Roles of auxin and ethylene in aerenchyma formation in sugarcane roots.

Although the cross-talk between auxin and ethylene has been described during plant development, the role played by auxin upon gene expression during aerenchyma formation is poorly understood. Root aerenchyma formation results from the opening of gas spaces in the cortex. It is part of a developmental program (constitutive) or due to ethylene treatment or abiotic stress (induced) such as flooding and nutrient starvation. This process relies on programmed cell death and cell wall modifications. Here we followed development of aerenchyma formation in sugarcane along 5 cm from the root apex. As a constitutive process, the aerenchyma formation was observed in the cortex from the 3rd cm onwards. This occurred despite 1-methylcyclepropene (1-MCP) treatment, an inhibitor of ethylene perception. However, this process occurred while ethylene (and auxin) levels decreased. Within the aerenchyma formation zone, the concentration of ethylene is lower in comparison to the concentration in maize. Besides, the ratio between both hormones (ethylene and auxin) was around 1:1. These pieces of evidence suggest that ethylene sensitivity and ethylene-auxin balance may play a role in the formation of aerenchyma. Furthermore, the transcriptional analysis showed that genes related to cell expansion are up-regulated due to 1-MCP treatment. Our results help explaining the regulation of the formation constitutive aerenchyma in sugarcane.

Cell wall changes during the formation of aerenchyma in sugarcane roots.

Background and Aims: Aerenchyma develops in different plant organs and leads to the formation of intercellular spaces that can be used by the plant to transport volatile substances. Little is known about the role of cell walls in this process, although the mechanism of aerenchyma formation is known to involve programmed cell death and some cell wall modifications. We assessed the role that cell wall-related mechanisms might play in the formation of aerenchyma in sugarcane roots. Methods: Sections of roots (5 cm) were subjected to microtomography analysis. These roots were divided into 1-cm segments and subjected to cell wall fractionation. We performed analyses of monosaccharides, oligosaccharides and lignin and glycome profiling. Sections were visualized by immunofluorescence and immunogold labelling using selected monoclonal antibodies against polysaccharide epitopes according to the glycome profiles. Key Results: During aerenchyma formation, gas spaces occupied up to 40 % of the cortex cross-section within the first 5 cm of the root. As some of the cortex cells underwent dissolution of the middle lamellae, leading to cell separation, cell expansion took place along with cell death. Mixed-linkage ß-glucan was degraded along with some homogalacturonan and galactan, culminating in the formation of cell wall composites made of xyloglucan, arabinoxylans, cellulose and possibly lignin. Conclusion: The composites formed seem to play a role in the physical-chemical properties of the gas chambers, providing mechanical resistance to forces acting upon the root and at the same time decreasing permeability to gases.