A mini binary vector series for plant transformation

Suppression of a vegetative MADS box gene of potato activates axillary meristem development

Potato MADS box 1 (POTM1) is a member of the SQUAMOSA-like family of plant MADS box genes isolated from an early stage tuber cDNA library. The RNA of POTM1 is most abundant in vegetative meristems of potato (Solanum tuberosum), accumulating specifically in the tunica and corpus layers of the meristem, the procambium, the lamina of new leaves, and newly formed axillary meristems. Transgenic lines with reduced levels of POTM1 mRNA exhibited decreased apical dominance accompanied by a compact growth habit and a reduction in leaf size. Suppression lines produced truncated shoot clusters from stem buds and, in a model system, exhibited enhanced axillary bud growth instead of producing a tuber. This enhanced axillary bud growth was not the result of increased axillary bud formation. Tuber yields were reduced and rooting of cuttings was strongly inhibited in POTM1 suppression lines. Both starch accumulation and the activation of cell division occurred in specific regions of the vegetative meristems of the POTM1 transgenic lines. Cytokinin levels in axillary buds of a transgenic suppression line increased 2- to 3-fold. These results imply that POTM1 mediates the control of axillary bud development by regulating cell growth in vegetative meristems.

Transport of cytokinins mediated by purine transporters of the PUP family expressed in phloem, hydathodes, and pollen of Arabidopsis

Nucleobases and derivatives like cytokinins and caffeine are translocated in the plant vascular system. Transport studies in cultured Arabidopsis cells indicate that adenine and cytokinin are transported by a common H+-coupled high-affinity purine transport system. Transport properties are similar to that of Arabidopsis purine transporters AtPUP1 and 2. When expressed in yeast, AtPUP1 and 2 mediate energy-dependent high-affinity adenine uptake, whereas AtPUP3 activity was not detectable. Similar to the results from cell cultures, purine permeases (PUP) mediated uptake of adenine can be inhibited by cytokinins, indicating that cytokinins are transport substrates. Direct measurements demonstrate that AtPUP1 is capable of mediating uptake of radiolabeled trans-zeatin. Cytokinin uptake is strongly inhibited by adenine and isopentenyladenine but is poorly inhibited by 6-chloropurine. A number of physiological cytokinins including trans- and cis-zeatin are also efficient competitors for AtPUP2-mediated adenine uptake, suggesting that AtPUP2 is also able to mediate cytokinin transport. Furthermore, AtPUP1 mediates transport of caffeine and ribosylated purine derivatives in yeast. Promoter-reporter gene studies point towards AtPUP1 expression in the epithem of hydathodes and the stigma surface of siliques, suggesting a role in retrieval of cytokinins from xylem sap to prevent loss during guttation. The AtPUP2 promoter drives GUS reporter gene activity in the phloem of Arabidopsis leaves, indicating a role in long-distance transport of adenine and cytokinins. Promoter activity of AtPUP3 was only found in pollen. In summary, three closely related PUPs are differentially expressed in Arabidopsis and at least two PUPs have properties similar to the adenine and cytokinin transport system identified in Arabidopsis cell cultures.

In planta side-chain glucosinolate modification in Arabidopsis by introduction of dioxygenase Brassica homolog BoGSL-ALK

Aliphatic glucosinolates and their derived isothiocyanates are important secondary metabolites in crucifers. Some of these compounds have beneficial activities such as carcinogen detoxification, pesticidal and antifungal properties, but others are anti-nutritional; the differences are largely due to side chain modifications. We report the cloning and in planta functionality analysis of BoGSL-ALK, a gene whose protein product influences side-chain modifications in the glucosinolate pathway. Expression of this Brassica gene was demonstrated in Arabidopsis thaliana by assaying RNA activity and monitoring changes in the glucosinolate profiles in leaves and seeds of transformed plants. Dependent on the proposed uses of the crops under development, the ability to regulate BoGSL-ALK expression is a key step towards engineering Brassica crops with specific glucosinolate content.

