Molecular Mechanism Of Light Transcription Factor HY5 Regulating Salt Tolerance In Plants

Light is a critical environmental factor which influencing all aspects of plant growth and development.As sessile organisms,plants have evolved multiple strategies to sense and cope with many forms of environmental stress conditions such as drought,salinity and extreme temperatures.The integration of light signaling and diverse abiotic stress response will facilitate plant survival under changing environmental conditions.To clarify the involvement of light in the salt stress response of plants,we investigated the effect of salt stress in wild-type Arabidopsis in the absence or presence of light.Then we found that the salt stress-induced growth inhibition of plants is critically affected by light,and with increasing light intensity the plant enhances tolerance to salt stress.Based on these results,we attempted to elucidate the molecular networks of light and salt stress.In this study,we found key b ZIP transcription factor,ELONGATED HYPOCOTYL 5(HY5),plays a critical role in light-enhanced salinity tolerance pathways in plant.By directly binding to Gbox motifs of Heat shock transcriptional factor A2(Hsf A2),HY5 represses its transcription to confer salinity tolerance.Furthermore,the phenotype assays showed that the hy5-215 mutation is epistatic to hsfa2 mutation in salt stress,they play opposite roles in salt stress pathway.Our results indicate that HY5 interacts with HDA9,a histone deacetylation factor,and acts synergistically to suppress the expression of Hsf A2.The HDA9 was found been recruited at Hsf A2G-Box loci,but vanished in hy5-215 background.Salt stress elevated HDA9 protein accumulation,meanwhile significantly enhanced the interaction between HY5 and HDA9,thereby resulting in inhibition of Hsf A2 expression under salt stress.Considering that Hsf A2 is the most highly heatinduced HSF in Arabidopsis,we next analyzed the salt sensitivity of wild-type(WT)plants in high temperature conditions.Compared with normal living temperature,the elevated temperature repressed salt tolerance of Arabidopsis seedlings.Consistently,the heat shock also results in the degradation of both HY5 and HDA9 proteins.Taking these results together,we proposed a hypothetical model that explains the roles of HY5,HDA9 and Hsf A2 in crosstalk between light signaling,temperature,and salt tress.Light and temperature,the two most essential environmental factors for plants,act antagonistically to modulate the salt stress response by a HY5-HDA9 complex mediated mechanism.Under normal temperature conditions,light signaling induces the accumulation of HY5,and salinity signaling induces the accumulation of HDA9,furthermore,the interaction between HY5 and HDA9 is significantly enhanced by salt stress.Next,HY5 directly binds to the G-box motifs in the Hsf A2 promoter to repress Hsf A2 gene expression with the cooperation of HDA9,resulting in the tolerance of salinity.On the contrary,upon high temperature,HY5 and HDA9 are degraded,these lead to dissociation of HY5-HDA9 from Hsf A2 loci,thereby resulting in enhanced sensitivity to salt stress.By using the model plant Arabidopsis thaliana,this study analyzed the molecular mechanism of light signal regulating plant salt tolerance through HY5,providing a new direction and theoretical basis for breeding salinity-tolerant crop cultivars.Functional loss mutants of rice HY5 homolog Osb ZIP18 and Hsf A2 homolog Os Hsf A2 c were created by CRISPR Cas9 technique,respectively.By analyzing their seedling traits,it was found that Osb ZIP18 mutation had no significant effect on plant height,root length and substance accumulation,but significantly reduced rice tolerance to salt stress.On the other hand,Os Hsf A2 c mutation significantly increased plant height of rice,and had no significant effect on root length and substance accumulation,but significantly increased salt tolerance of rice.These results indicated that Osb ZIP18 had a positive regulation effect on the salt stress pathway in rice,while Os Hsf A2 c negatively regulated the salt tolerance of rice.The functions of Osb ZIP18 and Os Hsf A2 c are consistent with At HY5 and At Hsf A2 in the salt stress pathway,which proves that the results of this study are conservative in model plants and crops.In conclusion,these results demonstrate that HY5 is a critical component integrating light signaling with salt stress and heat shock response.By regulating the proteins accumulation and interaction of HY5 and HDA9 fine-tune the Hsf A2 expression,which is the key for both heat shock and salinity response.More importantly,the functions of HY5 and Hsf A2 are conserved in model plants and crops.This study can contribute to a better understanding of crosstalk between light,temperature and salt stress in plants;also provides clues for breeding salinity-tolerant crop cultivars.