The novel function of HCS1 and its regulatory pathway in Arabidopsis is the focus of this dissertation. Due to the high similarity of HCS catalytic domain between Arabidopsis and humans, it provides the possibility that HCS1 is able to play novel roles for modulating histone proteins in Arabidopsis, like human HCS does. Biotinylation of histones enriched on a wide variety of cellular processes according to mammalian cell studies, such as heterochromatin formation, gene silencing, mitotic DNA condensation, and DNA repair (Stanley et al., 2001; Kothapalli et al., 2005; Camporeale et al., 2007). However, the biotinylation of histones have not been characterized in plants so far. Our in-vitro experiments indicate that HCS1 interacts with Arabidopsis histone H3 protein specifically, but unfortunately, no further in-vivo evidence of this interaction has been obtained. In addition, I was not able to detect a nucleus distribution for HCS1-CY protein (data not shown). The possible reason for this may be its low amount of in nucleus.

Since the homozygous T-DNA insertion mutants of HCS1 are embryonic-lethal, the analyses through transgenic RNAi technology show that the HCS1-RNAi mutants display a flowering delay phenotype under short day (SD) conditions compared to controls. Corresponding to this phenotype, the RNA levels of HCS1 decrease dramatically in mutant shoot apex. Many genes involved in controlling flowering time have spatial-limited expression patterns on shoot meristems. Therefore, HCS1 may affect flowering time by coordinating with floral-related gene networks specifically located in shoot apex. In plants, the processes of reproduction and seed development depend on flowering at the right time (Putterill et al., 2004). In addition to photo period, vernalisation and autonomous pathways, sugar signals are also known to affect flowering time in plants, but the mechanism is poorly understood (Eimert et al., 1995). Our results suggest that HCS1 is regulated by sucrose, glucose or fructose through an HXK-dependent pathway. Therefore, HCS1 may be a link between flowering time control and sugar signaling pathway. Our finding explores a new perspective on sugar signaling pathway, which may contribute the knowledge of how flower transition is regulated in plants.