Many investigators have suggested that abscisic acid (ABA) is the causative agent for root gravitropism. Differential ABA concentrations in the upper and lower halves of the root presumably result in the more rapid growth of the upper half, which leads to the positive gravitropism characteristic of primary roots. Most experimental evidence for this mechanism was derived from applications of exogenous ABA, which may not correctly depict the functions of ABA at physiological concentrations.
We used two carotenoid-deficient mutants, vp5 and w3, and the carotenogenesis inhibitor Fluridone for studies of primary root gravicurvature, as previous studies have shown ABA levels are reduced in carotenoid-deficient embryos. The mutants were in a Tx5855 background while Iochief is an unrelated sweet corn cultivar. Data related to ABA concentrations and gravicurvatures are presented in Tables 1 and 2, respectively.
The obvious conclusion to be drawn from these data is that ABA is not necessary for positive gravitropism of primary roots, since the Fluridone-treated and mutant seedlings were strongly graviresponsive in the absence of ABA.
Our second conclusion is that ABA is a carotenoid derivative, as we were not able to detect ABA in roots or leaves of carotenoid-deficient seedlings with an analysis sensitivity of 2 ng ABA g-1 fresh weight. This was not unexpected, as we had suspected for some time that the low levels of ABA found in carotenoid-deficient viviparous embryos were translocated from the maternal parent, and such ABA should be degraded in two-week-old seedlings.
Table 1. Abscisic acid content (ng ABA g-1 fresh wt.) ± s.d. of normal, Fluridone-treated and carotenoid-deficient mutant seedlings.
Table 2. Gravicurvatures (degrees) ± s.d. of primary roots of normal, Fluridone-treated and carotenoid-deficient seedlings.
J. D. Smith, Randy Moore* and Franklin Fong
*Baylor University, Waco, Texas
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