The Molecular Mechanism Of BR Signaling Regulate Plant Phosphate Deficiency Response

Phosphorus(P)is one essential macronutrient for plant growth and development.When phosphate supply is insufficient,plants will rapidly activate phosphatedeficiency adaptive responses,including increasing root secretion of organic acids and remodeling root architecture.STOP1(Sensitivity To Proton Rhizotoxicity 1)is a transcription factor with zinc finger structure,and ALMT1(Aluminium-activated Malate Transporter 1)is a transporter of malate secreted by roots.The signal transduction pathway of STOP1-ALMT1 is not only working under aluminum toxicity condition,but also phosphate deficiency.Phosphate-deficiency-induced malate secretion plays significant role in the utilization of fixed inorganic phosphate in soil and regulates the root architecture of plants under phosphate deficiency conditions.Brassinosteroid(BR),a hormone widely present in higher plants,plays vital role in plant growth and development and nutritional stress.Previous studies have shown that phosphate deficiency affects the synthesis of BR,and BR signaling also regulates phosphate deficiency.However,it is currently unclear how BR signaling participates in the process of phosphate deficiency.Therefore,research the response mechanism of BR signaling regulating plant phosphate deficiency which is greatly significant in plant phosphate signaling regulatory network completeness.In this study,Arabidopsis thaliana and Oryza sativa were used as the research objects to explore the molecular mechanism of BZR1(Brassinazole-Resistant 1),the circumnuclear transcription factor in BR signaling,regulating malate secretion under phosphate deficiency.Meanwhile,serine carboxypeptidase BRS1(BRI1 Suppressor 1)involved in BR signaling and root architecture changes under phosphorus deficiency condition.The main conclusions obtained as follows:1.BR is involved in the response to phosphate deficiency in ArabidopsisUnder phosphate deficiency,the relative expression levels of key genes At DWF4,At BR6ox1 and At DET2 in the BR synthesis pathway were significantly inhibited.At the same time,the cell membrane receptor At BRI1 and transcription factor At BZR1 that mediate BR signaling were also affected significant inhibition of phosphate deficiency.Correspondingly,the addition of BR can partially restore the phenotype of inhibition of primary root growth by phosphate deficiency,while the addition of BR inhibitors aggravated the primary root growth.In addition,the gain-of-function mutant of BZR1(bzr1-D)also significantly suppressed the shortened taproot phenotype under phosphate deficiency.These results indicated that BR signaling was involved in the response to phosphate deficiency in Arabidopsis.2.BZR1 inhibits ALMT1 expression and malate secretionTo research how BR signaling regulates taproot growth under phosphate deficiency,the expression of key genes controlling taproot growth under phosphate deficiency was examined in bzr1-D mutants.The results showed that both At ALMT1 and At LPR1 were significantly inhibited by BZR1.The direct inhibitory effect of BZR1 on At ALMT1 expression was further demonstrated by root staining and yeast onehybrid of pro At ALMT1::GUS material.Correspondingly,BZR1 also inhibited phosphate deficiency-induced malate secretion,while exogenous addition of malate could restore the phosphate deficiency phenotype of bzr1-D.Competitive expression experiments of STOP1 and BZR1 proved that these two transcription factors compete to bind the promoter of ALMT1 to regulate gene expression.The Arabidopsis transgenic material 35S::BZR1::m Cherry/UBQ10::STOP1::GFP was used to further prove that STOP1 and BZR1 regulate the process of plant malate secretion through the competitive mechanism.The BZR1-Os ALMT1 regulatory pathway existed in rice,and the secretion of malate in BZR1-D was also inhibited under phosphate deficiency,but no direct effect on root growth.3.BRS1 is involved in the regulation of root architecture of plants under phosphate deficiency stressStudies have shown that an At BRS1 gain-of-function mutant(brs1-D)inhibits the growth phenotype of bri1,the Arabidopsis BR receptor mutant,but the underlying molecular mechanism is unclear.In this study,we found that the expression of At BRS1 was also significantly induced by phosphate deficiency at the transcriptional level,and the brs1-D mutant exhibited obvious phenotypes of blocked primary root growth and increased root hair development under phosphate deficiency stress,so At BRS1 may mediate BR signal transduction under phosphate deficiency and participate in the regulation of root architecture.Since At BRS1 is a cell wall-localized carboxypeptidase protein,it was predicted that this protein interacts with multiple cell wall proteins,indicating that this protein may be involved in regulating the structure of the cell wall and thus affect the phosphate deficiency response process of plant roots.Secretoproteomics found that four At BRS1 homologous carboxypeptidase proteins in rice were also significantly induced by phosphate deficiency signals,but the molecular mechanism of their functions and effects still needs to be further detect.Taken together,this study demonstrates that BR signaling,through the transcription factor BZR1,negatively regulates malate secretion in plants.Under phosphate deficiency,plants can activate physiological processes such as malate secretion by inhibiting the synthesis and signal transduction of BR,and at the same time activate some positive regulators,thereby remodeling the root system architecture and improving phosphate deficiency adaptability.