Effect of Rice PCR1 on Grain Weight and Zn Accumulation

Zn is an essential element that all living organisms require for growth and development. Zn-containing proteins perform the functions of transcriptional regulators or showed enzymatic or transport activities. Some kind of Zn-binding proteins, like WRKY transcription factor in Arabidopsis and ubiquitin-related proteins in rice, are known to regulate organ size. Moreover, proteins containing a placenta-specific 8 domain (PLAC8) function as major organ size regulators in Solanum lycopersicum and Zea may, and putative metal ion transporters in Arabidopsis thaliana, Oryza sativa and Brassica juncea.  A. thaliana plant cadmium resistance 2 (AtPBR2), containing a PALC8 domain was implied to act as a Zn efflux transporter, and tomato fruit weight 2.2 (fw2.2) and maize cell number regulator 1 (ZmCNR1), also containing the PALC8 domain regulate fruit size. However, there is no reports documented a PALC8 containing protein that functions in organ size regulation and metal homeostasis.

Here, we found that plant cadmium resistance 1 (PCR1) influences both zinc (Zn) accumulation and grain weight in rice. Grain weight of OsPCR1 knockout and knockdown lines decreased than the wild type, while OsPCR1 overexpression lines produced heavier grains. Furthermore, the grains of OsPCR1 knockdown lines exhibited substantially higher Zn and lower cadmium (Cd) concentrations than the control. We identified some variation in the OsPCR1 amino acid sequence between the japonica and indica rice types using 15 different rice varieties. Japonica-type PCR1 had a shorter N-terminus than did PCR1 in the other rice types. Furthermore, japonica-type grains accumulated less Zn than did indica-type grains. Our study suggests that rice PCR1 maintains metal ion homeostasis and grain weight and might have been selected for during domestication.