Magnitude and Kinetics of Stem Elongation Induced by Exogenous Indole-3-Acetic Acid in Intact Light-Grown Pea Seedlings

Abstract

Exogenously applied indole-3-acetic acid (IAA) strongly promoted stem elongation over the long term in intact light-grown seedlings of both dwarf (cv Progress No. 9) and tal1 (cv Alaska) peas (Pisum safivum l.), with the relative promotion being far greater in dwarf plants. In dwarf seedlings, solutions of IAA (between 10-4 and 10-3 M), when continuously applied to the uppermost two internodes via a cotton wick, increased whole-stem growth by at least 6-fold over the first 24 h. The magnitude of growth promotion correlated with the applied IAA concentration from 10-6 to 10-3 M, particularly over the first 6 h of application. IAA applied only to the apical bud or the uppermost internode of the seedling stimulated a biphasic growth response in the uppermost internode and the immediately lower internode, with the response in the latter being greatly delayed. This demonstrates that exogenous IAA effedively promotes growth as it is transported through intact stems. IAA withdrawal and reapplication at various times enabled the separation of the initial growth response (ICR) and prolonged growth response (PCR) induced by auxin. The ICR was inducible by at least 1 order of magnitude lower IAA concentrations than the PCR, suggesting that the process underlying the ICR is more sensitive to auxin indudion. In contrast to the magnitude of the IAA effect in dwarf seedlings, applied IAA only doubled the growth in tal1 seedlings. These results suggest that endogenous IAA is more growth limiting in dwarf plants than in tal1 plants, and that auxin promotes stem elongation in the intact plant probably by the same mechanism of adion as in isolated stem segments. However, since dwarf plants to which IAA was applied failed to reach the growth rate of tal1 plants, auxin cannot be the only limiting factor for stem growth in peas.

Blockage of Brassinosteroid Biosynthesis and Sensitivity Causes Dwarfism in Garden Pea

“…Promo- (Yang et al, 1993), and there is a synergistic interaction between IAA and BRs in the promotion of cucumber hypocotyl sections (Katsumi, 1985). Furthermore, since application of brassinolide increases the endogenous IAA content of squash seedlings (Eun et al, 1989), it is evident that further studies are necessary at the biochemical leve1 to determine the IAA-BR relationship in the lkb mutant and other plant species.…”

The Control of Apical Bud Growth and Senescence by Auxin and Gibberellin in Genetic Lines of Peas

“…Law and Davies (1990) reported a good correlation between IAA level and the stem length of severa1 different pea genotypes, including slender lines in which GA is lacking. More recently, we (Yang et al, 1993(Yang et al, , 1995 demonstrated that applications of IAA enhance stem growth in intact pea plants, with the effect being more pronounced in dwarf plants containing lower auxin levels (Law and Davies, 1990). Therefore, it is clear that both GA and IAA are needed in stem elongation, so that when one of them is missing or present at low level the elongation response will be diminished.…”

Control of Internode Length in Pisum sativum (Further Evidence for the Involvement of Indole-3-Acetic Acid)

“…In the garden pea (Pisum sativum L.), quite large changes in intemode length have been attibuted (at least in part) to relatively small changes in IAA level (Law and Davies, 1990;. Furthermore, there is now little doubt that exogenous IAA can promote the elongation of intact pea seedlings (Hall et al, 1985;Yang et al, 1993).…”