The Tonoplast-Localized Transporter MTP8.2 Contributes to Manganese Detoxification in the Shoots and Roots of Oryza sativa L.

Abstract

Manganese (Mn) cation diffusion facilitators (Mn-CDFs) play important roles in the Mn homeostasis of plants. In rice, the tonoplast-localized Mn-CDF metal tolerance protein 8.1 (MTP8.1) is involved in Mn detoxification in the shoots. This study functionally characterized the Mn-CDF MTP8.2 and determined its contribution to Mn tolerance. MTP8.2 was found to share 68% identity with MTP8.1 and was expressed in both the shoots and roots, but its transcription level was lower than that of MTP8.1. Transient expression of the MTP8.2:green fluorescent protein (GFP) fusion protein and immunoblotting studies indicated that MTP8.2 was also localized to the tonoplast. MTP8.2 expression in yeast conferred tolerance to Mn but not to Fe, Zn, Co, Ni or Cd. MTP8.2 knockdown caused further growth reduction of shoots and roots in the mtp8.1 mutant, which already exhibits stunted growth under conditions of excess Mn. In the presence of high Mn, the MTP8.2 knockdown lines of the mtp8.1 mutant showed lower root Mn concentrations, as well as lower root:total Mn ratios, than those of wild-type rice and the mtp8.1 mutant. These findings indicate that MTP8.2 mediates Mn tolerance along with MTP8.1 through the sequestration of Mn into the shoot and root vacuoles.

Manganese in Plants: From Acquisition to Subcellular Allocation

“…OsMTP8.1 was highly expressed in older leaves, and it was induced and repressed by high and low Mn 2+ levels, respectively (Chen et al, 2013). OsMTP8.2 was expressed in roots and shoots, and it showed lower expression levels than OsMTP8.1 (Takemoto et al, 2017). The mtp8.1mtp8.2 double mutant suffered from necrotic leaf blades.…”

Rice reduces Mn uptake in response to Mn stress

“…4 Knockout of MTP8.1 resulted in significant growth reduction in addition to chlorosis and necrosis in leaves under high Mn conditions, 3 whereas no obvious Mn toxicity symptoms were presented in mtp8.2 leaves. 4 However, when MTP8.2 was knocked down in mtp8.1, the mutants (mtp8.2i/mtp8.1) exhibited higher Mn sensitivity than did mtp8.1, 4 indicating that MTP8.2 is also important for Mn tolerance in shoots. In roots, MTP8.1 and MTP8.2 were expressed at similar levels regardless of Mn condition.…”

“…2 We recently reported that Mn in old leaves is detoxified by two vacuolar transporters, MTP8.1 and MTP8.2. 3,4 Both MTP8.1 and MTP8.2 are constitutively expressed at higher levels in old than in young leaves, but the level of MTP8.1 is several times higher than that of MTP8.2. 4 Knockout of MTP8.1 resulted in significant growth reduction in addition to chlorosis and necrosis in leaves under high Mn conditions, 3 whereas no obvious Mn toxicity symptoms were presented in mtp8.2 leaves.…”

“…In roots, MTP8.1 and MTP8.2 were expressed at similar levels regardless of Mn condition. 4 MTP8.1 and/or MTP8.2 disruption led to impaired root elongation under excess Mn, 4 indicating that vacuolar sequestration by the transporters plays a crucial role in Mn detoxification in roots. Interestingly, the mtp8.2i/mtp8.1 mutants accumulated significantly lower amounts of Mn than did the wild type under excess Mn.…”

“…Interestingly, the mtp8.2i/mtp8.1 mutants accumulated significantly lower amounts of Mn than did the wild type under excess Mn. 4 However, these transporters are not involved in Mn uptake by roots. We hypothesized that rice plants may respond to excess Mn by reducing Mn uptake, thereby enhancing Mn tolerance.…”