Figure 5. Hypothetical model for the role of NLA and PHO2 genes and crosstalk between NO₃⁻ and Pi.

(A) In WT plants, NLA and PHO2 act as repressors for Pi uptake and Pi has a negative interaction with external NO₃⁻. The condition of low NO₃⁻ and high Pi is most conducive for higher Pi uptake. (B) In nla or pho2 mutants, the NLA or PHO2 gene is not functional and these plants have higher Pi uptake compared to WT plants. Low NO₃⁻ and high Pi conditions accelerate Pi accumulation causing Pi toxicity in these mutants. Further, NO₃⁻ and Pi content has antagonistic affects on flowering time. Purple leaves show Pi deficiency leading to anthocyanin accumulation, orange leaves indicate Pi toxicity, solid red circles indicate the Pi uptake system and dotted lines indicate less suppression. The size of the letter ‘Pi’ in leaves and arrow thickness in roots is correlated with Pi content in plants.