The Molecular Mechanisms Underlying TZP Regulation Of Far-red Light Signaling

Light plays an important role in the whole life cycle of plants in environmental factors such as light,temperature,CO2,water and nutrients which affect plant growth and development.Plants not only need light to provide energy for photosynthesis,but also can sense the light signal in nature through photoreceptor,and constantly adapt to the environment to promote growth and development.There are three main types of photoreceptors in plants:phytochromes(phys)are red/far-red(R/FR)receptors,there are five members(phyA-phyE)in Arabidopsis;cryptochromes(crys)and phototropins(phots)are blue/ultra violet A(UV-A)receptors;and ultraviolet B receptor UVR8.Photomorphogenesis is an important light signal response.Because the morphology of plant seedlings is easy to observe,photomorphogenesis of seedlings has gradually developed into a model system for studying the mechanism of plant light signal transduction.Phytochromes play an important role in regulating photomorphogenesis.Phytochrome A is the only far-red light receptor in plants and there are two main forms of phyA in plants,red light absorption Pr(inactive)form and far red light absorption Pfr(active)form.Upon red light irradiation,phyA changes its conformation,from Pr to Pfr,translocates from cytosol into the nucleus,and interacts with a series of important signalling components,ultimately resulting in signal transduction and photomorphogenesis.In far-red light,phyA mutant showed skotomorphogenesis phenotypes,with long hypocotyls,closed cotyledons and apical hooks,which completely deprived plants of the ability to sense far-red light.Therefore,we screened the mutant library based on the mutant phenotypes in far-red light,hoping to find new components of far-red light signal.In this study,we identified two Arabidopsis mutants,tzp-1 and tzp-2,that develop longer hypocotyl than wild-type seeldings in far-red light,and the phosphorylation of phyA was inhibited in the mutants.It was previously shown that phyA can be phosphorylated and degraded upon far-red light exposure,which might play into its role in far-red light signal transduction in plants.At present,there are few studies on the phosphorylation function of phyA.In this paper,we explore the mechanism of TZP involved in the regulation of far-red light signal and the important role of phyA phosphorylation in far-red light signal transduction.Our study found that:(1)a single gene,designated as TANDEM ZINC-FINGER/PLUS3(TZP),was shown to be responsible for the long hypocotyl phenotype in far-red light.TZP was previously shown to encode a nuclear protein involved in blue light signaling and phyB-dependent regulation of photoperiodic flowering,but not in far-red light signaling.(2)In this study,we found that the transcription of TZP increased dramatically in light conditions compared to darkness.TZP proteins were differentially modified in different light conditions and gradually accumulated after dark-light conversion treatment,indicating that TZP proteins were also induced and regulated by light.(3)Protein interaction experiments showed that TZP could interact with phyA,phyB and FHY1 in vivo.(4)More phyA and FHY1 proteins and less HY5 proteins accumulated in tzp mutants.The non-phosphorylated form of phyA increased and the phosphorylated form of phyA disappeard in tzp mutant,and the regulation of phyA phosphorylation by TZP occurred in the nucleus.(5)In Arabidopsis,COP1 interacts with phyA to promote its degradation after irradiation.The phosphorylated and non-phosphorylated forms of phyA interacted with COP1 in wild type are comparable,while only non-phosphorylated forms of phyA interacted with COP1 in tzp mutant,and coprecipitated at similar level compared with the non-phosphorylated forms of phyA interacted with COP1 in wild type.Previous studies have shown that COP1 preferentially degrades the phosphorylated form of phyA,while phyA inhibits the activity of COP 1/SPA complex,resulting in the accumulation of positive regulatory factor HY5,the direct substrate of COP 1/SPA complex,thereby inhibiting hypocotyl elongation.Therefore,the absence of phosphorylated form of phyA in tzp mutant slowed down the degradation of phyA by COP1,thus attenuated the inhibition of phyA on COP1/SPA complex,resulting in the decrease of HY5 accumulation and hypocotyl elongation.In summary,our study revealed that TZP participates in the regulation of far-red light signal transduction through the study of some important components in far-red light signal pathway.TZP can interact with phyA and regulate phyA phosphorylation,phyA(maybe mainly phosphorylated phyA)inactivates the COP1/SPA complex,leading to rapid accumulation of transcription factors(such as HY5)and initiation of photomorphogenesis.This study further suggests a role of phosphorylated phyA in inducing the far-red light response.TZP involves in blue light signaling and phyB-dependent regulation of photoperiodic flowering,our study also indicates that TZP plays an important role in plant response to far-red light,and provides theoretical foundation for the further study of the functional diversity of TZP in optical signal transduction and the mechanism of phyA phosphorylation in far-red light signal transduction.