An Apoplastic H-Type Thioredoxin Is Involved in the Stress Response through Regulation of the Apoplastic Reactive Oxygen Species in Rice

Thioredoxins (Trxs) are a multigenic family of proteins in plants that play a critical role in redox balance regulation through thiol-disulfide exchange reactions. There are 10 members of the h-type Trxs in rice (Oryza sativa), and none of them has been clearly characterized. Here, we demonstrate that OsTRXh1, a subgroup I h-type Trx in rice, possesses reduction activity in vitro and complements the hydrogen peroxide sensitivity of Trx-deficient yeast mutants. OsTRXh1 is ubiquitously expressed in rice, and its expression is induced by salt and abscisic acid treatments. Intriguingly, OsTRXh1 is secreted into the extracellular space, and salt stress in the apoplast of rice induces its expression at the protein level. The knockdown of OsTRXh1 results in dwarf plants with fewer tillers, whereas the overexpression of OsTRXh1 leads to a salt-sensitive phenotype in rice. In addition, both the knockdown and overexpression of OsTRXh1 decrease abscisic acid sensitivity during seed germination and seedling growth. We also analyzed the levels of hydrogen peroxide produced in transgenic plants, and the results show that more hydrogen peroxide is produced in the extracellular space of OsTRXh1 knockdown plants than in wild-type plants, whereas the OsTRXh1 overexpression plants produce less hydrogen peroxide under salt stress. These results show that OsTRXh1 regulates the redox state of the apoplast and influences plant development and stress responses.