Origin, timing, and gene expression profile of adventitious rooting in Arabidopsis hypocotyls and stems.

PREMISE OF THE STUDY: Adventitious root (AR) formation is indispensable for vegetative propagation, but difficult to achieve in many crops. Understanding its molecular mechanisms is thus important for such species. Here we aimed at developing a rooting protocol for direct AR formation in stems, locating cellular AR origins in stems and exploring molecular differences underlying adventitious rooting in hypocotyls and stems. METHODS: In-vitro-grown hypocotyls or stems of wild-type and transgenic ecotype Columbia (Col-0) of Arabidopsis thaliana were rooted on rooting media. Anatomy of AR formation, qRT-PCR of some rooting-related genes and in situ GUS expression were carried out during rooting from hypocotyls and stems. KEY RESULTS: We developed a rooting protocol for AR formation in stems and traced back root origins in stems by anatomical and in situ expression studies. Unlike rooting in hypocotyls, rooting in stems was slower, and AR origins were mainly from lateral parenchyma of vascular bundles and neighboring starch sheath cells as well as, to a lesser extent, from phloem cap and xylem parenchyma. Transcript levels of GH3-3, LBD16, LBD29, and LRP1 in hypocotyls and stems were similar, but transcript accumulation was delayed in stems. In situ expression signals of DR5::GUS, LBD16::GUS, LBD29::GUS, and rolB::GUS reporters in stems mainly occurred at the root initiation sites, suggesting their involvement in AR formation. CONCLUSIONS: We have developed an efficient rooting protocol using half-strength Lepoivre medium for studying AR formation in stems, traced back the cellular AR origins in stems, and correlated expression of rooting-related genes with root initiation sites.

Effects of transgenic rootstocks on growth and development of non-transgenic scion cultivars in apple.

Although cultivation of genetic modified (GM) annual crops has been steadily increasing in the recent 10 years, the commercial cultivation of GM fruit tree is still very limited and reports of field trials on GM fruit trees are rare. This is probably because development and evaluation of GM fruit trees require a long period of time due to long life cycles of trees. In this study, we report results from a field trial on three rolB transgenic dwarfing apple rootstocks of M26 and M9 together with non-transgenic controls grafted with five non-transgenic scion cultivars. We intended to investigate the effects of transgenic rootstock on non-transgenic scion cultivars under natural conditions as well as to evaluate the potential value of using the rolB gene to modify difficult-to-root rootstocks of fruit trees. The results showed that all rolB transgenic rootstocks significantly reduced vegetative growth including tree height regardless of scion cultivar, compared with the non-transgenic rootstocks. Flowering and fruiting were also decreased for cultivars grown on the transgenic rootstocks in most cases, but the fruit quality was not clearly affected by the transgenic rootstocks. Cutting experiment and RT-PCR analysis showed that the rolB gene was stably expressed under field conditions. PCR and RT-PCR analyses displayed that the rolB gene or its mRNA were not detectable in the scion cultivars, indicating no translocation of the transgene or its mRNA from rootstock to scion. Our results suggest that rolB modified rootstocks should be used in combination with vigorous scion cultivars in order to obtain sufficient vegetative growth and good yield. Alternatively, the rolB gene could be used to dwarf vigorous rootstocks of fruit trees or produce bonzai plants as it can significantly reduce the vegetative growth of plants.