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Variation in Transcript Abundance During Somatic Embryogenesis in Gymnosperms

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Variation in Transcript Abundance During Somatic Embryogenesis in Gymnosperms Tree Physiology 24, 1073–1085 © 2004 Heron Publishing—Victoria, Canada Variation in transcript abundance during somatic embryogenesis in gymnosperms CLAUDIO STASOLLA,1,2 PETER V. BOZHKOV,3 TZU-MING CHU,4 LEONEL VAN ZYL,5 ULRIKA EGERTSDOTTER,6 MARIA F. SUAREZ,3 DEBORAH CRAIG,5 RUSS D. WOLFINGER,4 SARA VON ARNOLD3 and RONALD R. SEDEROFF5 1 Department of Plant Science, University of Manitoba, Winnipeg, MB R3T 2N2, Canada 2 Corresponding author ([email protected]) 3 Department of Plant Biology and Forest Genetics, Uppsala Genetic Center, Swedish University of Agricultural Sciences, Box 7080, S-75007, 3 Uppsala, Sweden 4 SAS Institute Inc., Cary, NC 27513, USA 5 Forest Biotechnology Group, Department of Forestry, North Carolina State University, Raleigh, NC 27695-7247, USA 6 Institute of Paper Science and Technology, 500 10th Street NW, Atlanta, GA 30318, USA Received January 5, 2004; accepted March 13, 2004; published online August 2, 2004 Summary Somatic embryogenesis of Norway spruce (Picea Introduction abies L.) is a versatile model system to study molecular mecha- Plant embryogenesis begins with the division of the fertilized nisms regulating embryo development because it proceeds egg or zygote and culminates with the generation of a mature through defined developmental stages corresponding to spe- embryo, comprising an embryonic axis with shoot and root cific culture treatments. Normal embryonic development in- poles and cotyledon(s). In gymnosperms, the overall embryo volves early differentiation of proembryogenic masses (PEMs) development pathway can be divided in three distinct phases: into somatic embryos, followed by early and late embryogeny proembryogeny, which includes stages before the elongation leading to the formation of mature cotyledonary embryos. In of a suspensor; early embryogeny, which initiates with the some cell lines there is a developmental arrest at the PEM–so- elongation of the suspensor and terminates with the appear- matic embryo transition. To learn more about the molecular ance of the root meristem; and late embryogeny, which culmi- mechanisms regulating embryogenesis, we compared the tran- nates with the maturation of the embryo (Singh 1978). Despite script profiles of two normal lines and one developmentally ar- extensive knowledge on the physiological and molecular rested line. Ribonucleic acid, extracted from these cell lines at mechanisms regulating embryo maturation in angiosperms successive developmental stages, was analyzed on DNA (Kermode 1990, Girke et al. 2000, Ruuska et al. 2002), little is microarrays containing 2178 expressed sequence tags (ESTs) known about the mechanisms governing the early stages of (corresponding to 2110 unique cDNAs) from loblolly pine embryogenesis. In angiosperms, developmental arrest or aber- (Pinus taeda L.). Hybridization between spruce and pine spe- rations at the later stages of embryonic and post-embryonic de- cies on microarrays has been shown to be effective (van Zyl et velopment are typical consequences of disturbed regulation al. 2002, Stasolla et al. 2003). In contrast to the developmen- during the early stages of embryo development (Dunn et al. tally arrested line, the early phases of normal embryo develop- 1997, Scanlon et al. 1997, Hamann et al. 1999, Heckel et al. ment are characterized by a precise pattern of gene expression, 1999). The development of mutant analysis in Arabidopsis has i.e., repression followed by induction. Comparison of tran- identified several regulatory genes responsible for normal de- script levels between successive stages of embryogenesis al- velopment of the suspensor (Vernon and Meinke 1994, lowed us to identify several genes that showed unique expres- Yadegari et al. 1994, Zhang and Sommerville 1997, Rojo et al. sion responses during normal development. Several of these 2001) and embryo proper (reviewed by Jurgens 2001). In gym- genes encode proteins involved in detoxification processes, nosperms, long generation times make the selection of em- bryo-specific mutants practically impossible. methionine synthesis and utilization, and carbohydrate metab- Somatic embryogenesis, the process in which embryos, olism. The potential role of these genes in embryo develop- similar in morphology to their zygotic counterparts, are in- ment is discussed. duced to develop in culture from somatic cells, represents a suitable model system for investigating factors affecting em- Keywords: embryo development, gene expression, hybridiza- bryo growth. Through this process, a large number of embryos tion, microarray, Picea abies, Pinus taeda. at defined stages of development can easily be obtained. In 1074 STASOLLA ET