Studies on the control of transcriptional regulators in adaptive responses to environmental changes in Arabidopsis

Abstract

Since the sessile nature of the plants, they have developed various adaptive systems in response to environmental changes. Transcriptional control is one of the most important regulatory systems in adaptive responses. Transcriptional regulators such as transcription factors and transcription co-factors directly or indirectly control transcription of their downstream genes. Thus, modulating the activity of transcriptional regulators is a key issue in studies on transcription. By molecular and genetic studies, transcriptional regulators that play a key role in plant adaptive responses are identified. However, how plants control their activity in response to environmental changes is still largely unknown. In this study, regulatory mechanisms of two transcriptional regulators which are critical for temperature and immune responses are identified. In Chapter 1, a study on the modulation of PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) activity under high ambient temperature conditions is described. In Arabidopsis, the basic helix-loop-helix transcription factor PIF4 regulates high temperature-induced adaptive responses by modulating auxin biosynthesis. At high temperature, PIF4 directly activates expression of YUCCA8 (YUC8), a gene encoding an auxin biosynthetic enzyme, resulting in auxin accumulation. Here, I demonstrate that the RNA-binding protein FCA attenuates the PIF4 activity by inducing its dissociation from the YUC8 promoter at high temperature. At 28oC, the auxin content is elevated in FCA-deficient mutants that exhibit elongated stems but reduced in FCA-overexpressing plants that exhibit reduced stem growth. Therefore, the proposed mechanism is that the FCA-mediated regulation of YUC8 expression tunes down the PIF4-induced architectural changes to achieve thermal adaptation of stem growth at high ambient temperature. In Chapter 2, regulatory mechanism of NONEXPRESSER OF PR GENES 1 (NPR1) localization in response to pathogen infection is identified. In plants, necrotic lesions occur at the pathogen infection site through hypersensitive response (HR), which is followed by the activation of systemic acquired resistance (SAR) in distal tissues during defense responses. Salicylic acid (SA) plays a key role in inducing SAR by activating NPR1, but a high level of SA causes toxic effects to plant growth and development. During HR, SA biosynthesis increases in both local and systemic tissues. However, the SA level is only slightly elevated in systemic tissues, obscuring the role of SA in SAR. Here, I demonstrate that SNF1-RELATED PROTEIN KINASE 2.8 (SnRK2.8) phosphorylates NPR1 to facilitate its nuclear transport in systemic tissues which are under low SA conditions. Therefore, Arabidopsis requires the SnRK2.8-mediated activation system to induce immune responses while reducing growth defects by maintaining a low level of SA in systemic tissues.