GEM, , una proteína con dominio GRAM, es un regulador negativo de la señalización por ABA durante la germinación en Arabidopsis thaliana

Resumen

The GRAM domain is found in glucosyltransferases, Rab GTPases-activating proteins and myotubularins but its function remains poorly known in plants. We classified a new group of nine proteins with a specific sequence in the GRAM domain that we propose to name GER (GEM RELATED) subfamily, based on their similarity to GEM (GL2 EXPRESSION MODULATOR). In this work we have focused on GEM. GEM expression was mainly located in the root during the vegetative stage of the plant but in the reproductive stage GEM is also expressed in flowers and siliques during gametogenesis, embryogenesis and seed maturation. In the flower, GEM is located in developing microspores, the tapetum layer in the pollen sacs, dehiscent pollen grains and in the stigma papillae of the gynoecium. In siliques, GEM is located in the endosperm of developing seeds, similarly to ABA45, another GRAM protein of barley. All these plant locations are associated with the production of ABA and therefore we hypothesized that GEM could play a role in ABA signaling processes. We have demonstrated that GEM expression is induced by ABA and has a role in seed germination, related to its expression in the endosperm. ABA activates GEM expresion in an ABRE site-dependent manner and its regulation requires the activity of ABI1 phosphatase, SnRK2 2.2 / 2.3/ 2.6 kinases, and the ABI3 and ABI5 transcription factors, which specifically recognize the ABRE motifs. Similarly, other members of the subfamily GER, such as GER5, are also ABA responsive genes but only GER4 has ABRE motifs in its promoter. Global transcriptional studies in gem-1 and GEMoe plants have shown a strong deregulation of ABA-responsive genes. Germination studies in these plants indicate that GEM acts as a negative regulator of ABA signaling by inhibiting dormancy and promoting germination even at high salt concentrations. ABA signaling during root growth and drought tolerance is also promoted by GEM. These transcriptional studies have also shown a strong enrichment in genes controlled by the circadian clock. We have concluded that GEM negatively regulates ABA-mediated induction of ABAR, a gene involved in ABA signaling and circadian clock regulation. GEM is also expressed in a circadian manner being, expressed most probably due to daily variation in ABA content in plants. However, GEM is not directly regulated by the main circadian oscillator. Changes in the flowering behavior of gem-1 and GEMoe plants suggest a connection to ABA-mediated processes that link flowering regulators with the photoperiod, e.g., intermediate day photoperiod produces a delayed flowering in GEMoe plants. Our subcellular localization studies show that GEM is part of the peripheral pool of membrane proteins as well as in the nucleus, and it binds various phosphoinotide phosphate. Thus, we propose that GEM could act as a signal transduction component in the ABA response.