The ring D-modified gibberellin [GA], 16,17-dihydro
GA5, can retard stem growth in
Lolium temulentum L. while promoting flowering (Evans
et al., 1994,
Planta193, 107–114). Using
[1,2,3-3 H]GA20 to study
the final biosynthetic step to GA1 (a known effector of
shoot elongation in higher plants), it was shown that C-3b-hydroxylation of
GA20 to GA1 is blocked by
16,17-dihydro GA5 but is little affected by
GA5. Another late-stage biosynthetic inhibitor, the
acylcyclohexanedione, LAB 198 999, also blocked GA1
formation. Furthermore, endogenous levels of GA20 built
up after application of 16,17-dihydro GA5. Consequently,
growth retardation by 16,17-dihydro GA5 and LAB 198 999
is likely to be the result of their inhibition of GA20
3b-hydroxylation to GA1. Another fate for
GA20 in Lolium is its
C-2b-hydroxylation to growth-inactive GA29. This
conversion was also inhibited by 16,17-dihydro GA5 but
less so by LAB 198 999. The analogous step involving 2b-hydroxylation of
GA1 to GA8 appeared to be
insensitive to either growth retardant.
When [3H]GA20 was
injected into the cavity within the young intact sheathing leaves, there was
an appreciable metabolism of this GA20 to
GA1 and thence to GA8
(ca 10% and 30% respectively within 5 h).
For excised shoot tips, however, [3H]GA20 was
converted rapidly and virtually completely to GA29 in
3–5 h. Interestingly, with these excised shoot tips,
GA3 and GA5 as well as
16,17-dihydro GA5 when applied via the agar strongly
inhibited 2b-hydroxylation of GA20 to
GA29. In contrast, while 16,17-dihydro
GA5 blocked GA20 metabolism to
GA29 in intact sheath/stem tissue, this conversion
was not inhibited by GA5. These differences in
structural specificity for GAs which inhibit 2b-hydroxylation as opposed to
3b-hydroxylation are in accordance with these two Ring-A hydroxylation steps
being catalysed by different enzymes. Finally, the differences in
GA20 metabolism between intact versus excised tissue
raise the possibility that tissue wounding with excision enhanced the activity
of the GA20 2b-hydroxylase(s).