Phosphorus, an essential macronutrient, is taken up by plant roots in the form of inorganic phosphate (Pi) from soil. To deal with phosphate deficiency, plants have highly controlled mechanisms to maintain phosphate homeostasis, which include acquisition of Pi from soil, its storage, remobilization and translocation from roots to shoots, as well as optimization of metabolic processes that use Pi. In rice, root architecture alteration is one of the most important adaptations, which includes elongation of primary and adventitious roots in respose to Pi starvation. Other major players are phosphate transporters, transcription factors and PSI (Phosphate Starvation Induced) genes. The Pi signalling network includes miR399 and PHO2, PHR1 and IPS genes. MiR399 and PHO2 operates downstream of PHR1. IPS transcripts do not have an open reading frame; instead, they contain 23 bp region complementary to miR399 and operate via target mimicry mechanism to regulate PHO2. miR399 moves from shoot to root via phloem and represses PHO2 (Lin et al., 2008; Pant et al., 2008). Repression of PHO2 causes increases in expression of root Pi transporters which facilitate the acquisition of Pi by roots and its translocation from root to shoot (Rouached et al., 2014).
