyl-CoA intermediates in the pathway to cutin synthesis. The lacs2 phenotype, like the phenotypes of some other cutin mutants, is very pleiotropic, causing reduced leaf size and plant growth, reduced seed production, and lower rates of seedling germination and establishment. The LACS2 gene and the corresponding lacs2 mutant will help in future studies of the cutin synthesis pathway and in understanding the consequences of reduced cutin production on many aspects of plant biology.

In-depth analysis of the thylakoid membrane proteome of Arabidopsis thaliana chloroplasts: new proteins, new functions, and a plastid proteome database

An extensive analysis of the Arabidopsis thaliana peripheral and integral thylakoid membrane proteome was performed by sequential extractions with salt, detergent, and organic solvents, followed by multidimensional protein separation steps (reverse-phase HPLC and one- and two-dimensional electrophoresis gels), different enzymatic and nonenzymatic protein cleavage techniques, mass spectrometry, and bioinformatics. Altogether, 154 proteins were identified, of which 76 (49%) were alpha-helical integral membrane proteins. Twenty-seven new proteins without known function but with predicted chloroplast transit peptides were identified, of which 17 (63%) are integral membrane proteins. These new proteins, likely important in thylakoid biogenesis, include two rubredoxins, a potential metallochaperone, and a new DnaJ-like protein. The data were integrated with our analysis of the lumenal-enriched proteome. We identified 83 out of 100 known proteins of the thylakoid localized photosynthetic apparatus, including several new paralogues and some 20 proteins involved in protein insertion, assembly, folding, or proteolysis. An additional 16 proteins are involved in translation, demonstrating that the thylakoid membrane surface is an important site for protein synthesis. The high coverage of the photosynthetic apparatus and the identification of known hydrophobic proteins with low expression levels, such as cpSecE, Ohp1, and Ohp2, indicate an excellent dynamic resolution of the analysis. The sequential extraction process proved very helpful to validate transmembrane prediction. Our data also were cross-correlated to chloroplast subproteome analyses by other laboratories. All data are deposited in a new curated plastid proteome database (PPDB) with multiple search functions (http://cbsusrv01.tc.cornell.edu/users/ppdb/). This PPDB will serve as an expandable resource for the plant community.

BRITTLE CULM1, which encodes a COBRA-like protein, affects the mechanical properties of rice plants

Plant mechanical strength is an important agronomic trait. To understand the molecular mechanism that controls the plant mechanical strength of crops, we characterized the classic rice mutant brittle culm1 (bc1) and isolated BC1 using a map-based cloning approach. BC1, which encodes a COBRA-like protein, is expressed mainly in developing sclerenchyma cells and in vascular bundles of rice. In these types of cells, mutations in BC1 cause not only a reduction in cell wall thickness and cellulose content but also an increase in lignin level, suggesting that BC1, a gene that controls the mechanical strength of monocots, plays an important role in the biosynthesis of the cell walls of mechanical tissues.

BRITTLE CULM1, which encodes a COBRA-like protein, affects the mechanical properties of rice plants

Plant mechanical strength is an important agronomic trait. To understand the molecular mechanism that controls the plant mechanical strength of crops, we characterized the classic rice mutant brittle culm1 (bc1) and isolated BC1 using a map-based cloning approach. BC1, which encodes a COBRA-like protein, is expressed mainly in developing sclerenchyma cells and in vascular bundles of rice. In these types of cells, mutations in BC1 cause not only a reduction in cell wall thickness and cellulose content but also an increase in lignin level, suggesting that BC1, a gene that controls the mechanical strength of monocots, plays an important role in the biosynthesis of the cell walls of mechanical tissues.

Expression of antisense acyl carrier protein-4 reduces lipid content in Arabidopsis leaf tissue

Arabidopsis plants were transformed with acyl carrier protein (ACP)-4 in antisense conformation driven by the cauliflower mosaic virus 35S promoter. It was hypothesized that reduction of ACP4 in leaf tissue would result in a reduction in lipid biosynthesis and, in addition, affect fatty acid composition and leaf physiology. Several transgenic lines have been generated with reduced ACP4 protein in leaf tissue. Dramatic reductions in ACP4 resulted in a reduction of leaf lipid content (22%-60%) based on fresh leaf weight and a bleached appearance and reduced photosynthetic efficiency. In addition, a decrease in 16:3 as a percentage of the total fatty acid composition was noted. There were no changes in leaf lipid class distribution; however, there was a decrease in the relative amount of 16:3 in monogalactosyldiacylglycerol. These results suggest that ACP4 plays a major role in the biosynthesis of fatty acids for chloroplast membrane development. Alterations in the ACP isoform profile of Arabidopsis leaf also appear to alter the flow of fatty acids between the prokaryotic and eukaryotic pathways for assembly of galactolipids. However, it has not yet been determined if the changes in fatty acid composition are due to changes in the profile of ACP isoforms, or if they are actually a reaction to a reduction in fatty acid precursors.

