Spatial Regulation of Silicon Accumulation in Peduncle Confers Sheathed Spike in Barley

Peduncle, the uppermost internode in cereals, connects the stem to the inflorescence and is critical for the transport of water, nutrients and photosynthetic assimilates. While peduncle length associates with plant height and its elongation is primarily regulated by phytohormones, we report a previously unrecognized mechanism involving the spatial distribution of silicon (Si). We identified a barley mutant, sheathed spike 2 (ss2), characterized by a specifically shortened peduncle that traps the spike within the flag leaf sheath. Positional cloning and analysis of allelic mutants revealed that the wild-type SS2 gene encodes a putative silicon efflux transporter. SS2 is expressed throughout the lifecycle, with higher transcriptional levels in the rachis and stem internodes, and its encoded protein localizes to the plasma membrane. We demonstrate that SS2 is required for polarized Si partitioning. Unlike wild-type plants, which ultimately deposit Si in spikes, the ss2 mutant exhibits an 8-fold increase in Si accumulation in the peduncle and a significant increase in the flag leaf. Hydroponic experiments without Si supply restored normal peduncle elongation in the ss2 mutant, demonstrating that local Si hyper-accumulation directly inhibits elongation. The conserved role of SS2 was supported by diversity analysis across barley and common wheat, as well as by the similar sheathed spike phenotype in tetraploid wheat lines carrying non-functional SS2 homologues. Collectively, our findings uncover an evolutionarily conserved, silicon-dependent mechanism that regulates peduncle elongation and spike emergence in Triticeae crops like barley and wheat.