Polar auxin transport: an early invention

Abstract

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.

Fig. 1.

C. corallina as grown in the culture facility used in this study. Isolated internode cells were used in the auxin transport assays. Bar, about 2.5 cm.

Fig. 2.

Experimental set up for measuring polar auxin transport in Chara internodes. (A) Schematic representation of the experimental set up. (B) A Chara internode–node boundary with attached peripheral cells: bar, 5 μm. (C) Detailed schematic representation of a seal at the receiver well.

Fig. 3.

Polarity of auxin efflux in Chara internodes. (A) A typical example showing ³H-indole-3-acetic acid (IAA) added to the bridge compartment and the efflux into the donor and receiver wells was monitored for 1 h. The cumulative accumulation of auxin is plotted against time for the apical side (donor compartment, filled square) and the basal side (receiver compartment, filled circle) of the internode. Besides untreated controls, internodes of Chara were pre-incubated for 30 min with 5 × 10⁻⁶ M naphthylphthalamic acid. Subsequently, ³H-IAA was added to the bridge compartment and the efflux of IAA in donor (filled triangle) and receiver (×) compartments was determined. (B) A representative example of ³H-IAA added to the donor well, then the receiver well was sampled and ³H-IAA determined by LSC. Cumulative IAA accumulation in the receiver well was only observed in internodes placed in a polar orientation (filled circle). Reversion of the internode in a non-polar orientation displayed no transport of auxin through the internode (filled triangle). (C) Polar auxin transport in two connected internodes of Chara. Measurements of IAA accumulation in the receiver were performed as described in (B).

Fig. 4.

(A) Polar auxin transport in two species of Chara. Auxin transport was measured in Chara vulgaris var. longibracteata (filled square), which contains cortex cells surrounding its internode cell, and in Chara corallina (filled circle), without a cortex. Inset shows the amount of indole-3-acetic acid (IAA) accumulated in the bridge compartments for C. corallina and C. vulgaris after 270 min. (B) Effect of cytochalasin H on polar auxin transport. Addition of the cytoplasmic streaming inhibitor cytochalasin H (30 μM) to Chara internodes after 270 min (arrow) did not inhibit auxin transport (filled circle). Pre-incubation (30 min) with cytochalasin H (30 μM) before ³H-IAA was added in continued presence of this inhibitor did not alter polar auxin transport (filled square).