Protein Homologous to Human CHD1, Which Interacts with Active Chromatin (HMTase) from Onion Plants

Abstract

Onions are grown as an annual plant for commercial purposes; although since they are biennial it takes two seasons to grow from seed to seed. Bolting (flowering) of onion plants is determined by two factors, the size of the plant and cold temperatures. The critical size for bolting occurs when the onion reaches the five-leaf stage of growth. If onions are seeded in early fall, warm temperatures will result in sufficient size for bolting in the subsequent winter. Early transplants and some onion varieties are especially susceptible to bolting during cold temperatures. However, cold temperatures are not the sole prerequisite for bolting. If onions are not at the critical size in their development, they do not recognize cold as a signal to initiate bolting. Thus, sowing and transplanting at the correct time of year is the most important factor to avoid premature bolting. Genetic and molecular studies of Arabidopsis have revealed a complicated network of signaling pathways involved in flowering time (Boss et al., 2004; Macknight et al., 2002; Putterill et al., 2004). Four genetic pathways, which are known as the photoperiod, autonomous, vernalization, and gibberellin (GA) pathway, have been identified based on the phenotypes of flowering time mutants (Koornneef et al., 1998). The photoperiod pathway includes genes whose mutants show a late flowering phenotype under long day (LD) conditions that is not responsive to vernalization treatments. This pathway contains genes encoding photoreceptors such as PHYTOCHROME (PHY), components of the circadian clock, clock associated genes such as GIGANTEA (GI) (Fowler et al., 1999; Park et al., 1999), and the transcriptional regulator CONSTANS (CO) (Putterill et al., 1995). FLOWERING LOCUS T (FT) (Kardailsky et al., 1999; Kobayashi et al., 1999) and SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) (Lee et al., 2000) are targets of CO (Samach et al., 2000). The autonomous pathway includes genes whose mutants show a late flowering independently of day length that can be rescued by vernalization. Genes included in this pathway are FCA, FY, FVE, FLOWERING LOCUS D (FLD), FPA, FLOWERING LOCUS K (FLK), and LUMINIDEPENDENS (LD) (Ausin et al., 2004; He et al., 2003; Kim et al., 2004; Lee et al., 1994; Lim et al., 2004; Macknight et al., 1997; Schomburg et al., 2001; Simpson et al., 2003). They regulate FLOWERING LOCUS C (FLC) (Michaels and Amasino, 1999), a floral repressor, through several different mechanisms