Overexpression of PaHAP3A stimulates differentiation of ectopic embryos from maturing somatic embryos of Norway spruce

摘要

Somatic embryogenesis can be used for large-scale propagation of plants. In most conifers, it is only possible to establish embryogenic cultures from zygotic embryos or young seedlings. There is, however, a great interest to propagate selected trees with valuable traits via somatic embryos. To be able to establish embryogenic cultures from adult tissues, more knowledge about the molecular regulation of totipotency and embryogenic potential is needed. In Arabidopsis (Arabidopsis thaliana), LEAFY COTYLEDON1 (LEC1) is required for somatic embryogenesis, and overexpression of LEC1 can stimulate the formation of embryo-like structures from vegetative tissues. We have previously characterized a conifer LEC1-type gene, PaHAP3A, which is preferentially active during embryo development in Norway spruce (Picea abies). In this work, we show, by using histochemical GUS assays, that PaHAP3A is expressed in germinated embryos at presumptive sites from which embryogenic tissue differentiate during initiation of embryogenic cultures. Furthermore, we have overexpressed PaHAP3A, using both constitutive and inducible promoters, in order to elucidate whether elevated transcript levels of PaHAP3A are sufficient to induce embryonic properties after germination. In contrast to its angiosperm counterpart, PaHAP3A does not stimulate embryonic features in vegetative tissues. However, overexpression of PaHAP3A during the maturation stage leads to the differentiation of ectopic embryos from maturing somatic embryos. Our results not only indicate at least partial conservation between the conifer and angiosperm LEC1-type genes but also suggest that PaHAP3A is dependent of the cellular status to confer its presumptive role as an important factor influencing the competence of the tissue to initiate somatic embryos.