A group from CUHK Shenzhen Research Institute, No. 10 Yuexing 2nd Road, Nanshan, Shenzhen, China, has reported that rice Phosphate Response Regulator 2 (PHR2) is required for root colonization by AM fungi (AMF). Root colonization of PHR2 mutants is drastically reduced at low Phosphate (Pi), while ectopic PHR2 expression partially rescues root colonization at high Pi.
https://www.nature.com/articles/s41467-022-27976-8
When plants obtain sufficient phosphate, SPX (SYG1/PHO81/XPR1) proteins prevent nuclear translocation of PHR2, as well as PHR2 binding to promoters of phosphate starvation -induced genes including AM relevant genes. This causes low exudation of strigolactone and poor expression of genes required for perception of Myc-Factors and fungal entry, thereby preventing full symbiosis development.
Upon phosphate starvation, SPX proteins, are degraded. Consequently, PHR2 is active, can bind to P1BS elements in promoters, and transcriptionally activate genes important for AM, such as CCD7 involved in strigolactone biosynthesis for activation of the fungus in the rhizosphere prior to contact, genes encoding receptors involved in the perception of fungal signals prior to root contact such as CERK1 which is necessary for the establishment of the mycorrhizal interaction as well as for resistance to the rice blast fungus, and SYMRK which is required for both fungal and bacterial recognition, the transcription factor NSP2, ZAS involved in apocarotenoid biosynthesis promoting root colonization, and the AM-specific phosphate transporter gene PT11 (localized to the peri-arbuscular membrane) required for Pi uptake from the fungus. Consequently, at low phosphate, roots exude increased amounts of strigolactone and can perceive fungal signals, the fungus is activated to colonize the roots and the symbiosis can function through nutrient transporters localizing to the peri-arbuscular membrane. Thus, symbiosis establishment appears to be enabled as a result of the PHR2-regulated phosphate starvation response.
Arb.: Arbuscules; Ves.: Vesicles; phr2(C): CRISPR-Cas9 generated phr2 mutant