Improved efficiency of the walnut somatic embryo gene transfer system

A red fluorescent protein (DsRED) from Discosoma sp. as a reporter for gene expression in walnut somatic embryos

KEY MESSAGE

An improved scorable marker was developed for somatic embryo transformation. This method is more reliable than GFP and provides more efficient embryo selection than β-glucuronidase assays (GUS, MUG). Reporter genes are widely used to select transformed cells and tissues. Fluorescent proteins have become an integral part of live-cell imaging research over the past 10 years. DsRED is an ideal reporter for avoiding plant chlorophyll autofluorescence and for double labeling in combination with other scorable markers. In this study, we transformed walnut somatic embryos with a construct containing the DsRED-expressing binary vector pKGW-RR to assess the effect of this red fluorescent protein visual reporter on both embryos and regenerated plants. Results showed that DsRED expression was apparent with maximum brightness at 7-10 days after initiation. Fourteen of twenty-four surviving somatic embryos were bright red. These E0 embryos generated 25 wholly fluorescent E1 embryos and 43 wholly fluorescent E2 embryos at 2 weeks intervals. The germination percentage of DsRED-positive embryos was greater than 80% and gave rise to 45 fluorescent transgenic walnut plants. The regenerated transgenic plants expressed DsRED in all tissues examined including transverse sections of vegetative organs. The percentage of transformed plants that developed roots (48.3%) was similar to control shoots (53%). For transformation of walnut somatic embryos, the DsRED-based reporter system is more stable and reliable than green fluorescent protein (GFP) and, since it is a directly read and non-destructive assay, it provides a more efficient means of monitoring transformation than β-glucuronidase (GUS).

Walnut (Juglans)

Walnut species are important nut and timber producers in temperate regions of Europe, Asia, South America, and North America. Trees can be impacted by Phytophthora, crown gall, nematodes, Armillaria, and cherry leaf roll virus; nuts can be severely damaged by codling moth, husk fly, and Xanthomonas blight. The long generation time of walnuts and an absence of identified natural resistance for most of these problems suggest biotechnological approaches to crop improvement. Described here is a somatic embryo-based transformation protocol that has been used to successfully insert horticulturally useful traits into walnut. Selection is based on the combined use of the selectable neomycin phosphotransferase (nptII) gene and the scorable uidA gene. Transformed embryos can be germinated or micropropagated and rooted for plant production. The method described has been used to establish field trials of mature trees.

Novel roles for the polyphenol oxidase enzyme in secondary metabolism and the regulation of cell death in walnut

The enzyme polyphenol oxidase (PPO) catalyzes the oxidation of phenolic compounds into highly reactive quinones. Polymerization of PPO-derived quinones causes the postharvest browning of cut or bruised fruit, but the native physiological functions of PPOs in undamaged, intact plant cells are not well understood. Walnut (Juglans regia) produces a rich array of phenolic compounds and possesses a single PPO enzyme, rendering it an ideal model to study PPO. We generated a series of PPO-silenced transgenic walnut lines that display less than 5% of wild-type PPO activity. Strikingly, the PPO-silenced plants developed spontaneous necrotic lesions on their leaves in the absence of pathogen challenge (i.e. a lesion mimic phenotype). To gain a clearer perspective on the potential functions of PPO and its possible connection to cell death, we compared the leaf transcriptomes and metabolomes of wild-type and PPO-silenced plants. Silencing of PPO caused major alterations in the metabolism of phenolic compounds and their derivatives (e.g. coumaric acid and catechin) and in the expression of phenylpropanoid pathway genes. Several observed metabolic changes point to a direct role for PPO in the metabolism of tyrosine and in the biosynthesis of the hydroxycoumarin esculetin in vivo. In addition, PPO-silenced plants displayed massive (9-fold) increases in the tyrosine-derived metabolite tyramine, whose exogenous application elicits cell death in walnut and several other plant species. Overall, these results suggest that PPO plays a novel and fundamental role in secondary metabolism and acts as an indirect regulator of cell death in walnut.

Influence of abscisic acid and sucrose on somatic embryogenesis in Cactus Copiapoa tenuissima Ritt. forma mostruosa

Having produced the embryos of cactus Copiapoa tenuissima Ritt. forma monstruosa at the globular stage and callus, we investigated the effect of abscisic acid (ABA) in the following concentrations: 0, 0.1, 1, 10, and 100  μ M on successive stages of direct (DSE) and indirect somatic embryogenesis (ISE). In the indirect somatic embryogenesis process we also investigated a combined effect of ABA (0, 0.1, 1  μ M) and sucrose (1, 3, 5%). The results showed that a low concentration of ABA (0-1  μ M) stimulates the elongation of embryos at the globular stage and the number of correct embryos in direct somatic embryogenesis, while a high ABA concentration (10-100  μ M) results in growth inhibition and turgor pressure loss of somatic embryos. The indirect somatic embryogenesis study in this cactus suggests that lower ABA concentrations enhance the increase in calli fresh weight, while a high concentration of 10  μ M ABA or more changes calli color and decreases its proliferation rate. However, in the case of indirect somatic embryogenesis, ABA had no effect on the number of somatic embryos and their maturation. Nevertheless, we found a positive effect of sucrose concentration for both the number of somatic embryos and the increase in calli fresh weight.