Light-dependent death of maize lls1 cells is mediated by mature chloroplasts

We reported previously the isolation of a novel cell death-suppressing gene from maize (Zea mays) encoded by the Lls1 (Lethal leaf spot-1) gene. Although the exact metabolic function of LLS1 remains elusive, here we provide insight into mechanisms that underlie the initiation and propagation of cell death associated with lls1 lesions. Our data indicate that lls1 lesions are triggered in response to a cell-damaging event caused by any biotic or abiotic agent or intrinsic metabolic imbalance--as long as the leaf tissue is developmentally competent to develop lls1 lesions. Continued expansion of these lesions, however, depends on the availability of light, with fluence rate being more important than spectral quality. Double-mutant analysis of lls1 with two maize mutants oil-yellow and iojap, both compromised photosynthetically and unable to accumulate normal levels of chlorophyll, indicated that it was the light harvested by the plant that energized lls1 lesion development. Chloroplasts appear to be the key mediators of lls1 cell death; their swelling and distortion occurs before any other changes normally associated with dying cells. In agreement with these results are indications that LLS1 is a chloroplast-localized protein whose transcript was detected only in green tissues. The propagative nature of light-dependent lls1 lesions predicts that cell death associated with these lesions is caused by a mobile agent such as reactive oxidative species. LLS1 may act to prevent reactive oxidative species formation or serve to remove a cell death mediator so as to maintain chloroplast integrity and cell survival.

Silencing of phosphoethanolamine N-methyltransferase results in temperature-sensitive male sterility and salt hypersensitivity in Arabidopsis

S-Adenosyl-L-methionine:phosphoethanolamine N-methyltransferase (PEAMT; EC 2.1.1.103) catalyzes the key step in choline (Cho) biosynthesis, the N-methylation of phosphoethanolamine. Cho is a vital precursor of the membrane phospholipid phosphatidylcholine, which accounts for 40 to 60% of lipids in nonplastid plant membranes. Certain plants use Cho to produce the osmoprotectant glycine betaine, which confers resistance to salinity, drought, and other stresses. An Arabidopsis mutant, t365, in which the PEAMT gene is silenced, was identified using a new sense/antisense RNA expression system. t365 mutant plants displayed multiple morphological phenotypes, including pale-green leaves, early senescence, and temperature-sensitive male sterility. Moreover, t365 mutant plants produced much less Cho and were hypersensitive to salinity. These results demonstrate that Cho biosynthesis not only plays an important role in plant growth and development but also contributes to tolerance to environmental stresses. The temperature-sensitive male sterility caused by PEAMT silencing may have a potential application in agriculture for engineering temperature-sensitive male sterility in important crop plants.

Purple acid phosphatases of Arabidopsis thaliana. Comparative analysis and differential regulation by phosphate deprivation

Purple acid phosphatases (PAPs) are members of the metallo-phosphoesterase family. They are characterized by the presence of seven conserved amino acid residues involved in coordinating the dimetal nuclear center in their reactive site. We compared the 29 PAPs predicted for Arabidopsis thaliana in their varieties of potential metal-ligating residues. Although 24 members possessed sets of metal-ligating residues typical of known PAPs, 1 member lacked four of the seven residues. For the remaining four members, potential metal-ligating residues were generally more similar to those in metal-dependent exonucleases and related proteins. Evidence was obtained for the expression of the majority of the 29 PAPs. To facilitate future investigations, a scheme for naming Arabidopsis PAPs and a system for classifying the 29 PAPs are proposed. The cDNA sequences and the responses to phosphate deprivation of seven Arabidopsis PAPs (AtPAP7-AtPAP13) were characterized. For some AtPAPs analyzed, there were fully processed transcripts as well as splice variants. The splice variants of AtPAP10 were found to associate with polyribosomes and may be translated into a NH(2)-terminal truncated protein. Phylogenetic investigations showed that AtPAPs 7 and 8, together with similar enzymes from other plant species, formed the low molecular weight plant PAP group. Members of this group were more closely related to PAPs from mammalian cells. AtPAPs 9-13, together with kidney bean PAP, formed the high molecular weight PAP group. In phosphate deprivation experiments, gene transcription of AtPAP11 and AtPAP12 was induced and increased, respectively, whereas that of the remaining five AtPAPs was not affected by phosphate deprivation. The present work demonstrates that structure variation and expression regulation of plant PAPs are more complex than previously described and provides a framework for comprehensive molecular genetic and biochemical studies of all Arabidopsis PAPs in the future.

Mutants of Arabidopsis reveal many roles for membrane lipids

Polyunsaturated acyl lipids constitute approximately 50% of the hydrophobic membrane barriers that delineate the compartments of cells. The composition of these lipids is critically important for many membrane functions and, thus, for proper growth and development of all living organisms. In the model plant Arabidopsis, the isolation of mutants with altered lipid compositions has facilitated biochemical and molecular approaches to understanding lipid metabolism and membrane biogenesis. Just as importantly, the availability of a series of plant lines with specific changes in membrane lipids have provided a new resource to study the structural and adaptive roles of lipids. Now, the sequencing of the Arabidopsis genome, and the development of reverse-genetics approaches provide the tools needed to make additional discoveries about the relationships between lipid structure and membrane function in plant cells.

