The specificity of auxin transport in intact pea seedlings (Pisum sativum L.)

Applicability of the chemiosmotic polar diffusion theory to the transport of indol-3yl-acetic acid in the intact pea (Pisum sativum L.)

The transport of exogenous indol-3yl-acetic acid (IAA) from the apical tissues of intact, light-grown pea (Pisum sativum L. cv. Alderman) shoots exhibited properties identical to those associated with polar transport in isolated shoot segments. Transport in the stem of apically applied [1-(14)C]-or [5-(3)H]IAA occurred at velocities (approx. 8-15 mm·h(-1)) characteristic of polar transport. Following pulse-labelling, IAA drained from distal tissues after passage of a pulse and the rate characteristics of a pulse were not affected by chases of unlabelled IAA. However, transport of [1-(14)C]IAA was inhibited through a localised region of the stem pretreated with a high concentration of unlabelled IAA or with the synthetic auxins 1-napthaleneacetic acid and 2,4-dichlorophenoxyacetic acid, and label accumulated in more distal tissues. Transport of [1-(14)C]IAA was also completely prevented through regions of the intact stem treated with N-1-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid.Export of IAA from the apical bud into the stem increased with total concentration of IAA applied (labelled+unlabelled) but approached saturation at high concentrations (834 mmol·m(-3)). Transport velocity increased with concentration up to 83 mmol·m(-3) IAA but fell again with further increase in concentration.Stem segments (2 mm) cut from intact plants transporting apically applied [1-(14)C]IAA effluxed 93% of their initial radioactivity into buffer (pH 7.0) in 90 min. The half-time for efflux increased from 32.5 to 103.9 min when 3 mmol·m(-3) NPA was included in the efflux medium. Long (30 mm) stem sections cut from immediately below an apical bud 3.0 h after the apical application of [1-(14)C]IAA effluxed IAA when their basal ends, but not their apical ends, were immersed in buffer (pH 7.0). Addition of 3 mmol·m(-3) NPA to the external medium completely prevented this basal efflux.These results support the view that the slow long-distance transport of IAA from the intact shoot apex occurs by polar cell-to-cell transport and that it is mediated by the components of IAA transmembrane transport predicted by the chemiosmotic polar diffusion theory.

The indirect role of 2,4-D in the maintenance of apical dominance in decapitated sunflower seedlings (Helianthus annuus L.)

When sufficient 2,4-D to maintain apical dominance for at least 21d was applied to the cut stem interface of sunflower seedlings which had been decapitated in the epicotyl, it could not be detected in the vicinity of the inhibited axillary buds 7d after application. Rather the 2,4-D concentrated at the stump apex where it was associated with formation of meristematic tissue. The results indicate an indirect role for 2,4-D in the maintenance of apical dominance in this system, possibly involving the induced meristematic activity.

Transport of exogenous auxin in two-branched dwarf pea seedlings (Pisum sativum L.) : Some implications for polarity and apical dominance

Dwarf pea plants bearing two cotyledonary shoots were obtained by removing the epicotyl shortly after germination, and the patterns of distribution of (14)C in these plants was investigated following the application of [(14)C]IAA to the apex of one shoot. Basipetal transport to the root system occurred, but in none of the experiments was (14)C ever detected in the unlabelled shoot even after transport periods of up to 48 h. This was true both of plants with two equal growing shoots and of plants in which one shoot had become correlatively inhibited by the other, and in the latter case applied whether the dominant or subordinate shoot was labelled. In contrast, when [(14)C]IAA was applied to a mature foliage leaf of one shoot transfer of (14)C to the other shoot took place, although the amount transported was always low. Transport of (14)C from the apex of a subordinate shoot on plants bearing one growing and one inhibited shoot was severely restricted compared with the transport from the dominant shoot apex, and in some individual plants no transport at all was detected. Removal of the dominant shoot apex rapidly restored the capacity of the subordinate shoot to transport apically-applied [(14)C]IAA, and at the same time led to rapid cambial development and secondary vascular differentiation in the previously inhibited shoot. Applications of 1% unlabelled IAA in lanolin to the decapitated dominant shoot maintained the inhibition of cambial development in the subordinate shoot and its reduced capacity for auxin transport. These results are discussed in relation to the polarity of auxin transport in intact plants and the mechanism of correlative inhibition.