Agrobacterium-mediated plant transformation: the biology behind the "gene-jockeying" tool

Agrobacterium tumefaciens and related Agrobacterium species have been known as plant pathogens since the beginning of the 20th century. However, only in the past two decades has the ability of Agrobacterium to transfer DNA to plant cells been harnessed for the purposes of plant genetic engineering. Since the initial reports in the early 1980s using Agrobacterium to generate transgenic plants, scientists have attempted to improve this "natural genetic engineer" for biotechnology purposes. Some of these modifications have resulted in extending the host range of the bacterium to economically important crop species. However, in most instances, major improvements involved alterations in plant tissue culture transformation and regeneration conditions rather than manipulation of bacterial or host genes. Agrobacterium-mediated plant transformation is a highly complex and evolved process involving genetic determinants of both the bacterium and the host plant cell. In this article, I review some of the basic biology concerned with Agrobacterium-mediated genetic transformation. Knowledge of fundamental biological principles embracing both the host and the pathogen have been and will continue to be key to extending the utility of Agrobacterium for genetic engineering purposes.

Efficient disruption of a polyketide synthase gene ( pks1) required for melanin synthesis through Agrobacterium-mediated transformation of Glarea lozoyensis

Glarea lozoyensis produces pneumocandin B(0), a potent inhibitor of fungal glucan synthesis. This industrially important filamentous fungus is slow-growing, is very darkly pigmented, and has not been easy to manipulate genetically. Using a PCR strategy to survey the G. lozoyensis genome for polyketide synthase (PKS) genes, we have identified pks1, a gene that consists of five exons interrupted by four introns of 56, 400, 50 and 341 bp. It encodes a 2124-amino acid protein with five catalytic modules: ketosynthase, acyltransferase, two acyl carrier sites, and thioesterase/Claisen cyclase. The transcriptional initiation and termination sites were found 43 bp upstream of the translational start codon and 295 bp downstream of the translational stop codon, respectively. Cluster analysis of 37 fungal ketosynthase modules grouped the Pks1p with PKSs involved in the biosynthesis of 1,8-dihydroxynaphthalene melanin. Disruption of pks1 yielded knockout mutants that displayed an albino phenotype, suggesting that pks1 encodes a tetrahydroxynaphthalene synthase. Gene replacement was achieved by Agrobacterium-mediated transformation, which proved to be simple and efficient. Loss of pigmentation occurred in more than half the transformants, and examination of six non-pigmented transformants showed that the functional genomic copy of the pks1 gene had been replaced by the disruption cassette in each case. A putative 1215-bp ORF (dsg) devoid of introns was present downstream from pks1. BLAST analysis of the 405-amino acid sequence of its predicted product showed a high degree of similarity with Zn(II)(2)Cys(6) binuclear cluster DNA-binding proteins, a class of fungal transcription factors involved in the regulation of polyketide production and other pathways.

Identification of an Arabidopsis mitochondrial succinate-fumarate translocator

Complementation of a yeast acr1 mutant carrying a deletion of the succinate/fumarate carrier gene enabled functional identification of a mitochondrial succinate translocator from Arabidopsis thaliana (AtmSFC1). Thus complementation of yeast mutants is applicable also for identification and characterization of organellar transporters. Reverse transcription polymerase chain reaction and promoter-GUS fusion showed expression of AtmSFC1 in 2 day old dark grown seedlings, which declined in cotyledons during further development, consistent with a role in export of fumarate for gluconeogenesis during lipid mobilization at early germination of Arabidopsis seeds. In mature plants, expression was found in developing and germinating pollen, suggesting a role in ethanolic fermentation.

Expression pattern of a nuclear encoded mitochondrial arginine-ornithine translocator gene from Arabidopsis

BACKGROUND

Arginine and citrulline serve as nitrogen storage forms, but are also involved in biosynthetic and catabolic pathways. Metabolism of arginine, citrulline and ornithine is distributed between mitochondria and cytosol. For the shuttle of intermediates between cytosol and mitochondria transporters present on the inner mitochondrial membrane are required. Yeast contains a mitochondrial translocator for ornithine and arginine, Ort1p/Arg11p. Ort1p/Arg11p is a member of the mitochondrial carrier family (MCF) essential for ornithine export from mitochondria. The yeast arg11 mutant, which is deficient in Ort1p/Arg11p grows poorly on media lacking arginine.

