Stimulus Response Coupling in Auxin Action

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Dormant barley aleurone shows heterogeneity and a specific cytodifferentiation

In response to gibberellic acid, aleurone layers isolated from dormant barley (Hordeum distichum L. cv. Triumph) kernels produced significantly less alpha-amylase than aleurones from non-dormant kernels. Light microscopical investigations using the dye acridine orange as well as electron microscopical studies showed that the relatively low alpha-amylase production by aleurones from dormant kernels appears to be due to the majority of the cells remaining quiescent after GA3 incubation. The cells that did show ultrastructural signs of GA3-induced activation occurred in clusters that spanned all three cell layers of the dormant aleurone. These cells showed a characteristic ultrastructure, which was different from that described previously for GA3 incubated non-dormant aleurone layers. Dictyosomes, which are extremely difficult to visualise in non-dormant barley aleurone, were abundant. The presence of alpha-amylase in the Golgi could be demonstrated by immuno-gold labelling in these cells. Remarkably, most endosperm-facing aleurone cells in the activated clusters showed no cell wall degradation. This cytodifferentiation pattern could be mimicked in aleurone cells from non-dormant grains by imbibition of the grains in abscisic acid (ABA) prior to GA3 incubation. These data support the notion of heterogeneity of aleurone cells with respect to GA3 sensitivity, and suggest that ABA is involved in the impaired GA3 response of aleurone cells from dormant barley. © 1997 Academic Press Limited

Polar auxin transport: an early invention

In higher plants, cell-to-cell polar auxin transport (PAT) of the phytohormone auxin, indole-3-acetic acid (IAA), generates maxima and minima that direct growth and development. Although IAA is present in all plant phyla, PAT has only been detected in land plants, the earliest being the Bryophytes. Charophyta, a group of freshwater green algae, are among the first multicellular algae with a land plant-like phenotype and are ancestors to land plants. IAA has been detected in members of Charophyta, but its developmental role and the occurrence of PAT are unknown. We show that naphthylphthalamic acid (NPA)-sensitive PAT occurs in internodal cells of Chara corallina. The relatively high velocity (at least 4–5 cm/h) of auxin transport through the giant (3–5 cm) Chara cells does not occur by simple diffusion and is not sensitive to a specific cytoplasmic streaming inhibitor. The results demonstrate that PAT evolved early in multicellular plant life. The giant Chara cells provide a unique new model system to study PAT, as Chara allows the combining of real-time measurements and mathematical modelling with molecular, developmental, cellular, and electrophysiological studies