OsSEC24, a functional SEC24-like protein in rice, improves tolerance to iron deficiency and high pH by enhancing H+ secretion mediated by PM-H+-ATPase

Iron is abundant in the soil, but its low solubility in neutral or alkaline soils limits its uptake. Plants can rely on rhizosphere acidification to increase iron solubility. OsSEC27p was previously found to be a highly up-regulated gene in iron-deficient rice roots. Here, pH-dependent complementation assays using yeast mutants sec24Δ/SEC24 and sec27Δ/SEC27 showed that OsSEC27 could functionally complement SEC24 but not SEC27 in yeast; thus, it was renamed as OsSEC24. We found that OsSEC24-transgenic tobacco plants increased the length and number of roots under iron deficiency at pH 8.0. To explore how OsSEC24 confers tolerance to iron deficiency, we utilized transgenic tobacco, rice and rice protoplasts. H+ flux measurements using Non-invasive Micro-test Technology (NMT) indicated that the transgenic OsSEC24 tobacco and rice enhanced H+ efflux under iron deficiency. Conversely, the application of plasma membrane PM-H+-ATPase inhibitor vanadate elucidated that H+ secretion increased by OsSEC24 was mediated by PM-H+-ATPase. OsPMA2 was used as a representative of iron deficiency-responsive PM-H+-ATPases in rice root via RT-PCR analysis. In transgenic rice protoplasts OsPMA2 was packaged into OsSEC24 vesicles after export from the ER through confocal-microscopy observation. Together, OsSEC24 vesicles, along with PM-H+-ATPases stimulate roots formation under iron deficiency by enhancing rhizosphere acidification.