Contribution of the arbuscular mycorrhizal symbiosis to the regulation of radial root water transport in maize plants under water deficit

In roots, water flows radially through three parallel pathways: apoplastic, symplastic and transcellular (the last two referred as the cell-to-cell), with a different contribution depending on the environmental conditions. Thus, during drought, the cell-to-cell pathway, which is largely regulated by aquaporins, dominates. While it is accepted that water can flow across roots following the apoplastic, symplastic and transcellular pathways, the relative contribution of these pathways to whole root hydraulic conductivity is not well stablished. In addition, the symbiosis with arbuscular mycorrhizal (AM) fungi was reported to modify root water transport in host plants. This study aims to understand if the AM symbiosis alters radial root water transport in the host plant and whether this modification is due to alteration of plant aquaporins activity or amounts and/or changes in apoplastic barriers. Hence, the combined effect of mycorrhizal fungus, water deficit and application of the aquaporin inhibitor sodium azide (NaN3) on radial root water transport of maize plants was analyzed. The development of Casparian bands in these roots was also assessed. NaN(3 )clearly inhibited osmotic root hydraulic conductivity (Lo). However, the inhibitory effect of sodium azide on Lo was lower in AM plants than in non-AM plants, which together with their higher relative apoplastic water flow values suggests a compensatory mechanism for aquaporin activity inhibition in AM plants, leading to a higher hydrostatic root hydraulic conductivity (Lpr) compared to non-AM plants. This effect seems to be related to the mycorrhizal regulation of aquaporins activity through posttranslational modifications. The development of Casparian bands increased with drought and AM colonization, although this did not decrease water flow values in AM plants. The work provides new clues on the differential mycorrhizal regulation of root water transport.