Flowering is the developmental transition from a vegetative to a reproductive phase, which is the most critical event in the plant life cycle. Flowering at proper time is important for crop yield and fruit maturation. Plants sense the photoperiod time to adjust their flowering time. In long-day plant Arabidopsis, GIGANTEA (GI) activates CONSTANS (CO), and CO enhances Flowering Time (FT) to promote flowering time under prolonged photoperiod. CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) binds GI and blocks the GI activation. Nevertheless, the activators of GI remain unclear. 

Recently, Prof. DING Yong's team from the School of Life Science and Center of molecular plants of Chinese academy of sciences, the University of Science and Technology of China (USTC) identified Phosphorylation of histone H2A at serine 95, which is a plant-specific modification. Phosphorylation of H2A at Serine 95 by MUT9P-LIKE-KINASE (MLK4) promotes flowering time and H2A.Z deposition in Arabidopsis.

This work was published in The Plant Cell entitled as "Phosphorylation of Histone H2A at Serine 95: A Plant-specific Mark Involved in Flowering Time Regulation and H2A.Z Deposition" on August 8th, 2017. 

In eukaryotic cells, the covalent modifications in chromatin participate in development with activation or block of gene transcription. Unlike animal, plant starts its life with a seed. Plants might have specific histone modification sites if the chromatin really contains development information. The primary question was to understand how plant-specific histone modifications and other effectors work together to regulate the specific developmental processes. 

MLK4 has H2A phosphorylation activity at serine 95 in vitro and in vivo, and this site specifically exists in plants, but not in yeast, Drosophila, mouse, or humans. Loss of MLK4 function results in late-flowering plants under long-day condition, whereas the phenotype is not obvious under short-day condition.

This observation indicated that MLK4 is involved in photoperiod pathway. CCA1 interacts with MLK4, and allow MLK4 to bind GI. Point mutation of MLK4 lost the phosphorylation activity and failed to rescue the late-flowering phenotype caused by MLK4 mutations.

Therefore, phosphorylation of H2A at serine 95 modulated by MLK4 play a critical role in the regulation of flowering time. In addition, MLK4 in vivo interacts with YAF9a, a co-subunit of the Swi2/Snf2-related ATPase (SWR1) and NuA4 complexes, which incorporate the histone variant H2A.Z into chromatin and H4 acetylase activity, respectively. Mutations in MLK4 decreased the serine 95 phosphorylation of histone H2A, H2A.Z deposition, and transcripts level of GI, which, in turn delay the flowering time. 

This work not only finds a new plant-specific modification site and its function but also offers an outstanding example of how histone modification affects plant flowering time and histone variant exchange. 

Phosphorylation of H2A serine 95 modulated by MLK4 is required for flowering time, H2A.Z deposition, and H4 acetylation in Arabidopsis (Image by DING Yong) 

This work is supported by the National Natural Science Foundation of China, Chinese Thousand Youth Talents Plan, and he Strategic Priority Research Program "Molecular Mechanism of Plant Growth and Development" of CAS.