The roles of selectivity filters in determining aluminum transport by AtNIP1;2

Structural assessment of OsNIP2;1 highlighted critical residues defining solute specificity and functionality of NIP class aquaporins

INTRODUCTION

Nodulin-26-like intrinsic proteins (NIPs) are integral membrane proteins belonging to the aquaporin family, that facilitate the transport of neutral solutes across the bilayer. The OsNIP2;1 a member of NIP-III class of aquaporins is permeable to beneficial elements like silicon and hazardous arsenic. However, the atomistic cross-talk of these molecules traversing the OsNIP2;1 channel is not well understood.

OBJECTIVE

Due to the lack of genomic variation but the availability of high confidence crystal structure, this study aims to highlight structural determinants of metalloid permeation through OsNIP2;1.

METHODS

The molecular simulations, combined with site-directed mutagenesis were used to probe the role of specific residues in the metalloid transport activity of OsNIP2;1.

RESULTS

We drew energetic landscape of OsNIP2;1, for silicic and arsenous acid transport. Potential Mean Force (PMF) construction illuminate three prominent energetic barriers for metalloid passage through the pore. One corresponds to the extracellular molecular entry in the channel, the second located on ar/R filter, and the third size constriction in the cytoplasmic half. Comparative PMF for silicic acid and arsenous acid elucidate a higher barrier for silicic acid at the cytoplasmic constrict resulting in longer residence time for silicon. Furthermore, our simulation studies explained the importance of conserved residues in loop-C and loop-D with a direct effect on pore dynamics and metalloid transport. Next we assessed contribution of predicted key residues for arsenic uptake, by functional complementation in yeast. With the aim of reducing arsenic uptake while maintaining beneficial elements uptake, we identified novel OsNIP2;1 mutants with substantial reduction in arsenic uptake in yeast.

CONCLUSION

We provide a comprehensive assessment of pore lining residues of OsNIP2;1 with respect to metalloid uptake. The findings will expand mechanistic understanding of aquaporin's metalloid selectivity and facilitate variant interpretation to develop novel alleles with preference for beneficial metalloid species and reducing hazardous ones.

The plasma membrane-localized OsNIP1;2 mediates internal aluminum detoxification in rice

Aluminum (Al) toxicity significantly restricts crop production on acidic soils. Although rice is highly resistant to Al stress, the underlying resistant mechanisms are not fully understood. Here, we characterized the function of OsNIP1;2, a plasma membrane-localized nodulin 26-like intrinsic protein (NIP) in rice. Aluminum stress specifically and quickly induced OsNIP1;2 expression in the root. Functional mutations of OsNIP1;2 in two independent rice lines led to significantly enhanced sensitivity to Al but not other metals. Moreover, the Osnip1;2 mutants had considerably more Al accumulated in the root cell wall but less in the cytosol than the wild-type rice. In addition, compared with the wild-type rice plants, the Osnip1;2 mutants contained more Al in the root but less in the shoot. When expressed in yeast, OsNIP1;2 led to enhanced Al accumulation in the cells and enhanced sensitivity to Al stress, suggesting that OsNIP1;2 facilitated Al uptake in yeast. These results suggest that OsNIP1;2 confers internal Al detoxification via taking out the root cell wall's Al, sequestering it to the root cell's vacuole, and re-distributing it to the above-ground tissues.

AtTIP2;2 facilitates resistance to zinc toxicity via promoting zinc immobilization in the root and limiting root-to-shoot zinc translocation in Arabidopsis thaliana

Zinc (Zn) is an essential micronutrient for plants. However, excess Zn is toxic to non-accumulating plants like Arabidopsis thaliana. To cope with Zn toxicity, non-accumulating plants need to keep excess Zn in the less sensitive root tissues and restrict its translocation to the vulnerable shoot tissues, a process referred to as Zn immobilization in the root. However, the mechanism underlying Zn immobilization is not fully understood. In Arabidopsis, sequestration of excess Zn to the vacuole of root cells is crucial for Zn immobilization, facilitated by distinct tonoplast-localized transporters. As some members of the aquaporin superfamily have been implicated in transporting metal ions besides polar but non-charged small molecules, we tested whether Arabidopsis thaliana tonoplast intrinsic proteins (AtTIPs) could be involved in Zn immobilization and resistance. We found that AtTIP2;2 is involved in retaining excess Zn in the root, limiting its translocation to the shoot, and facilitating its accumulation in the leaf trichome. Furthermore, when expressed in yeast, the tonoplast-localized AtTIP2;2 renders glutathione (GSH)-dependent Zn resistance to yeast cells, suggesting that AtTIP2;2 facilitates the across-tonoplast transport of GSH-Zn complexes. Our findings provide new insights into aquaporins' roles in heavy metal resistance and detoxification in plants.

Involvement of cytokinins in STOP1-mediated resistance to proton toxicity

STOP1 (sensitive to proton rhizotoxicity1) is a master transcription factor that governs the expression of a set of regulatory and structural genes involved in resistance to aluminum and low pH (i.e., proton) stresses in Arabidopsis. However, the mechanisms and regulatory networks underlying STOP1-mediated resistance to proton stresses are largely unclear. Here, we report that low-pH stresses severely inhibited root growth of the stop1 plants by suppressing root meristem activities. Interestingly, the stop1 plants were less sensitive to exogenous cytokinins at normal and low pHs than the wild type. Significantly, low concentrations of cytokinins promoted root growth of the stop1 mutant under low-pH stresses. Moreover, lateral and adventitious root formation was stimulated in stop1 and by low-pH stresses but suppressed by cytokinins. Further studies of the expression patterns of a cytokinin signaling reporter suggest that both the loss-of-function mutation of STOP1 and low-pH stresses suppressed cytokinin signaling outputs in the root. Furthermore, the expression of critical genes involved in cytokinin biosynthesis, biodegradation, and signaling is altered in the stop1 mutant in response to low-pH stresses. In conclusion, our results reveal a complex network of resistance to low-pH stresses, which involves coordinated actions of STOP1, cytokinins, and an additional low-pH-resistant mechanism for controlling root meristem activities and root growth upon proton stresses.