Overexpression of the MYB-related transcription factor GCC7 in Arabidopsis thaliana leads to increased levels of P-i and changed P-dependent gene regulation

A proper concentration and turnover of inorganic phosphate (P-i) is essential to maintain cellular processes. Consequently, plants have mechanisms to control P-i homeostasis and to alleviate P-i limitation. The MYB-related transcription factor, PHR1, is important for gene induction during P-i starvation. PHR1 belongs to a family, characterised by the presence of a GARP-and a coiled coil domain. We propose that this family, with 15 members in Arabidopsis thaliana (L.) Heynh., be termed the GCC-family. In this study, transgenic plants overexpressing one member, GCC7, and a T-DNA knockout mutant, gcc7, are characterised. We find overexpressor plants to accumulate more P-i in shoots, irrespective of the P-i supply. Therefore, GCC7 was characterised in relation to P-i starvation. We conclude that GCC7 is not strictly required for a P-starvation response since the gcc7 mutant responds to P-i limitation. However, overexpression of GCC7 strongly interferes with the P-starvation response, abolishing induction of several P-responsive genes including AT4, IPS1 and several P-transporter genes, whereas GCC7 does not directly interfere with the PHR1(GCC1) dependent regulation of miR399d. Thus GCC7 influences P-accumulation and P-dependent gene regulation, but GCC7 has a function entirely different from PHR1.