Prof. Ferenc Nagy: "Molecular aspects of the photoreceptor phytochrome regulated growth and development in Arabidopsis thaliana"

Plants are sessile organisms and have to adapt to changes in their environment. Light is one the most variable environmental factors and it regulates development of plants through their entire life cycle, from seed germination to seed setting. To be able to monitor qualitative and quantitative changes in their natural light environment higher plants evolved a number of photoreceptors including the red/far-red light absorbing phytochromes, the blue-light sensing cryptochromes, phototropins, FKF1/LKP1/ZTL1 proteins and the molecularly yet unidentified UVB photoreceptor(s).

The model plant Arabidopsis thaliana has five genes encoding the PHYA, PHYB, PHYC, PHYD and PHYE phytochromes. These photoreceptors have specialised but partly overlapping function in controlling photomorphogenesis (light regulated development). PHYA is responsible for seed germination and seedling establishment whereas PHYB is the major photoreceptor acting in mature plants.

Phytochromes are 120 kD proteins, in vivo exist as dimers. Their chromophore is a linear tetra-pyrrole that is covalently attached to each monomer. Phytochromes are synthesized in their biologically inactive Pr (red-light absorbing) form in the dark. Absorption of red light induces formation of the active Pfr (far-red light absorbing) conformer which then can be converted back into the inactive Pr form by far-red illumination. This red/far-red induced conformation change is the hallmark of these photosensory molecules.

Phytochromes control expression of about 2500-3000 genes representing approximately 10-15% of the Arabidopsis genome. Spatial and temporal modulation of the expression of these genes underlies phytochrome induced physiological responses and leads then ultimately to phytochrome controlled growth and development. The molecular machinery mediating phytochrome induced and controlled signalling is inevitably complex and it has been the subject of intensive research in the last twenty years. I will discuss the main features of the phytochrome molecule itself, describe the basic components of the complex signalling network controlled by phytochromes by highlighting the most important recent break-throughs and finally I introduce those research topics which are in the centre of the ongoing work in my laboratory.