[Differentially expressed genes during Larix somatic embryomaturation and the expression profile of partial genes]

To study the molecular mechanism of Larix somatic embryogenesis, a differentially expressed cDNA library of Larix somatic embryo in the period of maturation was constructed using suppression subtractive hybridization (SSH). The cDNA from the cultures at the stage of somatic embryo maturation of embryogenic cell line Y35 of L. leptolepis xL. principis-rupprechtii was used as the tester and the cDNA from its subcultured callus was used as the driver. Eight hundreds randomly selected positive clones were sequenced, and 468 UniGenes were obtained finally. According to their function, these ESTs were classified into 19 categories and were involved in many biological process related to plant growth and development such as metabolism, transcription, signal transduction, transport facilitation, cell growth and division, cell structure, cell fate, protein synthesis or degradation, defense etc. Real-time PCR results of several ESTs showed that they were all differentially expressed at the different stages during cell line Y35 somatic embryo maturation.

In vitro regeneration and Agrobacterium-mediated genetic transformation of Euonymus alatus

An in vitro plant regeneration method and an Agrobacterium tumefaciens-mediated genetic transformation protocol were developed for Euonymus alatus. More than 60% of cotyledon and 70% of hypocotyl sections from 10-day-old seedlings of E. alatus produced 2-4 shoots on woody plant medium (WPM) supplemented with 5.0 mg/l 6-benzylaminopurine (BA) plus 0.2 mg/l alpha-naphthalene acetic acid (NAA), and 77% of shoots produced roots on WPM medium with 0.3 mg/l NAA and 0.5 mg/l Indole-3-butyricacid (IBA). On infection with Agrobacterium tumefaciens strain EHA105 harboring a gusplus gene that contained a plant recognizable intron from the castor bean catalase gene to ensure plant-specific beta-glucuronidase (GUS) expression, 16% of cotyledon and 15% of hypocotyl explants produced transgenic shoots using kanamycin as a selection agent, and 67% of these shoots rooted. Stable insertion of T-DNA into the host genome was determined with organ- and tissue-specific expression of the gusplus gene and further confirmed with a PCR-based molecular analysis.

A chimeric gene encoding the methionine-rich 2S albumin of the Brazil nut (Bertholletia excelsa H.B.K.) is stably expressed and inherited in transgenic grain legumes

The coding region of the 2S albumin gene of Brazil nut (Bertholletia excelsa H.B.K.) was completely synthesized, placed under control of the cauliflower mosaic virus (CaMV) 35S promoter and inserted into the binary vector plasmid pGSGLUC1, thus giving rise to pGSGLUC1-2S. This was used for transformation of tobacco (Nicotiana tabacum L. cv. Petit Havanna) and of the grain legume Vicia narbonensis L., mediated by the supervirulent Agrobacterium tumefaciens strain EHA 101. Putative transformants were selected by screening for neomycin phosphotransferase (NPT II) and beta-glucuronidase (GUS) activities. Transgenic plants were grown until flowering and fruiting occurred. The presence of the foreign gene was confirmed by Southern analysis. GUS activity was found in all organs of the regenerated transgenic tobacco and legume plants, including the seeds. In the legume, the highest expression levels of the CaMV 35S promoter-controlled 2S albumin gene were observed in leaves and roots. 2S albumin was localized in the vacuoles of leaf mesophyll cells of transgenic tobacco. The Brazil nut protein was present in the 2S fraction after gel filtration chromatography of the legume seed proteins and could be clearly identified by immunoblotting. Analysis of seeds from the R2 progenies of the legume and of transgenic tobacco plants revealed Mendelian inheritance of the foreign gene. Agrobacterium rhizogenes strain RifR 15834 harbouring the binary vector pGSGLUC1-2S was also used to transform Pisum sativum L. and Vicia faba L. Hairy roots expressed the 2S albumin-specific gene. Several shoots were raised but they never completely rooted and no fertile plants were obtained from these transformants.

Repetitive somatic embryogenesis and plant recovery in white clover

Breeding and selection was used to generate a population of white clover (Trifolium repens L.) from cultivar Osceola with a high embryogenic capacity. Somatic embryos were obtained from immature cotyledons of white clover placed onto EC6 basal medium containing 40 mg L(-1) of 2,4-D and 6% sucrose. The effects of 2,4-D at 20 and 40 mg L(-1) and of the carbohydrates, sucrose and maltose, were evaluated for their influence in the establishment of repetitive somatic embryogenesis. To determine the optimal protocol for plant recovery from somatic embryos, the effects of MS vs. EC6 basal salts, sucrose vs. maltose, B5 vitamins vs. yeast extract, and inclusion or exclusion of activated charcoal were evaluated. Repeated subculture of white clover somatic embryos on EC6 basal medium containing 6% sucrose with 2,4-D at 20 or 40 mg L(-1) effectively maintains repetitive embryogenesis. Medium containing MS salts with 6% maltose as the carbohydrate source was the most efficient for plant recovery.