Abstract
MicroR159 (miR159) is ancient, being present in the majority of land plants where it targets a class of regulatory genes called GAMYB or GAMYB-like via highly conserved miR159-binding sites. These GAMYB genes encode R2R3 MYB domain transcription factors that transduce the gibberellin (GA) signal in the seed aleurone and the anther tapetum. Here, GAMYB plays a conserved role in promoting the programmed cell death of these tissues, where miR159 function appears weak. By contrast, GAMYB is not involved in GA-signaling in vegetative tissues, but rather its expression is deleterious, leading to the inhibition of growth and development. Here, the major function of miR159 is to mediate strong silencing of GAMYB to enable normal growth. Highlighting this requirement of strong silencing are conserved RNA secondary structures associated with the miR159-binding site in GAMYB mRNA that promotes miR159-mediated repression. Although the miR159-GAMYB pathway in vegetative tissues has been implicated in a number of different functions, presently no conserved role for this pathway has emerged. We will review the current knowledge of the different proposed functions of miR159, and how this ancient pathway has been used as a model to help form our understanding of miRNA biology in plants.
Highlights
Associated with the emergence and diversification of land plants, is a core set of conserved microRNA families that arose early in terrestrial plant evolution and which are conserved in modern day plant species [1]
GAMYB positively transduces the GA signal to activate expression of α-amylase and other hydrolytic enzymes [74], as well as promote programmed cell death (PCD) in the aleurone [75]. This latter function appears conserved in Arabidopsis, as a myb33.myb65.myb101 triple mutant has attenuated vacuolation in aleurone cells, a PCD-mediated process that is positively regulated by GA [41]
De-regulated expression of GAMYB in leaves results in strong perturbation of growth. This was shown in mir159ab, as well as transgenic Arabidopsis expressing miRNA decoys to inhibit miR159 function, either MIMIC159 (MIM159), Short target tandem MIMIC159 (STTM159), or SPONGE159 (SP159) [58,76], or with STTM159 rice [61,62], which all result in the similar phenotypic defect of stunted growth
Summary
Associated with the emergence and diversification of land plants, is a core set of conserved microRNA (miRNA) families that arose early in terrestrial plant evolution and which are conserved in modern day plant species [1]. This conservation implies these endogenous gene regulators are fundamental to plant biology and have been indispensable for the conquest of plant life on land. One such core family is microR159 (miR159), which has been extensively studied in multiple, diverse plant species. We will highlight the major functions identified for miR159, its use as a model for gaining greater insights into miRNA biology in general, and highlight the many outstanding questions surrounding this ancient gene regulator
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