Suberin, Glycerol Esters and Iron Plaques: Key Apoplastic Barriers in Rice Roots

Rice (Oryza sativa) is predominantly cultivated in flooded conditions, a practice known as wetland or paddy rice farming, which is crucial for achieving optimal growth and high yields. However, rice cultivation in flooded soils poses several challenges, including anoxic conditions and high iron (Fe) concentrations (Zhong et al. 2024), especially in acidic soils. Under these conditions, Fe is reduced to its soluble and bioavailable ferrous form (Fe²⁺), increasing the risk of phytotoxicity. Though all flooded soils are characterized by low oxygen (O₂) availability, not all of them exhibit high Fe concentrations. In contrast, neutral or alkaline flooded soils tend to precipitate Fe as insoluble hydroxides or oxides (e.g., Fe(OH)₃), limiting its bioavailability. To cope with these hypoxic conditions, rice has developed key adaptations, including an efficient internal aeration mechanism facilitated by aerenchyma tissues (large and interconnected air-filled spaces), which facilitate the long-distance (shoot-to-root) transport of O₂, hence enhancing respiration in submerged roots. Additionally, the development of root barriers, such as the formation of suberized layers in the exodermis, serves to impede radial oxygen loss (ROL), preventing the diffusion of O₂ from the root into the O₂-deprived rhizosphere (Li et al. 2024). These barriers allow effective longitudinal O₂ diffusion to support respiration and nutrient uptake while also preventing the entry of toxic compounds into the roots (Zandi et al. 2023). This intricate interplay between rice roots and their surrounding environment has long fascinated researchers, particularly regarding their ability to adapt to stressful conditions such as low O₂ levels and elevated Fe concentrations in flooded soils. A recent study by Jiménez et al. (2025) delves deep into the effects of high Fe conditions on the formation of apoplastic barriers in rice roots, which are essential for regulating water and gas exchange in the plant. Specifically, researchers investigate the formation of apoplastic barriers in rice roots grown in stagnant or high-Fe conditions, with distinct properties and chemical compositions. This study highlights the pivotal roles of suberin, glycerol esters and iron plaques in this complex adaptive mechanism. Jiménez et al. (2025) investigated ROL from intact rice roots by analysing the root chemistry and Fe plaque formation across genotypes differing in Fe toxicity tolerance. Rice plants were grown in aerated, deoxygenated stagnant solutions and aerated solutions with high Fe concentrations (Figure 1). High Fe conditions led to partial ROL barriers with suberization and dense Fe plaque formation. Under stagnant conditions, stronger barriers formed due to additional accumulation of glycerol esters (Figure 1). Although glycerol esters also formed under high Fe conditions, their levels were not significantly elevated compared to controls. These findings align with Kotula et al. (2009) and Jiménez et al. (2024), reinforcing the idea that suberin and its associated compounds play a critical role in ROL regulation. A notable finding was that Fe-tolerant and Fe-sensitive genotypes showed similar suberization in the exodermis and Fe plaque formation responses, with no significant differences in ROL restriction. This suggests that root barrier traits under high Fe are mainly influenced by environmental factors rather than genetic predisposition. The role of iron plaques as a physical barrier to ROL is particularly interesting; they are formed by the oxidation of Fe²⁺ to Fe³⁺ at the root surface. These plaques not only restrict O₂ loss but also aid in nutrient uptake and detoxification. However, the study reveals a nuanced relationship between Fe plaque formation and Fe tolerance. For instance, the pronounced accumulation of Fe³⁺ in root apoplastic regions, coupled with the absence of visible toxicity symptoms in the tolerant genotype ‘Milyang 23’, indicates the presence of an efficient Fe compartmentation mechanism. This mechanism likely involves enhanced apoplastic sequestration and immobilization of Fe³⁺ within the root cortex and sclerenchyma, thereby restricting systemic translocation and conferring superior tolerance to high Fe conditions. In contrast, the Fe-sensitive ‘Ciherang’ displayed limited Fe³⁺ accumulation, correlating with visible toxicity symptoms in its shoots. Further, Jiménez et al. (2025) also explore the biochemical composition of root barriers, emphasizing the significance of glycerol esters and their isomers. Their findings showed that the formation of tight ROL barriers was correlated with the presence of specific isomers, particularly those esterified at the second carbon of the glycerol backbone (e.g., isomer 2; sn-2; MAG-2). These isomer 2 compounds were strongly associated with tight barrier formation (Jiménez et al. 2024, 2025). These results are based on previous studies in rice and other species, where suberin-associated lipids are known to act as protective barriers against desiccation and other stresses (Ogorek et al. 2023). In conclusion, Jiménez et al. (2025) demonstrate that the formation of apoplastic barriers to ROL in rice roots varies in structure and chemical composition depending on environmental stressors. Tight ROL barriers induced by stagnant, low-O₂ conditions were characterized by increased deposition of suberin and specific glycerol ester isomers (particularly sn-2 monoacylglycerols), whereas high Fe conditions triggered partial barriers comprising suberization and extensive Fe plaque formation, independent of genotype Fe tolerance. These results reveal that barrier formation is a general adaptive response to environmental constraints rather than a direct mechanism of Fe tolerance. This study offers valuable insights for improving rice cultivation in flood-prone regions by enhancing apoplastic barrier traits, such as increased suberization and glycerol ester accumulation. These traits could be improved through targeted genetic engineering, potentially involving the lactate dehydrogenase (Ldh) (Chatterjee et al. 2025), to improve root resilience under submergence and anoxic conditions. From a breeding perspective, modifying root barrier traits, such as suberization in the exodermis and Fe plaque formation, could facilitate the development of flood-tolerant rice cultivars. However, further research is needed to clarify the molecular basis of glycerol ester synthesis and its deposition and can also explore the interplay between root barrier formation and other stresses, like salinity and drought. Understanding these interactions could enhance rice resilience across various agro-ecological situations. The authors have nothing to report. The authors declare no conflicts of interest. The authors have nothing to report.