Knockout of Arabidopsis accelerated-cell-death11 encoding a sphingosine transfer protein causes activation of programmed cell death and defense

We describe the lethal, recessive accelerated-cell-death11 Arabidopsis mutant (acd11). Cell death in acd11 exhibits characteristics of animal apoptosis monitored by flow cytometry, and acd11 constitutively expresses defense-related genes that accompany the hypersensitive response normally triggered by avirulent pathogens. Global transcriptional changes during programmed cell death (PCD) and defense activation in acd11 were monitored by cDNA microarray hybridization. The PCD and defense pathways activated in acd11 are salicylic acid (SA) dependent, but do not require intact jasmonic acid or ethylene signaling pathways. Light is required for PCD execution in acd11, as application of an SA-analog to SA-deficient acd11 induced death in the light, but not in the dark. Epistatic analysis showed that the SA-dependent pathways require two regulators of SA-mediated resistance responses, PAD4 and EDS1. Furthermore, acd11 PR1 gene expression, but not cell death, depends on the SA signal tranducer NPR1, suggesting that the npr1-1 mutation uncouples resistance responses and cell death in acd11. The acd11 phenotype is caused by deletion of the ACD11 gene encoding a protein homologous to a mammalian glycolipid transfer protein (GLTP). In contrast to GLTP, ACD11 accelerates the transfer of sphingosine, but not of glycosphingolipids, between membranes in vitro.

A deficiency of coproporphyrinogen III oxidase causes lesion formation in Arabidopsis

We isolated an Arabidopsis lesion initiation 2 (lin2) mutant, which develops lesion formation on leaves and siliques in a developmentally regulated and light-dependent manner. The phenotype of the lin2 plants resulted from a single nuclear recessive mutation, and LIN2 was isolated by a T-DNA tagging approach. LIN2 encodes coproporphyrinogen III oxidase, a key enzyme in the biosynthetic pathway of chlorophyll and heme, a tetrapyrrole pathway, in Arabidopsis. The lin2 plants express cytological and molecular markers associated with the defense responses, usually activated by pathogen infection. These results demonstrate that a porphyrin pathway impairment is responsible for the lesion initiation phenotype, which leads to the activation of defense responses, in Arabidopsis. Lesion formation was not suppressed, and was even enhanced when accumulation of salicylic acid (SA) was prevented in lin2 plants by the expression of an SA-degrading salicylate hydroxylase (nahG) gene. This suggests that the lesion formation triggered in lin2 plants is determined prior to or independently of the accumulation of SA but that the accumulation is required to limit the spread of lesions in lin2 plants.

Promotive effect of brassinosteroids on cell division involves a distinct CycD3-induction pathway in Arabidopsis

Brassinosteroids (BRs) are steroid hormones that play an essential role in plant growth and development. However, the contradictory results of previous studies make their role in cell division unclear. Using a cDNA array, we identified genes that respond to BR in the det2 suspension culture of Arabidopsis, and found that epi-brassinolide upregulated transcription of the CycD3, a D-type plant cyclin gene through which cytokinin activates cell division. RNA gel-blot analysis and cell culturing showed that epi-brassinolide may promote cell division through CycD3, and can substitute cytokinin in culturing of Arabidopsis callus and suspension cells. The CycD3 induction by epi-brassinolide was further shown to involve de novo protein synthesis, but no protein phosphorylation or dephosphorylation. Induction was also found to occur in cells of a BR-insensitive mutant, bri1, suggesting that BR induces CycD3 transcription through a previously unknown signal pathway in plants.

Indole-3-glycerol phosphate, a branchpoint of indole-3-acetic acid biosynthesis from the tryptophan biosynthetic pathway in Arabidopsis thaliana

The phytohormone indole-3-acetic acid (IAA) plays a vital role in plant growth and development as a regulator of numerous biological processes. Its biosynthetic pathways have been studied for decades. Recent genetic and in vitro labeling evidence indicates that IAA in Arabidopsis thaliana and other plants is primarily synthesized from a precursor that is an intermediate in the tryptophan (Trp) biosynthetic pathway. To determine which intermediate(s) acts as the possible branchpoint for the Trp-independent IAA biosynthesis in plants, we took an in vivo approach by generating antisense indole-3-glycerol phosphate synthase (IGS) RNA transgenic plants and using available Arabidopsis Trp biosynthetic pathway mutants trp2-1 and trp3-1. Antisense transgenic plants display some auxin deficient-like phenotypes including small rosettes and reduced fertility. Protein gel blot analysis indicated that IGS expression was greatly reduced in the antisense lines. Quantitative analyses of IAA and Trp content in antisense IGS transgenic plants and Trp biosynthetic mutants revealed striking differences. Compared with wild-type plants, the Trp content in all the transgenic and mutant plants decreased significantly. However, total IAA levels were significantly decreased in antisense IGS transgenic plants, but remarkably increased in trp3-1 and trp2-1 plants. These results suggest that indole-3-glycerol phosphate (IGP) in the Arabidopsis Trp biosynthetic pathway serves as a branchpoint compound in the Trp-independent IAA de novo biosynthetic pathway.