Shoot-Silicon-Signal protein to regulate root silicon uptake in rice
Naoki Yamaji, Namiki Mitani‐Ueno, Toshiki Fujii, Tomonori Shinya, Ji Feng Shao, Shota Watanuki, Yasunori Saitoh, Jian Feng
Abstract
Plants accumulate silicon to protect them from biotic and abiotic stresses. Especially in rice (Oryza sativa), a typical Si-accumulator, tremendous Si accumulation is indispensable for healthy growth and productivity. Here, we report a shoot-expressed signaling protein, Shoot-Silicon-Signal (SSS), an exceptional homolog of the flowering hormone “florigen” differentiated in Poaceae. SSS transcript is only detected in the shoot, whereas the SSS protein is also detected in the root and phloem sap. When Si is supplied from the root, the SSS transcript rapidly decreases, and then the SSS protein disappears. In sss mutants, root Si uptake and expression of Si transporters are decreased to a basal level regardless of the Si supply. The grain yield of the mutants is decreased to 1/3 due to insufficient Si accumulation. Thus, SSS is a key phloem-mobile protein for integrating root Si uptake and shoot Si accumulation underlying the terrestrial adaptation strategy of grasses. Shoot-Silicon-Signal (OsFTL12) protein is expressed only in the rice shoot response to Si deficiency. Yet, the protein is also detected in phloem sap and root, stimulating the root Si uptake. This signal integrates shoot Si demand and root Si uptake.