The Effects of Polyethylene Glycol on Gene Expression of Developing White Spruce Somatic Embryos

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Abstract
Somatic embryogenic cultures of white spruce (Picea glauca) represent a valuable system to study molecular mechanisms regulating embryo development because many embryos of defined developmental stages can be generated. The inclusion of polyethylene glycol (PEG) in the maturation medium can improve the number and quality of embryos produced. To learn more about the mechanism of action of PEG, we analyzed transcript profiles of stage-specific embryos matured without (control) or with (PEG treated) PEG. RNA extracted from maturing spruce embryos was analyzed on DNA microarrays containing 2,178 cDNAs from loblolly pine (Pinus taeda). The efficiency of heterologous hybridization between spruce and pine species on microarrays has been documented previously (L. van Zyl, S. von Arnold, P. Bozhkov, Y. Chen, U. Egertsdotter, J. MacKay, R. Sederoff, J. Shen, L. Zelena, D. Clapham [2002] Comp Funct Genomics 3: 306–318). Several pine genes, including the apparent homologs to the Arabidopsis genes ZWILLE, FIDDLEHEAD, FUSCA, and SCARECROW, increased in expression after PEG treatments. These genes are known to be involved in the formation of the embryo body plan and in the control of the shoot and root apical meristems. The increased transcript levels of these genes in immature PEG-treated embryos suggest that PEG may improve the quality of spruce somatic embryos by promoting normal differentiation of the embryonic shoot and root. Changes in the transcript levels of many genes involved in sucrose catabolism and nitrogen assimilation and utilization were also observed between control and PEG-treated embryos.