Molecular regulation of somatic embryogenesis in potato: an auxin led perspective.

Potato internodal segments (INS) treated with the auxin 2,4-dichlorophenoxyacetic acid can be induced to develop somatic embryos upon their transfer to an auxin-free medium, while the continuous presence of auxin in the medium suppresses the progression of embryogenically-induced somatic cells to embryos. We have employed these contrasting pathways, in combination with potato microarrays representing circa 10,000 genes, to profile global gene expression patterns during the progression of somatic embryogenesis in potato. The induction phase, characterised by the presence of auxin, was analysed by the direct comparison of RNA isolated from freshly excised (0 days) and embryogenically induced (14 days) INS explants. RNAs from embryo-forming (withdrawal of auxin after 14 days) and embryo-inhibitory (continuous presence of auxin) conditions, isolated over a range of time-points until the emergence of somatic embryos, were compared in a loop design to identify auxin responsive genes putatively involved in the process of somatic embryogenesis. A total of 402 transcripts were found to be showing significant differential expression patterns during somatic embryogenesis 'induction' phase, 524 during 'embryo-transition' phase, while 44 transcripts were common to both phases. Functional classification of these transcripts, using Gene Ontology vocabularies (molecular and biological), revealed that a significant proportion of transcripts were involved in processes which are more relevant to somatic embryogenesis such as apoptosis, development, reproduction, stress and signal transduction. This is the first study profiling global gene expression patterns during true somatic embryogenesis initiated from mature and completely differentiated explants and has enabled the description of stage-specific expression patterns of a large number of genes during potato somatic embryogenesis (PSE). The significance of the key identified genes during critical stages of somatic embryogenesis is discussed.

Cloning and molecular characterisation of a potato SERK gene transcriptionally induced during initiation of somatic embryogenesis.

Somatic embryogenesis offers great potential in plant propagation, long-term germplasm conservation, and as a suitable model system for deciphering early events during embryogenesis. The up-regulation and ectopic expression of a SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) gene has been shown to mark and enhance embryogenic competence in somatic cells of model plant species. We have cloned and characterised a SERK gene (StSERK1) from potato (Solanum tuberosum L.), an important crop plant. Sequence analysis of StSERK1 revealed high levels of similarity to other plant SERKs, as well as a conserved intron/exon structure which is unique to members of the SERK family. Furthermore, StSERK clustered most closely with SERK gene family members such as MtSERK1, CuSERK1, AtSERK1, and DcSERK, implicated in evoking somatic embryogenesis. Monitoring of SERK expression during progression of potato somatic embryogenesis revealed increased StSERK expression during the induction phase. Subsequently, during the embryo transition phases, StSERK expression was unchanged and did not vary among embryo-forming and inhibitory conditions. However, in isolated somatic embryos StSERK expression was again up-regulated. In other plant parts (leaves, true potato seeds, microtubers and flower buds), StSERK showed different levels of expression. Expression analysis suggests that the isolated StSERK could be a functional SERK orthologue. The possible role of SERK as a marker of pluripotency, rather than embryogenesis alone, is discussed.

Genetic transformation of barley microspores using anther bombardment.

Bombardment of intact anthers of commercial barley (Hordeum vulgare) varieties resulted in 0.5-1.0% of transformed microspores of which 20-40% continued in androgenic development (0.2% of all bombarded microspores). Using a system based on bombardment of anthers is therefore likely to be more technically efficient than the use of a microspore isolation, transformation and regeneration system. Bombardment of anthers has a number of technical and scientific advantages over existing systems for gene transfer and can be considered as a alternative method to existing methods for genetic transformation in barley.

Stability of potato (Solanum tuberosum L.) plants regenerated via somatic embryos, axillary bud proliferated shoots, microtubers and true potato seeds: a comparative phenotypic, cytogenetic and molecular assessment.

The stability, both genetic and phenotypic, of potato (Solanum tuberosum L.) cultivar Desiree plants derived from alternative propagation methodologies has been compared. Plants obtained through three clonal propagation routes-axillary-bud-proliferation, microtuberisation and a novel somatic embryogenesis system, and through true potato seeds (TPS) produced by selfing were evaluated at three levels: gross phenotype and minituber yield, changes in ploidy (measured by flow cytometry) and by molecular marker analysis [measured using AFLP (amplified fragment length polymorphism)]. The clonally propagated plants exhibited no phenotypic variation while the TPS-derived plants showed obvious phenotypic segregation. Significant differences were observed with respect to minituber yield while average plant height, at the time of harvesting, was not significantly different among plants propagated through four different routes. None of the plant types varied with respect to gross genome constitution as assessed by flow cytometry. However, a very low level of AFLP marker profile variation was seen amongst the somatic embryo (3 out of 451 bands) and microtuber (2 out of 451 bands) derived plants. Intriguingly, only AFLP markers generated using methylation sensitive restriction enzymes were found to show polymorphism. No polymorphism was observed in plants regenerated through axillary-bud-proliferation. The low level of molecular variation observed could be significant on a genome-wide scale, and is discussed in the context of possible methylation changes occurring during the process of somatic embryogenesis.

