UBA2c Protein Functions In Arabidopsis Thermomorphogenesis Pathway

While temperature can determine the rate of growth and development of plants,plants can continuously monitor their environment and quickly adjust the expression levels of growth-related genes when being exposed to higher temperatures.Although some genes that function in thermomorphogenesis have been discovered,the mechanisms by which plants sense temperature are not well understood yet.In Arabidopsis thaliana,the MADS-box transcription factor gene FLOWERING LOCUS M(FLM)plays a key role in the regulation of flowering time in response to ambient temperature changes.FLM is regulated by temperature-dependent alternative splicing.The expression levels of the main splicing variants are up or down-regulated in response to the increase of ambient temperature.The different temperature-dependent FLM variants ratio determine the differential regulation of downstream growth-related genes has been well studied.However,it is not known how FLM is transcriptionally regulated.By forward genetic screening,we identified a nonsense mutation in UBA2c;the mutant showed typical phenotypes related to thermomorphogenesis,such as early flowering,hypocotyl elongation and petiole elongation.Based on transcriptome sequencing,we found that the transcript level of the temperature sensor gene FLM was reduced in uba2c mutant.By EMSA and Chromatin Immunoprecipitation,we proved that UBA2c can bind DNA in vitro and interact with FLM chromatin in vivo.UBA2c can also promote the occupancy of Pol Ⅱ at the FLM chromatin,thereby facilitating the transcription of FLM.Our ChIP-seq results showed that in the uba2c mutant,the H3K27me3 level was increased and the H3K4me3 and H3K36me3 levels were reduced on the FLM chromatin.The function of UBA2c in regulating the transcription and histone modifications of FLM is at least partially dependent on the histone H3K27 methyltransferase CLF.By comparing the effect of uba2c on the FLM transcription under different temperatures,we found that the effect of uba2c on FLM transcription is stronger in the low temperature than in the high temperature.The binding of UBA2c to FLM chromatin was not affected by the temperature change.Intriguingly,we observed that the number of UBA2c formed speckles in the nucleus is induced by the higher temperature.This result suggests that the induction of the speckle number in response to the higher temperature is likely to be one of the reasons for the different effects of UBA2c on the FLM transcription under the different temperatures.UBA2c encodes a protein containing a PrLD domain and two RRM domains.Firstly,we discovered that when the conserved RRM domain was mutated,UBA2c lost abilities in the DNA binding and in the regulation of FLM transcription and histone modifications,suggesting that the integrity of the RRM domain is essential for the function of the UBA2c protein.Secondly,we found that PrLD domain is necessary for the function of the UBA2c protein and the ability of UBA2c to form nucleus speckles is dependent on the PrLD domain.Collectively,by using genetic analysis,we identified UBA2c as a new protein that is functional in thermomorphogenesis.UBA2c can facilitate Pol Ⅱ occupancy on FLM chromatin,thereby regulating FLM transcription.Both the RRM domain and PrLD domain of UBA2c are proved to be essential for the function of UBA2c.The higher temperature-induced UB A2c speckles in the nucleus could be one of the reasons of the different transcription regulation efficiency of UBA2c under the different temperatures.Our work,on the one hand,enhanced the understanding of the regulatory mechanisms of plant thermomorphogenesis,and on the other hand,provided clues for studying the functions of other RRM and PrLD domains containing proteins.