RESULTS

High-level expression of a nuclear encoded Arabidopsis thaliana homolog (AtmBAC2) of Ort1p/Arg11p was able to suppress the growth deficiency of arg11. RT-PCR analysis demonstrated expression of AtmBAC2 in all tissues with highest levels in flowers. Promoter-GUS fusions showed preferential expression in flowers, i.e. pollen, in the vasculature of siliques and in aborted seeds. Variable expression was observed in leaf vasculature. Induction of the promoter was not observed during the first two weeks in seedlings grown on media containing NH4NO3, arginine or ornithine as sole nitrogen sources.

CONCLUSION

AtmBAC2 was isolated as a mitochondrial transporter for arginine in Arabidopsis. The absence of expression in developing seeds and in cotyledons of seedlings indicates that other transporters are responsible for storage and mobilization of arginine in seeds.

High affinity amino acid transporters specifically expressed in xylem parenchyma and developing seeds of Arabidopsis

Arabidopsis amino acid transporters (AAPs) show individual temporal and spatial expression patterns. A new amino acid transporter, AAP8 was isolated by reverse transcription-PCR. Growth and transport assays in comparison to AAP1-5 characterize AAP8 and AAP6 as high affinity amino acid transport systems from Arabidopsis. Histochemical promoter-beta-glucuronidase (GUS) studies identified AAP6 expression in xylem parenchyma, cells requiring high affinity transport due to the low amino acid concentration in xylem sap. AAP6 may thus function in uptake of amino acids from xylem. Histochemical analysis of AAP8 revealed stage-dependent expression in siliques and developing seeds. Thus AAP8 is probably responsible for import of organic nitrogen into developing seeds. The only missing transporter of the family AAP7 was nonfunctional in yeast with respect to amino acid transport, and expression was not detectable. Therefore, AAP6 and -8 are the only members of the family able to transport aspartate with physiologically relevant affinity. AAP1, -6 and -8 are the closest AAP paralogs. Although AAP1 and AAP8 originate from a duplicated region on chromosome I, biochemical properties and expression pattern diverged. Overlapping substrate specificities paired with individual properties and expression patterns point to specific functions of each of the AAP genes in nitrogen distribution rather than to mere redundancy.

Interaction between long-distance transport factor and Hsp70-related movement protein of Beet yellows virus

Systemic spread of viruses in plants involves local movement from cell to cell and long-distance transport through the vascular system. The cell-to-cell movement of the Beet yellows virus (BYV) is mediated by a movement protein that is an Hsp70 homolog (Hsp70h). This protein is required for the assembly of movement-competent virions that incorporate Hsp70h. By using the yeast two-hybrid system, in vitro coimmunoprecipitation, and in planta coexpression approaches, we show here that the Hsp70h interacts with a 20-kDa BYV protein (p20). We further demonstrate that p20 is associated with the virions presumably via binding to Hsp70h. Genetic and immunochemical analyses indicate that p20 is dispensable for assembly and cell-to-cell movement of BYV but is required for the long-distance transport of virus through the phloem. These results reveal a novel activity for the Hsp70h that provides a molecular link between the local and systemic spread of a plant virus by docking a long-distance transport factor to virions.

