Exploring the molecular hub in plant elongation responses: regulation of PHYs and PIFs

Light and temperature signalling response is a central mediator of plant growth plasticity. This thesis aimed to provides new molecular insights into control of plant growth. We investigated the role of phytochrome and PIF4 genes activity as function of light quality and temperature. First time the study shows that PHYD is a constitutive repressor of PHYA-reporter activity and that PHYB and PHYA reporter activity is strongly up-regulated under FR, while this response to FR is not affected by the classical FR sensor PHYA but by PHYB and PHYE. Our findings of the light sensitivity of phytochrome gene expression especially under the artificial LED light conditions provide crucial new fundamental insights that may be used to control plant growth, yield and quality in greenhouses and indoor farming industries. Next chapter, we discovered that the positive feedback regulation of PIF4 expression by BZR1 is actually broken by PIF4 itself, which acts as a negative regulator of its own expression. The negative action of PIF4 can also compete with the positive action of BZR1. Overall this adds a new layer to the regulation of transcription of PIF4 and shows that overall PIF4 transcription may be determined by the relative levels of PIF4 to BZR1 protein. We also investigated the role of MED25 in transcriptional regulation of PIF4. We showed that PIF4 expression is upregulated in med25 mutant at ambient temperature, while PIF4 expression is super induced compared to WT at warm temperature. However, this upregulation of PIF4 expression is uncoupled from induction of PIF4 target gene YUCCA8 and from the elongation response. The regulation of PIF4 gene transcription and regulation of PIF4 target genes by PIF4 has an additional layer, as histone modifications at these promoters also become part of the equation. We provide evidence that MED25 may actually recruit histone modifying activity for PIF4 target genes. In last chapter, we describe a novel strategy by which plants can be transformed with a single construct to obtain overexpression of the transgene and silencing of an endogenous target gene of interest. The concept of a transgene containing an aimiRNA could be useful for simultaneous manipulation of several gene activities, which could be an important tool for plant biotechnology