Epigenetic Dynamics: Role of Epimarks and Underlying Machinery in Plants Exposed to Abiotic Stress

Table 1

Different classes of histone methyl transferases (HMTs).

	Class I	Class II	Class III	Class IV	Class V
					SUVH	SUVR
Members	These are homologues of E(Z) (enhancer of zeste from Drosophila); three E(Z)-like proteins are encoded by Arabidopsis genome; CURLY LEAF (CLF); MEDEA (MEA); SWINGER (SWN)	This class is constituted by SDG8, SDG4	This class contains homologues of Trithorax; therefore, members are called ATX (Arabidopsis Trithorax-like) proteins 1–5; Five genes encode Trithorax homologues, ATX 1–5	Arabidopsis has two members belonging to this class, ATXR5 and ATXR6	SUVH (SU(VAR 3-9)); Ten plant specific members	SU(VAR) 3-9 related
Domains	Contains SET (suppressor of variegation, enhancer of zeste and Trithorax) domain, E(Z) domain,; SANT domain (SW13, ADA2, NCOR, and TFIIIB-binding), and; CXC (cysteine rich region)	Contain SET domain and AWS motif (associated with SET)	Contain SET, post-SET domain,; PHD (plant homeodomain),; PWWP (proline-tryptophan-tryptophan-proline),; FYRN (F/Y rich N-terminus), and; FYRC (F/Y rich C-terminus)	Contain SET domain and PHD domain	Contain SET domain, pre-SET domain, post-SET domain, and SRA (SET and RING finger associated domain)	Contain all other domains except SRA domain; some members are reported to possess a novel WIYLD domain similar to DNA-binding protein
Role	CLF helps in flowering time regulation; MEA is involved in seed development; SWN is required for flower development	SDG8 is involved in FLC expression which regulates flowering time; SDG4 is involved in proper pollen and stamen development	Involved in drought stress response; atx1 mutant is hyposensitive to dehydration	Play role in cell cycle regulation	Mark inactive chromatin; gene silencing	Potential role in heterochromatic siRNA production machinery
Epigenetic Role	They have H3K27 methyltransferase activity	They carry out di- or trimethylation of H3K36	ATX1 and ATX2 are involved in H3K4me3 and H3K4me2; Function of ATX 3, 4, 5 is still not known	They carry out monomethylation of H3K27	H3K9 methylation; different SRA domains have preferential affinity for different cytosine methylation context (symmetric or asymmetric)	Function unclear yet; however, it resembles proteins involved in heterochromatin formation