Auxin pulse treatment holds the potential to enhance efficiency and practicability of somatic embryogenesis in potato.

The objective of the current study was to simplify existing somatic embryogenesis systems in potato (Solanum tuberosum L.) cv. Desiree. The project targeted the agar-based induction phase of the potato somatic embryogenesis process as the key area for improvement. Experiments were established to ascertain the effect of a 2,4-D (2,4 dichlorophenoxyacetic acid) pulse, applied to the primary internodal section explant source and its subsequent effect on embryo induction. Parameters tested were the duration of the auxin pulse in a range from 0 to 300 min, and the concentrations of 2,4-D applied, in a range from 0 to 5,120 microM. The mean number of somatic embryos formed per explant was recorded after 4 and 8 weeks culture. Our findings indicated that the somatic embryogenesis in potato internodal segments could be evoked by an auxin (2,4-D) pulse treatment over a wide concentration and duration range. The results further suggested that a simple 20 microM 2,4-D pulse treatment could replace a lengthy 2 week induction phase in potato somatic embryogenesis and thus improve the system's practicability for wider uptake.

Potato (Solanum tuberosum L.).

Potato (Solanum tuberosum L.) is a globally important crop plant producing high yields of nutritionally valuable food in the form of tubers. It has been the focus of substantial study because of its use both as a staple food crop and as a potentially significant source of compounds of interest. This has included the development and application of transgenic technology for introducing novel traits of fundamental and applied interest. This chapter describes a rapid, efficient, and cost-effective system for the routine transformation of this crop plant at rates above 40% efficiency, calculated as the mean number of Southern blot- confirmed independent transgenics per number of internodal explants originally plated. Internodal sections are co-cultivated with Agrobacterium tumefaciens and subjected to a two-stage callus induction/shoot outgrowth system under kanamycin selection. Shoot regeneration rates are high using the described method, and excised independent shoots rooting from the cut end of the stem after two further subcultures on kanamycin are 95% certain to be transformed. The transgenic status can be confirmed by molecular analysis and the plants grown on for tuber production enabling a wide spectrum of further studies.

Overexpression of a bacterial 1-deoxy-D-xylulose 5-phosphate synthase gene in potato tubers perturbs the isoprenoid metabolic network: implications for the control of the tuber life cycle.

Potato tubers were engineered to express a bacterial gene encoding 1-deoxy-D-xylulose 5-phosphate synthase (DXS) in order to investigate the effects of perturbation of isoprenoid biosynthesis. Twenty-four independent transgenic lines out of 38 generated produced tubers with significantly elongated shape that also exhibited an early tuber sprouting phenotype. Expression analysis of nine transgenic lines (four exhibiting the phenotype and five showing a wild-type phenotype) demonstrated that the phenotype was strongly associated with dxs expression. At harvest, apical bud growth had already commenced in dxs-expressing tubers whereas in control lines no bud growth was evident until dormancy was released after 56-70 d of storage. The initial phase of bud growth in dxs tubers was followed by a lag period of approximately 56 d, before further elongation of the developing sprouts could be detected. Thus dxs expression results in the separation of distinct phases in the dormancy and sprouting processes. In order to account for the sprouting phenotype, the levels of plastid-derived isoprenoid growth regulators were measured in transgenic and control tubers. The major difference measured was an increase in the level of trans-zeatin riboside in tubers at harvest expressing dxs. Additionally, compared with controls, in some dxs-expressing lines, tuber carotenoid content increased approximately 2-fold, with most of the increase accounted for by a 6-7-fold increase in phytoene.

Engineering ketocarotenoid biosynthesis in potato tubers.

Consumption of astaxanthin is increasingly associated with a range of health benefits. Attempts to engineer ketocarotenoid biosynthesis in plants have been successful although there are no reports of nutritionally significant levels of astaxanthin in plant storage organs. Thus, in this study, ketocarotenoid biosynthesis was engineered in potato tubers. Both Solanum tuberosum and Solanum phureja transgenic lines were produced that expressed an algal bkt1 gene, encoding a beta-ketolase, and accumulated ketocarotenoids. Two major ketocarotenoids were detected, ketolutein and astaxanthin. The level of unesterified astaxanthin reached ca. 14 microg g(-1) DW in some bkt1 expressing lines of S. phureja but was much lower in the S. tuberosum background. Co-transformation of S. tuberosum with crtB, encoding phytoene synthase, and the bkt1 gene was achieved in order to determine whether this would enhance the levels of S. tuberosum ketocarotenoid.

