Preliminary Study On The Role Of HSF4 In Root Development Under Stress

In addition to providing energy for plant photosynthesis,light is an important environmental signal that regulates plant growth and development.As a plant canopy grows and fills space,light over most of the visible spectrum is absorbed by leaves,while far-red light is filtered and reflected by neighboring plants,resulting in a reduction in the ratio of red to far-red(R:FR)light.According to the different ratio of red to far-red light,shade can be classified into mild shade and strong shade.And the responses of plant to these two kinds of shade are different.Mild shade is mainly sensed by the red-light photoreceptor,phytochrome B,and trigger a series of responses(shade avoidance response,SAR),including the elongation of hypocotyl and petiole.Strong shade attenuates shade-induced stem elongation.The far red-light photoreceptor,phytochrome A(PHYA),plays a critical role in inhibiting the SAR as phyA mutant displays a constitutive shade-avoidance hypocotyl phenotype under strong shade.In order to further understand the response of plants to shade as well as how PHYA regulates the SAR,we carried out a genome array and digged out 506 down-regulated genes in phyA mutant compared with wild type under strong shade.After analyzing the promoters of these genes,we discovered a conserved sequence enriched,which is GAAGAAGA.Fortunately,a transcription factor,Heat Shock Factor 4(HSF4)has been reported to bind to this cis element specifically.We then generate different kind of expressed materials of HSF4 to investigate its function in shade.First of all,we found that HSF4 was mainly expressed in root,detecting by realtime quantitative PCR and the expression of HSF4-YFP driven by HSF4 promoter.We also found that,pro-long strong shade will affect the elongation of plant roots,and phyA mutant exacerbates to it.Neither the meristematic zone length or mature cell length showed a decrease in the phyA compared with wild type under strong shade.Together,the inhibition of root elongation by strong shade is partially suppressed by PHYA.However,hsf4 mutant shown no significant difference with WT in root elongation under strong shade.We then generate hsf4 phyA double mutant,the result shown that the mutation of HSF4 can partially suppressed root phenotype of phyA in strong shade.Since reactive oxygen species(ROS)is important for root development.The accumulation of hydrogen peroxide as well as the protein level of HSF4 can be induced by strong shade in a short-term and then significantly reduced after long-term shade.This suggests that ROS may be the signal which regulates the expression of HSF4.Moreover,we found that the expression of HSF4 is rapidly induced by hydrogen peroxide in the cortex and epidermis of roots.In the meanwhile,after treating with hydrogen peroxide,the elongation of root was inhibited and the stem cell in root tip was dead.However,the root of hsf4 mutant is insensitive to the treatment of hydrogen peroxide.These results revealed that HSF4 is required for the responses to hydrogen peroxide in roots.In addition,we also found that heat shock will inhibit plant root elongation,while short-term heat treatment can endow root with induced thermo-tolerance.The expression of HSF4 was rapidly induced by heat shock,and hsf4 mutant were not only hypersensitive to heat shock stress,but also had reduced induced thermo-tolerance,suggesting that HSF4 may involve in the response to heat shock stress in roots.