Multiple lines of evidence suggest that the genes involved in gibberellin (GA) biosynthesis are regulated by bioactive GA levels. With the recent cloning of GA 2-oxidase genes from pea, we investigated whether this homeostatic regulation extends to the genes controlling GA deactivation in this species, utilizing two well-characterized GA-deficient mutants, ls and na and a GA-accumulating mutant, sln. The pea GA 2-oxidases showed feed-forward effects at the mRNA level, while the endogenous levels of GA 20 , GA 29 , GA 1 , and GA 8 showed no evidence of feed-forward regulation. Analyses of genomic Southern blots and expressed sequenced tag (EST) databases suggest that other GA 2-oxidases could possibly account for this lack of feed-forward on GA levels.
Information from well-known stem length mutants, both short and elongated, is discussed in the context of criteria necessary to demonstrate that the level of GA1 controls stem elongation in wild- type plants of the garden pea. Whilst this evidence is compelling, a mutant which over-produces GA1 would afford further insight, particularly into whether GA1 levels are saturating for growth in the wild-type. In this paper we further characterise the first reported garden pea mutant (sln) which possesses elevated levels of GA1. Evidence is presented from studies using this mutant that GA1 is normally limiting for growth over the early internodes in wild-type plants. In the developing seed, the mutant sin is shown to block the metabolism of [13C, 3H]GA29 to [13C, 3H]GA29-catabolite, particularly in the testa. Associated with this there were dramatically elevated GAGA29 levels in the dry seed from sln plants (400 times) compared with seeds from Sln plants. Upon germination, it appears that some of this GAGA20 is converted to GAGA1, which leads to substantial elongation of the early internodes. This hypothesis is supported by the observation that the inhibitor of an early step in GA biosynthesis, paclobutrazol, reduces elongation of sln plants when applied to developing seeds but not when applied at the start of germination. By contrast, prohexadione-calcium (BX-112), which inhibits the step GA20 to GA1, dramatically reduces internode length of sln plants when applied to seeds at the start of germination. Finally, application of GA20 to the dry seed of a wild-type (Sln) line (before sowing) resulted in a phenocopy of the sln 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 ...
Application of 2,4-dichlorophenoxyacetic acid (2,4-D) has been shown to be an effective control method for common scab of potato. Prior research targeted applications at or shortly after tuber initiation during the period of known susceptibility, with later treatment providing little or no control. This study examined the effect of a range of application dates and the influence of multiple applications at low and high rates on common scab control in both Russet Burbank and Desiree cultivars. We show that applications as early as 5 days after average plant emergence gave the greater disease control for both cultivars. These early treatments provide sufficient material to the tuber to induce resistance that lasts throughout the tuber susceptibility period with no requirement for, or benefit of repeated applications. Agronomic assessments suggested minor effects on total tuber mass produced with 2,4-D treatment but these were not consistent. Effective treatments result in levels of 2,4-D at harvest below the Australian maximum residue limit.
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