Role of acetyl-coenzyme A synthetase in leaves of Arabidopsis thaliana

Acetyl-coenzyme A synthetase (ACS) is a plastidic enzyme that forms acetyl-coenzyme A (acetyl-CoA) from acetate and coenzyme A using the energy from ATP. Traditionally it has been thought to be the major source for the production of acetyl-CoA destined for fatty acid formation. Recent work suggested that the accumulation of lipids in developing Arabidopsis seeds was more closely correlated with the expression of pyruvate dehydrogenase complex than with the expression of ACS, suggesting that most of the carbon for fatty acid formation in the plastids of seeds comes from pyruvate rather than from acetate. To explore the role of this enzyme, Arabidopsis plants with altered amounts of ACS were generated by overexpressing its cDNA in either the sense or the antisense configuration. The resulting plants had in vitro enzyme activities that ranged from about 5% to over 400% of wild-type levels. The rate of [1-14C]acetate conversion into fatty acids was closely related to the in vitro ACS activity, showing that the amount of enzyme clearly limited the capacity of leaves to convert exogenous acetate to fatty acids. There was, however, no relationship between the ACS level and the capacity of the plants to incorporate 14CO2 into 14C-labeled fatty acids. These data strongly support the idea that, although plants can convert acetate into fatty acids, relatively little carbon moves through this pathway under normal conditions.

Telomere length deregulation and enhanced sensitivity to genotoxic stress in Arabidopsis mutants deficient in Ku70

The Ku70/80 heterodimer is a critical component of the non-homologous end-joining (NHEJ) pathway and of the telomere cap in yeast and mammals. We report the molecular characterization of the KU70 and KU80 genes in Arabidopsis and describe the consequences of a Ku70 deficiency. Arabidopsis KU70/80 genes are ubiquitously expressed and their products form stable heterodimers in vitro. Plants harboring a T-DNA insertion in KU70 exhibit no growth or developmental defects under standard growth conditions. However, mutant seedlings are hypersensitive to gamma-irradiation-induced double-strand breaks. Unexpectedly, we found that mutants are hypersensitive to methyl methanosulfonate during seed germination, but lose this sensitivity in seedlings, implying that the requirement for NHEJ varies during plant development. Lack of Ku70 results in a dramatic deregulation of telomere length control, with mutant telomeres expanding to more than twice the size of wild type by the second generation. Furthermore, in contrast to the situation in mammals, chromosome fusions are not associated with a Ku deficiency in Arabidopsis. These findings imply that Ku may play a different role in capping plant and animal telomeres.

Null mutation of AtCUL1 causes arrest in early embryogenesis in Arabidopsis

The SCF (for SKP1, Cullin/CDC53, F-box protein) ubiquitin ligase targets a number of cell cycle regulators, transcription factors, and other proteins for degradation in yeast and mammalian cells. Recent genetic studies demonstrate that plant F-box proteins are involved in auxin responses, jasmonate signaling, flower morphogenesis, photocontrol of circadian clocks, and leaf senescence, implying a large spectrum of functions for the SCF pathway in plant development. Here, we present a molecular and functional characterization of plant cullins. The Arabidopsis genome contains 11 cullin-related genes. Complementation assays revealed that AtCUL1 but not AtCUL4 can functionally complement the yeast cdc53 mutant. Arabidopsis mutants containing transfer DNA (T-DNA) insertions in the AtCUL1 gene were shown to display an arrest in early embryogenesis. Consistently, both the transcript and the protein of the AtCUL1 gene were found to accumulate in embryos. The AtCUL1 protein localized mainly in the nucleus but also weakly in the cytoplasm during interphase and colocalized with the mitotic spindle in metaphase. Our results demonstrate a critical role for the SCF ubiquitin ligase in Arabidopsis embryogenesis.

A replication-competent chimera of plant and animal viruses

Human, animal, fungal, and plant viruses encode papain-like proteinases that function in polyprotein processing, RNA synthesis, and virus-host interactions. To compare the functional profiles of diverse papain-like proteinases, we replaced a proteinase gene of the beet yellows virus (BYV) with those derived from equine arteritis virus (EAV), foot-and-mouth disease virus (FMDV), and the fungal virus CHV1. We found that, although each of the foreign proteinases efficiently processed the viral polyprotein, only the EAV proteinase supported vigorous replication of the chimeric BYV in plant protoplasts. This result demonstrated that the proteinases of BYV and EAV, but not FMDV or CHV1, provide a function that is critical for genome replication and that is separable from polyprotein processing. Further characterization of the BYV-EAV chimera revealed that BYV proteinase is also required for virus invasion and cell-to-cell movement. Thus, the same viral protein can combine both replication-related functions shared by plant and animal viruses and specialized functions in virus-host interactions.

