Lysigenous aerenchyma, which is created by death and subsequent lysis of the cortical cells in roots, is essential for internal oxygen diffusion from shoot to the submerged root tips. Previously, we showed that rice (Oryza sativa) has higher cortex to stele ratio (CSR) and higher aerenchyma to cortex ratio (ACR) than wheat (Triticum aestivum) and maize (Zea mays ssp. mays). Moreover, rice roots increase CSR and ACR substantially more than wheat and maize roots in response to low-oxygen conditions. As root cortex is the tissue in which aerenchyma formation occurs, large CSR enhances oxygen diffusion in rice roots and reduces the respiratory costs per unit of root length of the root under soil flooding. Indeed, thicker rice roots with larger cortex area can transport more oxygen into the root tips than thinner rice roots. Interestingly, the stele area does not differ between under aerobic and low-oxygen conditions, and thus the cortex expansion is most important for the root adaptation to soil flooding. Nevertheless, the molecular mechanisms that underlie root cortex expansion in response to low-oxygen conditions remains unclear. In this presentation, we show that adventitious roots of a rice mutant defects in auxin signaling have smaller cortex area than those of its background wild type. Moreover, the level of cortex expansion in response to low-oxygen conditions is also reduced in this mutant. From these results, we will discuss how CSR is regulated in rice roots under flooding and non-flooding conditions.