Rice Protein Domain Boosts Growth by Taming Stress, Not Just Moving Nutrients

Phosphorus (P) is a critical nutrient for plant growth, and its scarcity severely limits agricultural yields. The PHOSPHATE 1 (PHO1) protein family, particularly OsPHO1;2 in rice, is known for its vital role in moving phosphate (Pi) from roots to shoots. Defects in this protein lead to severe growth problems and Pi deficiency, even when enough Pi is available externally. This study aimed to investigate the specific functions of the EXS domain and the combined 4TM+EXS domains of OsPHO1;2, focusing on their contribution to plant growth, their independence from direct Pi transport, and their influence on hormone signaling and gene regulation.

Researchers employed CRISPR/Cas9 gene editing to create rice lines expressing only the EXS or 4TM+EXS domains by precisely deleting other coding regions. Under varying Pi conditions, lines with the isolated EXS domain showed significantly improved growth during early development compared to full loss-of-function mutants, even though their shoot Pi levels were similar to the deficient mutants. Crucially, these EXS lines displayed lower levels of defense hormones like jasmonic acid, mirroring wild-type plants, and exhibited a less pronounced phosphate starvation response and defense pathway activity, as revealed by RNA sequencing. As lead author Mani notes in the paper, "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 implies the EXS domain promotes growth by modulating stress responses, rather than directly transporting Pi.

In contrast, lines expressing the 4TM+EXS domains behaved similarly to the complete loss-of-function mutants. While the EXS domain showed clear benefits for early growth, both EXS and 4TM+EXS lines shared common defects with the null mutant, such as issues with seed development and reduced total phosphorus in seeds. The recessive nature of the mutation was confirmed by the normal development of heterozygous plants. These findings highlight a novel signaling role for the EXS domain of OsPHO1;2, moving beyond its traditional transport function. This expanded understanding of Pi homeostasis could pave the way for breeding crops that are more efficient at utilizing phosphorus.