MAP kinase signalling cascade in Arabidopsis innate immunity

There is remarkable conservation in the recognition of pathogen-associated molecular patterns (PAMPs) by innate immune responses of plants, insects and mammals. We developed an Arabidopsis thaliana leaf cell system based on the induction of early-defence gene transcription by flagellin, a highly conserved component of bacterial flagella that functions as a PAMP in plants and mammals. Here we identify a complete plant MAP kinase cascade (MEKK1, MKK4/MKK5 and MPK3/MPK6) and WRKY22/WRKY29 transcription factors that function downstream of the flagellin receptor FLS2, a leucine-rich-repeat (LRR) receptor kinase. Activation of this MAPK cascade confers resistance to both bacterial and fungal pathogens, suggesting that signalling events initiated by diverse pathogens converge into a conserved MAPK cascade.

Selenate-resistant mutants of Arabidopsis thaliana identify Sultr1;2, a sulfate transporter required for efficient transport of sulfate into roots

To investigate how plants acquire and assimilate sulfur from their environment, we isolated and characterized two mutants of Arabidopsis thaliana deficient in sulfate transport. The mutants are resistant to selenate, a toxic analogue of sulfate. They are allelic to each other and to the previously isolated sel1 (selenate-resistant) mutants, and have been designated sel1-8 and sel1-9. Root elongation in these mutants is less sensitive to selenate than in wild-type plants. Sulfate uptake into the roots is impaired in the mutants under both sulfur-sufficient and sulfur-deficient conditions, but transport of sulfate to the shoot is not affected. The sel1 mutants contain lesions in the sulfate transporter gene Sultr1;2 located on the lower arm of chromosome 1. The sel1-1, sel1-3 and sel1-8 mutants contain point mutations in the coding sequences of Sultr1;2, while the sel1-9 mutant has a T-DNA insertion in the Sultr1;2 promoter. The Sultr1;2 cDNA derived from wild-type plants is able to complement Saccharomyces cerevisiae mutants defective in sulfate transport, but the Sultr1;2 cDNA from sel1-8 is not. The Sultr1;2 gene is expressed mainly in roots, and accumulation of transcripts increases during sulfate deprivation. Examination of transgenic plants containing the Sultr1;2 promoter fused to the GUS-reporter gene indicates that Sultr1;2 is expressed mainly in the root cortex, the root tip and lateral roots. Weaker expression of the reporter gene was observed in hydathodes, guard cells and auxiliary buds of leaves, and in anthers and the basal parts of flowers. The results indicate that Sultr1;2 is primarily involved in importing sulfate from the environment into the root.

The Arabidopsis xylem peptidase XCP1 is a tracheary element vacuolar protein that may be a papain ortholog

XCP1 is a xylem-specific papain-like cysteine peptidase in Arabidopsis. To determine whether XCP1 could be involved in tracheary element autolysis, promoter activity and localization of XCP1 were investigated using XCP1 promoter-beta-glucuronidase fusions and immunofluorescence confocal microscopy. A tracheary element expression pattern was detected for XCP1. Results from confocal microscopy and biochemical subcellular fractionation indicated that XCP1 was localized in the vacuole. Ectopic expression of XCP1 resulted in a reduction in plant size in some lines and early leaf senescence, as indicated by early loss of leaf chlorophyll. Reduced plant size was correlated with higher levels of XCP1, as shown by immunoblot and peptidase activity gel analyses. The XCP1 prodomain exhibits exceptionally high similarity (greater than 80%) to the prodomains of papain and other papain-like enzymes isolated from papaya (Carica papaya) laticifers when compared with all other reported papain-like enzymes. The potential for XCP1 and papain to perform common functions as catalysts of autolytic processing following cell death due to programmed suicide or to wounding is discussed.

