Mechanism Analysis Of AtHBs Regulate Primary Root Growth Inhibition In Aluminum Stress

During the long-time evolution,plants have gradually formed a series of stratigies to adapt environments and survive under adverse environmental conditions.Among the various biotic and abiotic stresses,aluminum stress is an important limiting factor affecting plant growth and development and crop yield in acid soil.Aluminum is the most abundant metal element in the earth's crust.Generally,aluminum is mainly found in the form of minerals,which has no significant effect for plant growth.However,when soil pH drops below 5.0 in acidic soils,the aluminum element is found in the form of Al3+,which will cause serious damage to plant roots and affect the normal growth and development of plants.The study found that the ZIP transcription factor STOP1 can regulate the secretion of organic acids by regulating the genes encoding organic acid transporters in the aluminum tolerance mechanism of plants,and then the organic acid chelate aluminum ions to reduce the toxicity of aluminum stress on plants.Our subjects AtHB7 and AtHB12 are members of the HD-Zip protein family.The homeodomain-leucine zipper proteins(HD-Zips)are unique proteins in plants and play an important role as transcription factors in plant growth and environmental adaptability.HD-Zip family members have been reported to be able to respond to drought,salt,hypothermia and viral infection.But whether they are associated with aluminum stress response has not been reported.Our transcriptome analysis showed that aluminum significantly induced the expression of AtHB7 and AtHB12.To investigate the relationship between AtHB7 and AtHB12 with aluminum stress,we constructed AtHB7 and AtHB12 overexpression lines and generated mutant materials for subsequent experiments.In the normal conditions,we found that 35S:AtHB7 and 35S:AtHB12 have elongated root phenotype which was result from the increased cell number in the meristem zone and enlarged cell size in the mature zone.Consistent with the role of AtHB7 and AtHB12 in root growth,GUS staining analysis showed AtHB7pro:GFP:GUS was mainly expressed in root cap cells and AtHB12pro:GFP:GUS was detected mainly in stele cells.To elucidate whether the transcriptional expression of AtHB7 and AtHB12 induced by aluminum stress is involved in the regulation of root elongation,we examined the root phenotypes of AtHB7 and AtHB12 overexpression lines under aluminum stress.The results showed that 35S:AtHB7 and hb12 had a reduced sensitivity to aluminum treatment,while the hb7 mutant and 35S:AtHB12 were hypersensitive to aluminum stress.After treatment with different ions and pH,it was found that AtHB7 and AtHB12 were specifical responded to aluminum stress and controlled root growth.Although AtHB7 and AtHB12 were involved in the inhibition of root elongation by aluminum stress in the opposite way,both of them showed up-regulated expression of aluminum in the apical transition region.AtHB7 and AtHB12,which may be up-regulated by aluminum stress,integrate different signals or interact with different regulatory factors,respectively,and participate in the regulation of root elongation under aluminum stress conditions.STOP1 is a key transcription factor in plant response to aluminum stress.We found that there was no significant difference in the up-regultion of AtHB7 and AtHB12 between the stop1 mutant and wild-type control,indicating that STOP1 is not involved in the up-regulated expression of AtHB7 and AtHB12 under aluminum stress.We also investigated whether auxin and ethylene signaling are involved in the regulation of AtHB7 and AtHB12 transcriptional expression.The results showed that under aluminum stress,auxin and ethylene signals positively regulated the expression of AtHB7 and negatively regulated the expression level of AtHB12.In summary,on the one hand,the expression of AtHB12 is up-regulated to inhibit root growth under aluminum stress.On the other hand,the up-regulation of AtHB7 alleviates the inhibition of root growth caused by aluminum stress.In response to aluminum stress,both auxin and ethylene control the trade off between growth and inhibition through the regulation of AtHB7 and AtHB12 expression to balance AtHB7 and AtHB12 in vivo.