Molecular hydrogen positively influences root gravitropism involving auxin signaling and starch accumulation

Although geoscience of natural hydrogen (H₂), hydrogen-producing soil bacteria, and especially plant-based H₂, has been observed, it is not clear whether or how above H₂ resources influence root gravitropic responses. Here, pharmacological, genetic, molecular, and cell biological tools were applied to investigate how plant-based H₂ coordinates gravity responses in Arabidopsis roots. Since roots show higher H₂ production than shoots, exogenous H₂ supply was used to mimic this function. After H₂ supplementation, the asymmetric expression of the auxin-response reporter DR5 driven by auxin influx and efflux carriers, and thereafter positive root gravitropism were observed. These positive responses in root gravitropism were sensitive to auxin polar transport inhibitors, and importantly, the defective phenotypes observed in aux1-7, pin1, and pin2 mutants were not significantly altered by exogenous H₂. The observed starch accumulation was matched with the reprogramming gene expression linked to starch synthesis and degradation. Transgenic plants expressing hydrogenase1 (CrHYD1) from Chlamydomonas reinhardtii not only displayed higher endogenous H₂ concentrations, the inducible AUX1 gene expression and starch accumulation, but also showed pronounced root gravitropism. Collectively, above evidence preliminarily provides a framework for understanding the molecular basis of the possible functions of both plant/soil-based and nature H₂ in root architecture.