Inhibited polar auxin transport results in aberrant embryo development in Norway spruce

Current hypotheses concerning the role of polar auxin transport in embryo development are entirely based on studies of angiosperms, while little is known about how auxin regulates pattern formation in gymnosperms. In this study, different developmental stages of somatic embryos of Norway spruce (Picea abies) were treated with the polar auxin transport inhibitor 1-N-naphtylphthalamic acid (NPA). Effects of the treatments on auxin content, embryo differentiation and programmed cell death (PCD) were analysed. During early embryo development, NPA-treatment led to increased indole-3-acetic acid (IAA) content, abnormal cell divisions and decreased PCD, resulting in aberrant development of embryonal tube cells and suspensors. Mature embryos that had been treated with NPA showed both apical and basal abnormalities. Typically the embryos had abnormal cotyledon formation and irregular cell divisions in the area of the root meristem. Our results show that polar auxin transport is essential for the correct patterning of both apical and basal parts of conifer embryos throughout the whole developmental process. Furthermore, the aberrant morhologies of NPA-treated spruce embryos are comparable with several auxin response and transport mutants in Arabidopsis. This suggests that the role of polar auxin transport is conserved between angiosperms and gymnosperms.