The phosphate starvation response regulator PHR2 antagonizes arbuscule maintenance in Medicago
• Phosphate starvation response (PHR) transcription factors play essential roles in regulating phosphate uptake in plants through binding to the P1BS cis-element in the promoter of phosphate starvation response genes. Recently, PHRs were also shown to positively regulate arbuscular mycorrhizal colonization in rice and lotus by controlling the expression of a large set of symbiotic genes. However, their role in arbuscule development has remained unclear. In Medicago we previously showed that arbuscule degradation is controlled by two SPX proteins that are highly expressed in arbuscule-containing cells. Since SPX proteins bind to PHRs and repress their activity in a phosphate-dependent manner, we investigated whether arbuscule maintenance is also regulated by PHR.  • Here, we show that MtPHR2 is a major regulator of the phosphate starvation response in Medicago. Knock-out of mtphr2 showed reduced phosphate starvation response, symbiotic gene expression and AM colonization levels. However, the arbuscules that formed showed less degradation, suggesting a negative role for PHR2 in arbuscule maintenance. This was supported by the observation that overexpression of MtPHR2 led to enhanced degradation of arbuscules. Although many arbuscule-induced genes contain P1BS elements in their promoters, we found that the P1BS cis-elements in the promoter of the symbiotic phosphate transporter MtPT4 are not required for arbuscule-containing cell specific expression. • These results indicate that MtSPX1 and MtSPX3 control arbuscule maintenance independent from PHR2. While MtPHR2 potentiates symbiotic gene expression and colonization, its activity in arbuscule-containing cells needs to be tightly controlled to maintain a successful symbiosis in Medicago. Overall design: To investigate gene differential expression between wild type R108 and Mtphr2 mutant under mycorrizal condition.