Regulation of Root Tissue Size and Adaptations to Hypoxia

Soil flooding restricts gas exchange between the air and the soil. Thus, the oxygen level gradually decreases in the soil through the respiration of plant roots and microorganisms. Internal gas spaces consisting of aerenchyma and intercellular spaces in the root cortex are essential for efficient oxygen diffusion from the shoot to the root tips. While many studies focus on the contribution of aerenchyma formation to internal oxygen diffusion, the size of the cortex, which provides a space for the formation of aerenchyma and intercellular space, should be also essential for plant adaptation to low-oxygen conditions. Indeed, rice (Oryza sativa L.) and other wetland species have a much higher cortex-to-stele ratio when compared with upland species, such as maize (Zea mays ssp. mays) and wheat (Triticum aestivum). As the root-tissue size shows plasticity in response to low-oxygen conditions, it could be a key for plants to overcome the trade-off between oxygen diffusion through the gas spaces in the cortex and water uptakes through the xylem vessels in the stele. In this chapter, I summarize the importance of root anatomical adaptation to soil flooding and discuss possible mechanisms underlying root tissue-size controls based on the fundamental mechanisms of root radial patterning.