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Phosphate (Pi) is an essential nutrient for all organisms. Root are underground organs, but the majority of the root biology studies have been done growing the root system in presence of light.Root illumination alters the Pi starvation response (PSR) at different levels. Thus, we have analyzed morphological, transcriptional and physiological responses to Pi starvation in dark?grown roots.We have identified new genes and pathways regulated by Pi starvation that were not described previously. We also show that Pi?starved plants increase the cis?zeatin (cZ)/trans?zeatin (tZ) ratio. Transcriptomic analyses show that tZ preferentially represses cell cycle and PSR genes while cZ induces genes involved in cell and root hair elongation and differentiation. In fact, cZ?treated seedlings show longer root system as well as longer root hairs than tZ?treated, increasing the total absorbing surface. Mutants with low cZ levels do not allocate free Pi in roots during Pi starvation.We propose that Pi?starved plants increase the cZ/tZ ratio to maintain basal CK responses and allocate Pi in the root system to sustain its growth. Therefore, cZ acts as a Pi?starvation response hormone that stimulates root and root hair elongation to enlarge root absorbing surface and to increase Pi levels in roots.