OsPHR3 affects the traits governing nitrogen homeostasis in rice

Phosphate (Pi) and Nitrogen (N) are essential macronutrients required for plant growth and development. In Arabidopsis thaliana (Arabidopsis), the transcription factor PHR1 acts as a Pi central regulator. PHL1 is a homolog of PHR1 and also plays a role in maintaining Pi homeostasis. In rice (Oryza sativa), OsPHR1-4 are the orthologs of PHR1 and have been implicated in regulating sensing and signaling cascades governing Pi homeostasis.

Here the role of OsPHR3 was examined in regulating the homeostasis of N under different Pi regimes. Deficiencies of different variants of N exerted attenuating effects on the relative expression levels of OsPHR3 in a tissue-specific manner. For the functional characterization of OsPHR3, its Tos17 insertion homozygous mutants i.e., osphr3-1, osphr3-2, and osphr3-3 were compared with the wild-type for various morphophysiological and molecular traits during vegetative (hydroponics with different regimes of N variants) and reproductive (pot soil) growth phases. During vegetative growth phase, compared with the wild-type, OsPHR3 mutants showed significant variations in the adventitious root development, influx rates of ¹⁵N-NO₃^- and ¹⁵N-NH₄⁺, concentrations of total N, NO₃^- and NH₄⁺ in different tissues, and the relative expression levels of OsNRT1.1a, OsNRT2.4, OsAMT1;1, OsNia1 and OsNia2. The effects of the mutation in OsPHR3 was also explicit on the seed-set and grain yield during growth in a pot soil. Although Pi deficiency affected total N and NO₃^- concentration, the lateral root development and the relative expression levels of some of the NO₃^- and NH₄⁺ transporter genes, its availability did not exert any notable regulatory influences on the traits governing N homeostasis.

OsPHR3 plays a pivotal role in regulating the homeostasis of N independent of Pi availability.