Figure 3.

Pathways involved in patterning and differentiation of the ground tissue. A) In A. thaliana primary roots, a target of the SHR-SCR protein complex, CYCD6;1, controls division of the cortex-endodermal initial cell, resulting in the 2 cell files of the ground tissue (cortex and endodermis). In older roots, periclinal divisions in the endodermal cell file form an extra ground tissue layer usually referred to as the middle cortex. The formation of the middle cortex is regulated by the hormones ethylene, gibberellic acid (GA), and abscisic acid (ABA) as well as reactive oxygen species (ROS). In another Brassicaceae species, C. hirsuta, the middle cortex development occurs earlier in development and is regulated by the transcription factor PHB. The expression of miR165/166 was found to be lower in the endodermis of this species compared with A. thaliana, allowing the expansion in expression domain of its target, PHB. B) A differentiated endodermis in A. thaliana roots consists of 2 barriers for solute transport, the lignin-rich Casparian strip and suberin lamellae (left). S. lycopersium and S. tuberosum roots also contain a similar endodermal layer (middle). In addition to this, these species also contain an outer exodermal layer that displays a characteristic polar lignin cap and suberin lamellae. In O. sativa, the exodermis contains Casparian strip and suberin lamellae similar to the endodermis (right). C) Ethylene levels increase in maize roots experiencing hypoxic conditions caused by water-logging. This elevated ethylene triggers cell death leading to the formation of aerenchyma. The transcription factor, bHLH121, promotes the development of aerenchyma in maize roots. D) Hypoxic conditions also trigger aerenchyma formation in the rice and wheat roots as well as in A. thaliana hypocotyls, with auxin and salicylic acid playing key roles in this process.