Abscissic acid and water stress.

Discussion The evidence presented here and previously (Neill et al., 1982a,b) strongly suggests that 1 ‘-deoxyabscisic acid (111) is the immediate precursor of abscisic acid in C. rosicola. Feeding studies have shown that 2-cis-a-ionylidene ethanol (I) and 2-cis-a-ionylidene acetic acid (11) are converted in good yield into 111 and abscisic acid. Thus they can be utilized as biosynthetic precursors. However, their presence as endogenous fungal metabolites has yet to be demonstrated. In addition I is converted to 4’-hydroxy-2-cis-a-ionylidene acetic acid (IV) (configuration of the hydroxy group unknown). This compound has been tentatively identitied by g.c.-m.s. as an endogenous fungal metabolite. Both 4’-epimers of IV are converted to I11 and to abscisic acid by the fungus. Thus, it is likely that IV is the direct precursor of 111. We have not yet investigated in detail the stereochemistry of the interconversions reported here. 1 ’-Deoxyabscisic acid isolated from the fungus has the same absolute configuration at the 1’-carbon as abscisic acid. This is assumed from the fact that the 0.r.d. curves of the two compounds are identical in sign. Thus it would appear that the final hydroxylation step proceeds with retention of configuration. All the synthetic compounds fed as putative intermediates were used as racemates. We have not yet fully investigated the optical properties of the products and residues obtained from these incubations to ascertain the degree of stereospecificity of the proposed pathway with regard to the configuration at C1 ’. The 2-lrans isomers of I and I1 are converted to 2-trans 111 but not to abscisic acid or 2-trans-abscisic acid (Table 1). This suggests that isomerization of the 2,3-double bond precedes or is concomitant with the formation of the first desaturated cyclic intermediate. These results also suggest that the early oxidizing enzymes are rather non-specific; on the other hand the final hydroxylase enzyme is sensitive to the geometry of the side-chain and will not utilize the 2-trans isomer of 111 as a substrate. The lack of incorporation of the 1’-ene (B)-compounds suggest that cyclization proceeds via the formation of a 2’-ene intermediate, analogous to &-ring formation in carotenoid biosynthesis. Thus we propose that some pathway through the matrix shown in Fig. 3 is the most likely route for the later stages of abscisic acid biosynthesis in C. rosicola. The failure to obtain incorporation of either the 2-cis or 2-trans isomers of farnesol and dehydrofarnesol indicates that the early stages of abscisic acid biosynthesis i.e. cyclization, desaturation of the 4 3 bond, and isomerization of the 2,3 double bond, utilize intermediates which are not the free alcohols. It would seem likely therefore that phosphorylated intermediates may be involved in all or some of these steps. The results reported with various plant tissues suggest that I1 and 111 can be extensively metabolized to compounds other than abscisic acid. This metabolism is very rapid and the failure to observe significant incorporation of I1 and 111 into abscisic acid may simply be a consequence of competitive metabolism as a result of breakdown of compartmentation on external application. The incorporation observed in Vicia probably reflects access of the precursors to the site of abscisic acid biosynthesis before excessive metabolism by other routes takes place.