Endogenous target mimics, microRNA167, and its targets ARF6 and ARF8 during somatic embryo development in Dimocarpus longan Lour.

Abstract

microRNA167 regulates vegetative and reproductive growth by controlling expression patterns of auxin response factors 6 and 8 (ARF6/8) in plants. However, their mutual regulatory roles during plant embryo development are still unknown. Here, we first identified miR167 family members, isolated the primary miR167 and its promoter, predicted endogenous target mimics (eTMs), identified miR167 targets, and analyzed their accumulation in Dimocarpus longan (longan). Four variants of dlo-miR167a and one dlo-miR167f were identified; their accumulation at the cotyledonary embryo stage suggests they may be required for cotyledon-shaped embryo morphogenesis in longan. We then isolated the miR167 precursor, primary transcript, and promoter. The promoter contained cis-acting elements responsive to stimuli such as light, fungal elicitors, heat, low temperature, drought, methyl jasmonate, salicylic acid, gibberellic acid, and circadian control. Two eTM homologs of a phosphatidylinositol 3- and 4-kinase family protein containing a FAT domain and an unknown protein were predicted for miR167. miR167 mediates transcript degradation in longan somatic embryos for DlARF8 but not cleaves for DlARF6. Moreover, both the eTMs were barely detectable in incomplete compact pro-embryogenic cultures and torpedo-shaped embryos, but reached peaks at globular embryos, which was largely reciprocal to that of their corresponding miR167, indicating a potential role in the negative control of miR167 expression; increased DlARF6 is required for somatic embryo development; miR167a/DlARF8 pair is essential to globular embryo formation and subsequent conversion to cotyledonary embryos. In addition, eTM-miR167-ARF6-8 signal transduction was found to play a conserved role in root development of the longan cultivar ‘Sijimi.’ We conclude that eTM down-regulates miR167 via cleavage of DlARF8 mRNAs in longan somatic embryo development, especially during late developmental stages.