**Masato Ejiri, Yuto Sawazaki and Katsuhiro Shiono \***

**Aerated Conditions**

Graduate School of Bioscience and Biotechnology, Fukui Prefectural University, 4-1-1 Matsuoka-Kenjojima, Eiheiji, Fukui 910-1195, Japan; s2093001@g.fpu.ac.jp (M.E.); s2073004@g.fpu.ac.jp (Y.S.)

**\*** Correspondence: shionok@fpu.ac.jp; Tel.: +81-776-61-6000

Received: 29 May 2020; Accepted: 10 July 2020; Published: 13 July 2020

**Abstract:** A barrier to radial oxygen loss (ROL), which reduces the loss of oxygen transported via the aerenchyma to the root tips, enables the roots of wetland plants to grow into anoxic/hypoxic waterlogged soil. However, little is known about its genetic regulation. Quantitative trait loci (QTLs) mapping can help to understand the factors that regulate barrier formation. Rice (*Oryza sativa*) *inducibly* forms an ROL barrier under stagnant conditions, while a few wetland plants *constitutively* form one under aerated conditions. Here, we evaluated the formation of a constitutive ROL barrier in a total of four accessions from two wild rice species. Three of the accessions were wetland accessions of *O. glumaepatula*, and the fourth was a non-wetland species of *O. rufipogon*. These species have an AA type genome, which allows them to be crossed with cultivated rice. The three *O. glumaepatula* accessions (W2165, W2149, and W1183) formed an ROL barrier under aerated conditions. The *O. rufipogon* accession (W1962) did not form a constitutive ROL barrier, but it formed an inducible ROL barrier under stagnant conditions. The three *O. glumaepatula* accessions should be useful for QTL mapping to understand how a constitutive ROL barrier forms. The constitutive barrier of W2165 was closely associated with suberization and resistance to penetration by an apoplastic tracer (periodic acid) at the exodermis but did not include lignin at the sclerenchyma.

**Keywords:** apoplastic barrier; barrier to radial oxygen loss (ROL); lignin; *Oryza glumaepatula*; *O. rufipogon*; rice (*O. sativa*); suberin; wild rice
