Salinity accelerates endodermal development and induces an exodermis in cotton seedling roots

The development of the endodermis was studied in 5- to 48-day-old cotton (Gossypium hirsutum L., cv. Acala SJ-2) seedling roots grown in vermiculite at different NaCl salinity levels. Sensitive fluorochromes (berberine-aniline blue for Casparian bands, and fluorol yellow-aniline blue for suberin lamellae) were used to detect cell wall modifications. Endodermal cells progressed through several developmental stages. In the primary stage, endodermal Casparian bands appeared synchronously in the radial walls. Then, suberin lamellae were laid down asynchronously in all cell walls, but always first in endodermal cells in the phloem sectors. Passage cells without suberin lamellae were usually present in the endodermal layer opposite protoxylem poles. The positions relative to the root tip at which Casparian bands and suberin lamellae developed were dependent on plant age and primary root length for control and sail-stressed seedlings. Salinity induced those structures to mature closer to the root tip for plants up to 10 days old, whereas in older plants no differences between treatments were observed. Exposure to high salinity (200 mM NaCl) induced the formation of an exodermis with Casparian bands and suberin lamellae close to the root base and in the transition zone to the hypocotyl. The exodermis, which never developed in control roots, may play a role in protecting the root from water loss and/or leakage of solutes important for osmotic adjustment. An exodermis was detected in 5- to 28-day-old seedlings. Older plants (48 days), with advanced secondary growth in the root stele, did not differentiate an exodermis.