Functional specialization and evolution of leader proteinases in the family Closteroviridae

Members of the Closteroviridae and Potyviridae families of the plant positive-strand RNA viruses encode one or two papain-like leader proteinases. In addition to a C-terminal proteolytic domain, each of these proteinases possesses a nonproteolytic N-terminal domain. We compared functions of the several leader proteinases using a gene swapping approach. The leader proteinase (L-Pro) of Beet yellows virus (BYV; a closterovirus) was replaced with L1 or L2 proteinases of Citrus tristeza virus (CTV; another closterovirus), P-Pro proteinase of Lettuce infectious yellows virus (LIYV; a crinivirus), and HC-Pro proteinase of Tobacco etch virus (a potyvirus). Each foreign proteinase efficiently processed the chimeric BYV polyprotein in vitro. However, only L1 and P-Pro, not L2 and HC-Pro, were able to rescue the amplification of the chimeric BYV variants. The combined expression of L1 and L2 resulted in an increased RNA accumulation compared to that of the parental BYV. Remarkably, this L1-L2 chimera exhibited reduced invasiveness and inability to move from cell to cell. Similar analyses of the BYV hybrids, in which only the papain-like domain of L-Pro was replaced with those derived from L1, L2, P-Pro, and HC-Pro, also revealed functional specialization of these domains. In subcellular-localization experiments, distinct patterns were observed for the leader proteinases of BYV, CTV, and LIYV. Taken together, these results demonstrated that, in addition to a common proteolytic activity, the leader proteinases of closteroviruses possess specialized functions in virus RNA amplification, virus invasion, and cell-to-cell movement. The phylogenetic analysis suggested that functionally distinct L1 and L2 of CTV originated by a gene duplication event.

Two-component circuitry in Arabidopsis cytokinin signal transduction

Cytokinins are essential plant hormones that are involved in shoot meristem and leaf formation, cell division, chloroplast biogenesis and senescence. Although hybrid histidine protein kinases have been implicated in cytokinin perception in Arabidopsis, the action of histidine protein kinase receptors and the downstream signalling pathway has not been elucidated to date. Here we identify a eukaryotic two-component signalling circuit that initiates cytokinin signalling through distinct hybrid histidine protein kinase activities at the plasma membrane. Histidine phosphotransmitters act as signalling shuttles between the cytoplasm and nucleus in a cytokinin-dependent manner. The short signalling circuit reaches the nuclear target genes by enabling nuclear response regulators ARR1, ARR2 and ARR10 as transcription activators. The cytokinin-inducible ARR4, ARR5, ARR6 and ARR7 genes encode transcription repressors that mediate a negative feedback loop in cytokinin signalling. Ectopic expression in transgenic Arabidopsis of ARR2, the rate-limiting factor in the response to cytokinin, is sufficient to mimic cytokinin in promoting shoot meristem proliferation and leaf differentiation, and in delaying leaf senescence.

A nuclear gene encoding mitochondrial Delta-pyrroline-5-carboxylate dehydrogenase and its potential role in protection from proline toxicity

Delta1-pyrroline-5-carboxylate (P5C), an intermediate in biosynthesis and degradation of proline (Pro), is assumed to play a role in cell death in plants and animals. Toxicity of external Pro and P5C supply to Arabidopsis suggested that P5C dehydrogenase (P5CDH; EC 1.2.1.12) plays a crucial role in this process by degrading the toxic Pro catabolism intermediate P5C. Also in a Deltaput2 yeast mutant, lacking P5CDH, Pro led to growth inhibition and formation of reactive oxygen species (ROS). Complementation of the Deltaput2 mutant allowed identification of the Arabidopsis P5CDH gene. AtP5CDH is a single-copy gene and the encoded protein was localized to the mitochondria. High homology of AtP5CDH to LuFIS1, an mRNA up-regulated during susceptible pathogen attack in flax, suggested a role for P5CDH in inhibition of hypersensitive reactions. An Arabidopsis mutant (cpr5) displaying a constitutive pathogen response was found to be hypersensitive to external Pro. In agreement with a role in prevention of cell death, AtP5CDH was expressed at a basal level in all tissues analysed. The highest expression was found in flowers that are known to contain the highest Pro levels under normal conditions. External supply of Pro induced AtP5CDH expression, but much more slowly than Pro dehydrogenase (AtProDH) expression. Uncoupled induction of the AtProDH and AtP5CDH genes further supports the hypothesis that P5C levels have to be tightly controlled. These results indicate that, in addition to the well-studied functions of Pro, for example in osmoregulation, the Pro metabolism intermediate P5C also serves as a regulator of cellular stress responses.

