Background Metal homeostasis is critical for plant growth, development and adaptation to environmental stresses and largely governed by a variety of metal transporters. The plant ZIP ( Z n-regulated transporter, I ron-regulated transporter-like P rotein) family proteins belong to the integral membrane transporters responsible for uptake and allocation of essential and non-essential metals. However, whether the ZIP family members mediate metal efflux and its regulatory mechanism remains unknown. Results In this report, we provided evidence that OsZIP1 is a metal-detoxified transporter through preventing excess Zn, Cu and Cd accumulation in rice. OsZIP1 is abundantly expressed in roots throughout the life span and sufficiently induced by excess Zn, Cu and Cd but not by Mn and Fe at transcriptional and translational levels. Expression of OsZIP-GFP fusion in rice protoplasts and tobacco leaves shows that OsZIP1 resides in the endoplasmic reticulum (ER) and plasma membrane (PM). The yeast ( Saccharomyces cerevisiae ) complementation test shows that expression of OsZIP1 reduced Zn accumulation. Transgenic rice overexpressing OsZIP1 grew better under excess metal stress but accumulated less of the metals in plants. In contrast, both oszip1 mutant and RNA interference (RNAi) lines accumulated more metal in roots and contributed to metal sensitive phenotypes. These results suggest OsZIP1 is able to function as a metal exporter in rice when Zn, Cu and Cd are excess in environment. We further identified the DNA methylation of histone H3K9me2 of OsZIP1 and found that OsZIP1 locus, whose transcribed regions imbed a 242 bp sequence, is demethylated, suggesting that epigenetic modification is likely associated with OsZIP1 function under Cd stress. Conclusion OsZIP1 is a transporter that is required for detoxification of excess Zn, Cu and Cd in rice. Electronic supplementary material The online version of this article (10.1186/s12870-019-1899-3) contains supplementary material, which is available to authorized users.

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Both zinc (Zn) and iron (Fe) are essential micro-nutrients for plant growth and development, yet their levels in plants are tightly regulated to prevent either deficiency or phytotoxicity. In agronomic reality, such an imbalance of metal bioavailability to crops occurs frequently. Thus, mining genetic resources to improve crop traits relevant to metal homeostasis is a great challenge to ensure crop yield and food quality. This study functionally identified an uncharacterized metallochaperone family HIPP protein gene Heavy Metal Associated Isoprenylated Plant Proteins 33 (OsHIPP33) in rice (Oryza sativa). OsHIPP33 resides in the nucleus and plasma membrane and constitutively expresses throughout the lifespan. Transcription of OsHIPP33 is not induced by deprivation of Zn and Fe but upregulated under excessive Zn and Fe stress. In a short-term (one month) hydroponic study with the normal Zn and Fe supply, there were no significant changes in the growth and metal accumulation between the knockout (OsHIPP33) or knockdown (RNA interference) mutant lines and wild-type, while the long-term field trials (for two successive years) demonstrated that the mutation of OsHIPP33 significantly compromised the rice growth and development (such as rice leave tissues, panicle length, spikelet fertility, seed weight per plant, 1000-grain weight, etc.), with the mature grain yield of OsHIPP33 and RNAi lines reduced by 52% and 12–15% respectively, compared with wild-type. Furthermore, the accumulation of Zn and Fe in rice straw, husk and brown rice was also reduced. These results suggest that the disruption of OsHIPP33 can dampen rice agronomic traits, signifying that OsHIPP33 expression is required for Zn and Fe homeostasis and subsequent production of rice grains.

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