Moderation of morphogenetic and oxidative stress responses in flax in vitro cultures by hydroxynonenal and desferrioxamine.

Hypocotyl segments of 7-day old seedlings of flax (Linum usitatissimum L.) cultivars Atalante, Flanders, Jitka, Szegedi 30 and Super were screened for organogenesis (shoot and root induction) and embryo-like structure production. A non-destructive assay for hydroxyl radicals (*OH), utilising DMSO as a radical trap, was used to determine *OH formation during tissue culture and morphogenesis. Desferrioxamine, an inhibitor of Fenton reaction, and 4-hydroxy-2-nonenal, a cytotoxic Lipid peroxidation product, were exogenously applied to flax cultures to determine the effect of antioxidative and prooxidative status on morphogenetic responses induced through the exogenous application of plant growth regulators. Flax genotypes varied in their response to treatments after exposure to different plant hormones. Hydroxyl radical (*OH) formation correlated with morphogenetic responses and this was affected by plant hormones. Desferrioxamine and 4-hydroxy-2-nonenal also moderated morphogenetic responses and influenced hydroxyl radical formation during in vitro propagation.

Metabolic engineering of high carotenoid potato tubers containing enhanced levels of beta-carotene and lutein.

In order to enhance the carotenoid content of potato tubers, transgenic potato plants have been produced expressing an Erwinia uredovora crtB gene encoding phytoene synthase, specifically in the tuber of Solanum tuberosum L. cultivar Desiree which normally produces tubers containing c. 5.6 microg carotenoid g(-1) DW and also in Solanum phureja L. cv. Mayan Gold which has a tuber carotenoid content of typically 20 microg carotenoid g(-1) DW. In developing tubers of transgenic crtB Desiree lines, carotenoid levels reached 35 microg carotenoid g(-1) DW and the balance of carotenoids changed radically compared with controls: beta-carotene levels in the transgenic tubers reached c. 11 microg g(-1) DW, whereas control tubers contained negligible amounts and lutein accumulated to a level 19-fold higher than empty-vector transformed controls. The crtB gene was also transformed into S. phureja (cv. Mayan Gold), again resulting in an increase in total carotenoid content to 78 microg carotenoid g(-1) DW in the most affected transgenic line. In these tubers, the major carotenoids were violaxanthin, lutein, antheraxanthin, and beta-carotene. No increases in expression levels of the major carotenoid biosynthetic genes could be detected in the transgenic tubers, despite the large increase in carotenoid accumulation. Microarray analysis was used to identify a number of genes that were consistently up- or down-regulated in transgenic crtB tubers compared with empty vector controls. The implications of these data from a nutritional standpoint and for further modifications of tuber carotenoid content are discussed.

Somatic embryogenesis in Solanum tuberosum L.: a histological examination of key developmental stages.

A potential novel method of producing high-quality potato ( Solanum tuberosum L.) nuclear seeds is through the process of somatic embryogenesis (SE). Somatic embryo formation has been successfully reported in many plant species, but in potato, reliable SE systems are still at the experimental stage. A key factor in the success of any SE system is the ability to discriminate SE-specific cellular structures from those emerging through an organogenic route. In the investigation reported here we attempted to discriminate the progression of specific stages of potato SE by histological means. Internodal segment (INS) explants from 4- to 6-week-old cv. Desiree in vitro cultures were successively cultured on SE induction (for 2 weeks) and expression/regeneration media (for 3 weeks) with and without 2,4-dichlorophenoxyacetic acid (5 microM). Microscopic examination of histological slides prepared using INS explants at different stages revealed the presence of characteristic globular, heart and torpedo stages in the potato SE system along with other associated unique features such as protoderm development and discrete vascular connections. These results confirm the occurrence of potato SE as per the accepted definition of the term.

Potato virus X-induced gene silencing in leaves and tubers of potato.

Virus induced gene silencing (VIGS) is increasingly used to generate transient loss-of-function assays and has potential as a powerful reverse-genetics tool in functional genomic programs as a more rapid alternative to stable transformation. A previously described potato virus X (PVX) VIGS vector has been shown to trigger silencing in the permissive host Nicotiana benthamiana. This paper demonstrates that a PVX-based VIGS vector is also effective in triggering a VIGS response in both diploid and cultivated tetraploid Solanum species. We show that systemic silencing of a phytoene desaturase gene is observed and maintained throughout the foliar tissues of potato plants and was also observed in tubers. Here we report that VIGS can be triggered and sustained on in vitro micropropagated tetraploid potato for several cycles and on in vitro generated microtubers. This approach will facilitate large-scale functional analysis of potato expressed sequence tags and provide a noninvasive reverse-genetic approach to study mechanisms involved in tuber and microtuber development.