A fatty acid desaturase modulates the activation of defense signaling pathways in plants

Salicylic acid (SA) plays an important role in activating various plant defense responses, including expression of the pathogenesis-related (PR) genes and systemic acquired resistance. A critical positive regulator of the SA signaling pathway in Arabidopsis is encoded by the NPR1 gene. However, there is growing evidence that NPR1-independent pathways can also activate PR expression and disease resistance. To elucidate the components associated with NPR1-independent defense signaling, we isolated a suppressor of the npr1-5 allele, designated ssi2. The recessive ssi2 mutation confers constitutive PR gene expression, spontaneous lesion formation, and enhanced resistance to Peronospora parasitica. In contrast, a subset of defense responses regulated by the jasmonic acid (JA) signaling pathway, including expression of the defensin gene PDF1.2 and resistance to Botrytis cinerea, is impaired in ssi2 plants. With the use of a map-based approach, the SSI2 gene was cloned and shown to encode a stearoyl-ACP desaturase (S-ACP DES). S-ACP DES is an archetypical member of a family of soluble fatty acid (FA) desaturases; these enzymes play an important role in regulating the overall level of desaturated FAs in the cell. The activity of mutant S-ACP DES enzyme was reduced 10-fold, resulting in elevation of the 18:0 FA content in ssi2 plants. Because reduced S-ACP DES activity leads to the induction of certain defense responses and the inhibition of others, we propose that a FA-derived signal modulates crosstalk between different defense signaling pathways.

The biological functions of glutathione revisited in arabidopsis transgenic plants with altered glutathione levels

A functional analysis of the role of glutathione in protecting plants from environmental stress was undertaken by studying Arabidopsis that had been genetically modified to have altered glutathione levels. The steady-state glutathione concentration in Arabidopsis plants was modified by expressing the cDNA for gamma-glutamyl-cysteine synthetase (GSH1) in both the sense and antisense orientation. The resulting plants had glutathione levels that ranged between 3% and 200% of the level in wild-type plants. Arabidopsis plants with low glutathione levels were hypersensitive to Cd due to the limited capacity of these plants to make phytochelatins. Plants with the lowest levels of reduced glutathione (10% of wild type) were sensitive to as little as 5 microM Cd, whereas those with 50% wild-type levels required higher Cd concentrations to inhibit growth. Elevating glutathione levels did not increase metal resistance. It is interesting that the plants with low glutathione levels were also less able to accumulate anthocyanins supporting a role for glutathione S-transferases for anthocyanin formation or for the vacuolar localization and therefore accumulation of these compounds. Plants with less than 5% of wild-type glutathione levels were smaller and more sensitive to environmental stress but otherwise grew normally.

Plant transformation technology. Developments and applications

Plant transformation has its roots in the research on Agrobacterium that was being undertaken in the early 1980s. The last two decades have seen significant developments in plant transformation technology, such that a large number of transgenic crop plants have now been released for commercial production. Advances in the technology have been due to development of a range of Agrobacterium-mediated and direct DNA delivery techniques, along with appropriate tissue culture techniques for regenerating whole plants from plant cells or tissues in a large number of species. In addition, parallel developments in molecular biology have greatly extended the range of investigations to which plant transformation technology can be applied. Research in plant transformation is concentrating now not so much on the introduction of DNA into plant cells, but rather more on the problems associated with stable integration and reliable expression of the DNA once it has been integrated.