AL. gymnosperms, the developmental pathway of somatic em- block at the PEM–somatic embryo transition. The employ- bryogenesis has been described for Norway spruce, consisting ment of a pine cDNA array for studies on gene expression in of a sequence of defined developmental stages corresponding spruce has been documented previously (van Zyl et al. 2002) to three specific treatments (Filonova et al. 2000a) (Fig- and implemented (Stasolla et al. 2003). The major objective of ure 1A). Maintenance of the embryogenic potential through this work was to identify genes involved in physiological re- proliferation of proembryogenic masses (PEMs) occurs in the sponses, which are required for the normal progression of em- presence of the exogenously supplied plant growth regulators bryo development. (PGR) auxin and cytokinin. Removal of these PGR from the medium reduces cell proliferation in PEMs and initiates early embryogeny through trans-differentiation of PEMs into so- Materials and methods matic embryos. From this developmental stage on, the path- way of somatic embryogenesis is similar to zygotic embryo Plant material development. Late embryogeny requires abscisic acid (ABA) Three lines of Norway spruce (Picea abies L. Karst.) were uti- and involves establishment of root and shoot meristems and lized: two normal lines, 61.21 (line N1) and 88.17 (line N2), differentiation of cotyledons (Figure 1A). A critical step dur- both of which passed through the PEM–somatic embryo tran- ing the overall embryogenic process appears to be the trans- sition, and one developmentally arrested line, 88.1 (line B), differentiation of PEMs into somatic embryos, requiring the where this transition was precluded (Figure 1A). All cell lines execution of several physiological events, including program- were stored in liquid nitrogen and thawed 5 months before med cell death (Filonova et al. 2000b, Bozhkov et al. 2002, starting the experiment. Maintenance of PEMs and initiation Smertenko et al. 2003). Normal passage of PEMs to somatic of embryo development were performed as previously re- embryos is precluded in some lines, resulting in a develop- ported (Bozhkov et al. 2002). Briefly, proliferation of PEMs mental arrest at the PEM stage (Filonova et al. 2000a, Smer- was stimulated using half-strength LP medium (modified after tenko et al. 2003). Preliminary studies on the molecular mech- Bozhkov and von Arnold 1998) supplemented with PGR anisms regulating the early phases of somatic embryogenesis auxin (9.0 µM 2,4-dichlorophenoxyacetic acid (2,4-D)) and have revealed several genes with differentially regulated ex- cytokinin (4.4 µM N6-benzyladenine (BA)). For maintenance pression between embryogenic and non-embryogenic tissue of embryogenic potential, PEMs were sub-cultured weekly (Bishop-Hurley et al. 2003, van Zyl et al. 2003). As an exten- into fresh PGR-containing medium. Trans-differentiation of sion of this work and to contribute to the understanding of the PEMs into somatic embryos was accomplished through two molecular events associated with gymnosperm embryo- successive washings of 3-ml settled cell aggregates with 10-ml genesis, this study compares the steady-state transcript levels samples of half-strength LP medium devoid of PGR, followed of more than 2000 cDNAs from loblolly pine in three cell lines by inoculation with 3 ml of washed cells into 47 ml of the same of Norway spruce. Of these lines, two were able to form so- PGR-free medium for 1 week. Continuation of embryo devel- matic embryos, whereas the third line had a developmental opment, i.e., generation of cotyledonary somatic embryos, Figure 1. (A) Developmental pathway of somatic embryo- genesis in Norway spruce (modified from Filonova et al. 2000a). In the two normal N1 and N2 lines, proembryogenic masses (PEMs) can success- fully differentiate into early somatic embryos (ESEs) upon removal of plant growth regu- lators (–PGR), and mature into somatic embryos (SE) and cot- yledonary embryos in the pres- ence of abscisic acid (ABA). Trans-differentiation of PEMs into ESEs is precluded in the developmentally blocked B line. Tissue was collected for gene expression analysis at six stages of embryo development. (B) Percentage composition of PEMs (solid bars) and ESEs + SEs (open bars) in the three cell lines during the first five stages of development. TREE PHYSIOLOGY VOLUME 24, 2004 EMBRYOGENESIS REGULATION IN GYMNOSPERMS 1075 was carried out by plating 1 ml of settled cell aggregates onto procedure developed by DeRisi (http://cmgm.stanford.edu/ Whatman No. 2 filter paper placed on solidified BMI-S1 mat- pbrown/protocols/index.html) was followed to label cDNA uration medium containing 30 µM ABA. Cotyledonary em- probes. The RNA from each sample was labeled with both Cy3 bryos were harvested after 35 days in culture. Sampling of and Cy5 dyes and used for reciprocal hybridizations.
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