Molecular genetic studies on the adaptive strategies of plants in changing environments

Abstract

Unlike animals, plants spend their entire lifetime in one position and are unable to escape from the unfavorable environmental conditions. As a consequence, plants have evolved diverse and effective strategies to monitor and adapt to various environmental conditions. For instance, it is well known that controlled cleavage of membrane-bound transcription factors ensures rapid transcriptional responses to abrupt environmental stresses in plants. In this study, I investigated diverse molecular mechanisms allowing plants to cope with environmental changes. In Chapter 1, I examined the regulatory mechanism of the membrane-bound transcription factor (MTF). NTL6 is a plasma membrane-associated transcription factor and positively regulates drought resistance in Arabidopsis. I found that SnRK2.8 directly interacts with NTL6 in the cytoplasm. SnRK2.8 phosphorylates NTL6 primarily at Thr142 and SnRK2.8-mediated phosphorylation is required for the nuclear import of NTL6. Futhermore, the drought-resistant phenotype of 35S:NTL6 transgenic plants was compromised in 35S:NTL6 X snrk2.8-1 plants. These observations indicate that SnRK2.8-mediated protein phosphorylation, in addition to a proteolytic processing event, is required for NTL6 function in drought-stress signaling. In Chapter 2, I investigated the roles of BCD1 in iron homeostasis under osmotic stress. The BCD1 gene is regulated by the iron availability: induced by excessive iron, but repressed by iron deficiency. It is also induced under osmotic stress conditions such as high salinity and drought. Whereas the activation-tagged mutant bcd1-1D accumulated a lower amount of iron, the iron level was elevated in the knockout mutant bcd1-1. I also found that the BCD1 protein is localized to the Golgi complex. I propose that the BCD1 transporter plays a role in the iron homeostasis by reallocating excess iron released from the damaged cells exposed to osmotic stress. In the study of iron in plants, the conventional histochemical staining methods, such as Perls staining are still widely used. I also adopted Perls staining to determine the localization of iron in Arabidopsis in Chapter 2. However, it suffers from relatively poor resolution and detection limit. To improve the detection of iron in plants, in Chapter 3, I described a nobel method for high-sensitivity fluorescence imaging of iron, which demonstrates the amount and distribution of iron in plant tissues more precisely than conventional methods. Changes in day-length accompanied by seasonal changes are one of the major environmental factors that affect flowering time. In Arabidopsis, the diurnal control of CONSTANS (CO) accumulation by the circadian clock and light signals is critical for day-length measurement and therefore, for the photoperiodic flowering. While diverse molecular mechanisms are known to regulate the diurnal CO dynamics, it has never been explored whether and how CO itself contributes to this process. In Chapter 4, I demonstrated that CO undergoes alternative splicing, producing two protein isoforms, the full-size COa that is equivalent to the canonical CO transcription factor and the C-terminally truncated COb. Notably, I found that COb, which is resistant to the E3 enzymes, facilitates COa degradation by modulating the accessibility of COa to E3 ubiquitin ligases, providing a self-regulatory role of CO in its own diurnal dynamics.