Figure 4.

Interrelationships between signals and signal cascades leading to induction, organization, and construction of polarized ingrowth walls in epidermal transfer cells of Vicia faba cotyledons. In cultured cotyledons, a combination of declining intracellular glucose levels and an auxin (IAA) regulated burst in ethylene production, initiates ingrowth wall formation within adaxial epidermal cells. The glucose/ethylene sequence is reproduced in planta and downstream events are deduced from findings obtained using cultured cotyledons. Prior to the onset of cotyledon growth, an extracellular invertase, localized to inner cells of the seed coat (dark green), hydrolyzes sucrose and the glucose product, sensed by a cotyledon epidermal cell (light green) hexokinase, blocks an ethylene signal cascade by down regulating ethylene insensitive 3 (EIN3), a key ethylene signal cascade transcription factor. Upon initiating expansion growth, the cotyledons crush inner cells of the seed coat (see images of seeds cut through their longitudinal plan illustrating cotyledon growth at two stages). Cell crushing results in a loss of extracellular invertase activity and hence the glucose signal. This is accompanied by an amplified wound-induced ethylene (C₂H₄) signal cascade, mediated through EIN3, driving expression of respiratory burst oxidases (Rbohs), trafficked to portions of the plasma membrane (blue) that line the outer periclinal walls of cotyledon epidermal cells. The Rbohs catalyze production of extracellular reactive oxygen species (ROS) that are further reduced to form an extracellular hydrogen peroxide (H₂O₂) signal that serves two known functions. These are activating expression of cell wall building machinery through an unknown signal cascade (arrows with broken shafts) and serving as a positional signal directing construction of the uniform wall (dark gray) polarized to the outer periclinal walls (light gray) of the cotyledon epidermal cells. How the polarized wall construction subsequently is constrained to loci from which wall ingrowth papillae arise remains to be determined (purple arrows with broken shafts) but likely involves directional control by cytosolic calcium signals.