Background: 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.
Results: 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.
Conclusions: OsPHR3 plays a pivotal role in regulating the homeostasis of N independent of Pi availability.
Keywords: Arabidopsis; Nitrogen variants; OsPHR3; Phosphate; Pi availability; Rice.