Figure 2.

Adventitious root development in response to flooding. Under aerated conditions, gaseous ethylene escapes from plant tissues, but during flooding, water acts as a physical barrier, trapping ethylene in the plant. GA enhances the ethylene-promoted adventitious root growth, while abscisic acid reduces the effect. Ethylene triggers reactive oxygen species production, and together they trigger epidermal programmed cell death for root emergence and cortical programmed cell death lysigenous aerenchyma formation. The main difference in some eudicots (e.g. tomato) is the requirement for de novo adventitious root initiation via auxin and ethylene signaling. In the cross section, epidermis and exodermis are combined, but the exodermis can be several cell layers adjacent to the epidermis. Yellow roots are adventitious roots, blue and pink roots are lateral roots, and white roots are primary roots. Pointed arrows represent positive interactions, and flat-ended arrows represent negative interactions.