l h e effects of altered endogenous indole-3-acetic (IAA) levels on elongation in garden pea (Pisum sativum 1.) plants were investigated. The auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA) and 9-hydroxyfluorene-9-carboxylic acid (HFCA) were applied to elongating internodes of wild-type and mutant Ikb plants. The Ikb mutant was included because elongating Ikb internodes contained 2-to 3-fold less free IAA than those of the wild type. In the wild type, TIBA reduced both the IAA level and internode elongation below the site of application. Both TIBA and HFCA strongly promoted the elongation of Ikb internodes and also raised IAA levels above the application site. The synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) also markedly increased internode elongation in Ikb plants and virtually restored petioles and tendrils to their wild-type length. In contrast, treatment of wildtype plants with TIBA, HFCA, or 2,4-D caused little or no increase in elongation above the application site. The ethylene synthesis inhibitor aminoethoxyvinylglycine also increased stem elongation in Ikb plants, and combined application of HFCA and aminoethoxyvinylglycine restored Ikb internodes to the wild-type length. It is concluded that the level of IAA in wild-type internodes is necessary for normal elongation, and that the reduced stature of Ikb plants is at least partially attributable to a redudion in free IAA level in this mutant.
In the garden pea (Pisum sativum L.), the mutant allele le blocks the conversion of GAZ0 to GAl (Ingram et al., 1984), reducing the endogenous level of GAl (Ross et al., 1992; Smith et al., 1992). This is responsible for the dwarf phenotype of le plants. It has been suggested that le codes for an altered form of GA20 3B-hydroxylase, the enzyme that catalyzes the step GAZo to GAl (Ross et al., 1989; MacMillan, 1990). Recently, however, Smith (1992) questioned this proposa1 in a model addressing severa1 important aspects of GA biosynthesis in pea. According to this model the apical bud itself does not metabolize GAZo to a significant extent. Instead, it is suggested that GAl production is confined largely to young stem tissue, where it is regulated by an inhibitor of GA2o 30-hydroxylation produced by the apical bud (and possibly by young, expanding leaflets). Smith (1992) proposed that in le plants the inhibitor is constitutively produced, thereby blocking GA1 production. Smiths model is based on the metabolism of [13C,3H]GAz0 by excised leaves, stems, and apical buds. The metabolites were chromatographed by HPLC as the free acids (Smith, 1992). In extracts from apical buds and leaves the level of radioactivity co-eluting with GAl (GA1-like) was very low. This level was higher in extracts from stems, but in a11 cases was not affected by the Le/le gene difference.We have investigated the metabolism of [13C,3H]GAz0 in plants either left intact or with their apical buds removed (decapitated). Furthermore, we have reproduced the experimental conditions used by Smith (1992) and have analyzed the resulting extracts by HPLC as the free acids and methyl esters, and by GC-MS. Our results do not support the in- MATERIALS A N D METHODSThe lines used were the isolines 205+ (Le, tall) and 205-(le, dwarf), developed from cross L2 X L77 at Hobart (Tasmania, Australia). The plants were grown, two per pot, in a heated glasshouse under an 18-h photoperiod (Reid and Potts, 1986). Experiments were conducted when the plants had expanded to five to seven leaves (including the scale leaves).The substrate in a11 experiments was [17-13C,3H~]GA~~ (1.27 X 10" Bq mmol-'; Ingram et al., 1984). Two experimental systems were used. First, substrate (2 r g in 10 PL of ethanol) was applied to the second uppermost fully expanded leaf of 12 tall and 12 dwarf plants. Immediately afterward, six tall and six dwarf plants were decapitated by excising just below the node situated three nodes up from the treated leaf. This left three intemodes and two leaves above the treated leaf in decapitated plants. After 2 d, the uppermost two of these intemodes were harvested together (stem portion), as were the stipules, petiole, and leaflets of the uppermost leaf (leaf portion). Intact plants were harvested by first excising the apical portion as before and then harvesting the next two intemodes down and (separately) the associated leaf, as for decapitated plants. A11 excised portions were then immersed in cold (-20°C) methanol.Second, the technique of Smith ...
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