This study was primarily concerned with the involvement of
gibberellins in the control of internode length in a range of genotypes
of Pisum sativum L. Shoots of tall peas (Le) are shown by lettuce
hypocotyl and rice seedling bioassays to contain a relatively large
amount of a polar gibberellin-like substance which is either absent from
or present in only small amounts in dwarf peas (Ls). This biochemical
difference can be attributed to a genetic difference at the Le locus.
Preliminary results using unrelated pure breeding tall and dwarf lines
were confirmed by the use of an F2 progeny segregating for the Le/le
pair of alleles. The polar gibberellin-like substance in shoots of tall
peas is tentatively identified by GCMS-MIM as GA1. In tall peas, the
GA1-like substance is located mainly in the actively growing and
elongating apical region of the shoot. Its level is much reduced in
mature leaf and stem tissue, perhaps explaining the lack of a graft
transmissible effect of the tall and dwarf phenotypes. GA20 is the
major active gibberellin in dwarf peas and the GA20-like substance is
also shown to be located mainly in the apical region of the shoot of
both tall and dwarf peas.
Shoots of the extremely short plants with mutant na are found by
bioassay to contain undetectable levels of gibberellin-like substances.
This is confirmed by the use of near isogenic lines differing at the Na
locus. Results from graft and feed experiments using the gibberellin
deficient na Le and na is plants suggest that the Le gene controls the
conversion of GA20 to the GA1-like compound by promoting
31β-hydroxylation (as evidenced by high activity in the rice seedling
bioassay using cv. Waito C). In contrast to their action in the shoot,
the na and Le genes do not influence the gibberellin-like content of
immature seed, suggesting that the action of genes controlling gibberellin
biosynthesis or metabolism can be organ or tissue specific.
In an F2 progeny, shoots of slender and dwarf segregates show no
large differences in gibberellin content that would explain the large
phenotypic difference. The slender phenotype (la cry8 ) can be mimicked
in dwarf peas with the application of non-limiting quantities of GA3 .
However, as slender peas appear insensitive to changes in gibberellin
levels, it is suggested that genes at the La and Cry loci are influencing
some process at or beyond the activation of the gibberellin receptor
site.
The gibberellin content of a range of different flowering genotypes
was examined to provide evidence for or against a reported link between
the flowering genes Sn and Hr and gibberellin metabolism. The results
do not support the theory that the primary action of these genes is to
control gibberellin metabolism. The results do show an apparent drop in
the level of the GA20-like substance with the onset of apical senescence
in some lines making the comparison of early flowering to later flowering
genotypes difficult.
In the garden pea ( Pisum sativum L.), shoots of the extremely short plants with the mutant na (phenotype nana) are found by bioassay to contain undetectable levels of gibberellin‐like substances. This is confirmed by the use of near isogenic lines differing at the Na locus. Thus, mutant na appears to block a step early in the pathway of gibberellin synthesis. It is suggested that the polar gibberellin‐like substance found in the apical portion of shoots of tall ( Le ) but not dwarf ( le ) peas could be GA 1 . Extracts of shoots of na Le peas treated with GA 20 (the major active gibberellin in dwarf peas) possess a large amount of GA 1 ‐like activity whereas extracts of shoots of na le peas treated with GA 20 possess a much reduced amount. Thus, gene Le may allow the conversion of a less active gibberellin (GA 20 ) into one more active in stimulating elongation in the pea (the GA 1 ‐like compound). In contrast to their influence in the shoot, the na and Le genes do not appear to be operative in controlling the gibberellin content of developing seed, indicating that organ specific gibberellin biosynthesis and metabolism occur in peas.
Tall peas ( Pisum sativum L.) contain a relatively large amount of a polar gibberellinlike substance(s) which is either absent or present in only small amounts in dwarf peas. This biochemical difference can be attributed to a genetic difference at the le locus. Preliminary results using unrelated pure breeding tall ( Le ) and dwarf ( le ) lines were confirmed by the use of an F 2 progeny segregating for the Le/le pair of alleles. The polar gibberellin‐like substance(s) in tall peas is located mainly in the actively growing and elongating apical region of the shoot. The level of this substance(s) is much reduced in mature stem and leaf tissue. The gibberellin‐like substance(s) in question remains unidentified although it can be characterised by activity in both the lettuce hypocotyl bioassay and the rice seedling bioassay. The dwarf rice cultivars ‘Tan‐ginbozu’ and ‘Waito C’ are both sensitive. The magnitude of the other major zone of gibberellin‐like activity (the GA 20 ‐like zone) did not vary greatly between tall and dwarf segregates but the apical regions of the shoots again contained significantly more activity than the mature regions.
Three further internode length mutants in peas ( Pisum sativum L.), lh , ls and lk , were examined to determine if they influenced gibberellin synthesis or sensitivity. Two mutants, lh and ls , showed pronounced elongation in response to applied GA 1 and extracts from their shoots contained little gibberellin‐like activity when assayed on the rice seedling (cv. Tan ginbozu) bioassay compared with similar extracts from essentially isogenic Lh and Ls plants. The third mutant, lk , was almost insensitive to applied GA 1 and at no dose rate did it become a phenocopy of normal Lk plants. Extracts from the shoots of lk and Lk segregants contained similar levels of gibberellinlike substances. All three mutants influenced growth in both the light and the dark, although only the effect of genes Lh and Ls were graft transmissible. These results suggest that lh and ls are mutants with reduced gibberellin synthesis, while lk is the first gibberellin‐insensitive dwarfing gene identified in peas.
Pea plants ( Pisum sativum L.) possessing the slender phenotype (conferred by the gene combination la cry s ) have extremely long, thin internodes and are phenotypically similar to dwarf plants (possessing genes La and/or Cry ) that have been treated with a non‐limiting dose of gibberellin (GA 3 ). In contrast to tall and dwarf plants, slender plants are virtually insensitive to treatment with AMO 1618, PP333 or GA 3 and addition of the “gibberellin‐less” mutant gene na does not alter the phenotype of slender plants. Na slender segregates possessed lower levels of gibberellin‐like substances than comparable dwarf segregates when extracts from shoots were assayed using the lettuce hypocotyl or rice seedling bioassays. In addition, na slenders possessed little or no gibberellin‐like activity even though they possessed a slender phenotype. Thus the gene combination la cry s causes slender plants to respond as if they are saturated with gibberellins for growth. In addition, the gene combinations la cry s and le la cry c (allele cry c is less extreme in effect than cry s ) are shown to be almost completely epistatic to the alleles at the na locus. All these results suggest that gibberellin levels are not important in determining the internode length of slender peas (genotype la cry s ). The possible mechanisms by which this could occur are discussed.
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