Histone methylation is a major component in numerous processes such as determination of flowering time, which is fine-tuned by multiple genetic pathways that integrate both endogenous and environmental signals. Previous studies identifiedSET DOMAIN GROUP 26(SDG26) as a histone methyltransferase involved in the activation of flowering, as loss of function ofSDG26caused a late-flowering phenotype inArabidopsis thaliana. However, the SDG26 function and the underlying molecular mechanism remain largely unknown. In this study, we undertook a genetic analysis by combining thesdg26mutant with mutants of other histone methylation enzymes, including the methyltransferase mutantsArabidopsis trithorax1(atx1),sdg25andcurly leaf(clf), as well as the demethylase double mutantlsd1-like1 lsd1-like2(ldl1 ldl2). We found that the early-flowering mutantssdg25,atx1andclfinteract antagonistically with the late-flowering mutantsdg26, whereas the late-flowering mutantldl1 ldl2interacts synergistically withsdg26. Based on microarray analysis, we observed weak overlaps in the genes that were differentially expressed betweensdg26and the other mutants. Our analyses of the chromatin of flowering genes revealed that the SDG26 protein binds at the key flowering integratorSUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1/AGAMOUS-LIKE 20(SOC1/AGL20), and is required for histone H3 lysine 4 trimethylation (H3K4me3) and histone H3 lysine 36 trimethylation (H3K36me3) at this locus. Together, our results indicate that SDG26 promotes flowering time through a distinctive genetic pathway, and that loss of function ofSDG26causes a decrease in H3K4me3 and H3K36me3 at its target geneSOC1, leading to repression of this gene and the late-flowering phenotype.Significance Statement
Our finding is of high interest because: (i) previous studies primarily report on FLC and FT, very little is known about chromatin regulation at SOC1; (ii) it provides ample understanding of regulatory mechanism of SDG26 in flowering time control; (iii) the uncovered mechanism may be extended to other plant species since SOC1 as well as SDG26 are evolutionarily conserved in higher plants.