Abscisic-acid metabolism in a wilty mutant of Nicotiana plumbaginifolia

Abstract

A mutant of Nicotiana plumbaginifolia, CKR1, isolated on the basis of its enhanced resistance to cytokinins was found to have a greater tendency to wilt than the wild type (Blonstein et al., 1991, Planta 183, 244-250). Further characterisation has shown that the wiltiness in the mutant is not caused by an insensitivity to abscisic acid (ABA) because the external application of ABA leads to stomatal closure and phenotypic reversion. The basal ABA level in the mutant is < 20% of that in the wild type. Following stress, the ABA level in wild-type leaves increases by approx 9-to 10-fold while the mutant shows only a slight increase. This deficiency in ABA is unlikely to be the consequence of accelerated catabolism as the levels of two major metabolites of ABA, phaseic and dihydrophaseic acid, are also much reduced in the mutant. The qualitative and quantitative distributions of carotenoids, the presumed presursors of ABA, are the same for the leaves of both wild type and mutant. Biosynthesis of ABA at the C15 level was investigated by feeding xanthoxin (Xan) to detached leaves. Wild-type leaves convert between 9-19% of applied Xan to ABA while the mutant converts less than 1%. The basal level of trans-ABA-alcohol (t-ABA-alc) is 3-to 10-fold greater in the mutant and increases by a further 2.5-to 6.0-fold after stress. This indicates that the lesion in the wilty mutant of N. plumbaginifolia affects the conversion of ABA-aldehyde to ABA, as in the flacca and sitiens mutants of tomato and the droopy mutant of potato (Taylor et al., 1988, Plant Cell Environ. 11, 739-745; Duckham et al., 1989, J. Exp. Bot. 217, 901-905). Wild-type tomato and N. plumbaginifolia leaves can convert trans-Xan into t-ABA-alc, and Xan into ABA, while those of flacca and the wilty N. plumbaginifolia mutant convert both Xan and t-Xan to t-ABA-alc.

Abscisic Acid Biosynthesis and Response

“…There are another two Arabidopsis genes with 46% similarity to the novel C-terminus of ABA3 which are 74% similar to each other, suggesting this domain is functionally important (possibly in determining target enzyme specificity). The flacca (flc) mutant of tomato may also be orthologous to aba3 and Npaba1 because all three mutants are ABAdeficient, cannot convert xanthoxal or ABA-aldehyde to ABA, lack AO and XDH but not NR activities, and in vitro sulfurylation with Na 2 S reactivates preexisting XDH and AO proteins in flc, and Npaba1, as in aba3, extracts Parry et al, 1991;Leydecker et al, 1995;Marin and Marion-Poll, 1997;Abaka et al, 1998;Sagi et al, 1999). The nar2a (Az34) MoCo mutant of barley lacks all AO, XDH and NR activities, suggesting an upstream lesion in the synthesis of the MoCo which all three enzyme activities require (Walker-Simmons et al, 1989;Rock, 1991).…”

A Single Genetic Locus, Ckr1, Defines Arabidopsis Mutants in which Root Growth Is Resistant to Low Concentrations of Cytokinin

“…Instead, the mutant seems less able to mount a response to general stresses such as high cytokinin concentrations. The mutant has a wilty phenotype, shows little ABA induction following stress, and apparently is blocked in the last step of ABA biosynthesis (12).…”

Isomeric Ratio and Level of Xanthoxin in Tomato Plants Measured by a New Analytical Method

“…Another candidate for the t-XAN metabolite is t-ABA-alcohol. 191 It has been reported that both the wild type and wilty mutant of tobacco plants are endowed with this pathway from t- XAN;20) however, the level of t-ABA-alcohol is usually too low to be identified in wild tomato plants. 19 ) In conclusion, without any plausible metabolite having been found so far, it is difficult to explain the fate of t-XAN, suggesting that t-XAN does not actually exist in plants.…”