The EXS domain of rice PHOSPHATE1 (OsPHO1;2) reduces the low phosphate response through jasmonate signaling

Phosphorus (P) is a vital macronutrient essential for plant growth, and its deficiency significantly hampers agricultural productivity. The PHOSPHATE 1 (PHO1) protein family, characterized by an N-terminal SPX domain, four transmembrane (4TM) domains, and a C-terminal EXS domain, is pivotal in transporting phosphate (Pi) from roots to shoots. Rice, PHO1;2 plays a crucial role in the Pi export process, and defects in this gene cause severe growth retardation and Pi deficiency symptoms even when external Pi levels are adequate. This study examined the roles of the EXS domain and the combined 4TM+EXS domains of OsPHO1;2 in supporting plant growth responses, independent of Pi transport activity, as well as their influence on hormone signaling and gene regulation. Using CRISPR/Cas9, rice lines expressing specific OsPHO1;2 domains (EXS or 4TM+EXS) were created by targeted deletion of particular domain-coding regions. Phenotypic analysis under Pi-sufficient and deficient conditions, as well as phosphate profiling, revealed that EXS lines exhibited notably better growth than loss-of-function mutants, ospho1;2, during early development, despite having similar shoot Pi levels to the null mutant. These lines showed lower levels of defense hormones (jasmonic acid) than ospho1;2 but were comparable to those in the wild type. Conversely, 4TM+EXS lines exhibited growth patterns similar to ospho1;2 mutants. RNA sequencing indicated that the phosphate starvation response (PSR) and defense pathways were less pronounced in the EXS lines compared to ospho1;2 mutants. However, both EXS and 4TM+EXS lines showed seed development defects and reduced total phosphorus content in seeds, mirroring the ospho1;2 phenotype. Heterozygous plants carrying one functional OsPHO1;2 allele displayed normal growth and seed development, indicating the mutations recessive nature. The findings suggest that the EXS domain of OsPHO1;2 can promote plant growth independently of Pi transport by decreasing PSR and modulating defense hormone pathways. This further suggests a signaling role for PHO1 domains beyond direct Pi translocation. Overall, these results enhance our understanding of Pi homeostasis and may help form strategies for breeding P-efficient crops.