Last year we reported that there were two separate sites of synthesis for abscisic acid (ABA) found in maize kernels, i.e., "in situ ABA" is synthesized within the kernel while "maternal ABA" is synthesized in the maternal plant and transported into the developing kernel. It now appears that there may be three sources, since "maternal ABA" may have two components. Cob tissue cultured in vitro synthesizes ABA which appears to be transported into ABA-deficient embryos. Thus, "maternal ABA" (synthesized in distal regions of the plant), "cob ABA" (synthesized in the cob tissue) and "in situ ABA" components contribute to the total ABA found in the kernel.
Coupling genetic and chemical inhibitors of ABA synthesis with cultural manipulation of seed development has enabled us to partition ABA into various combinations of component sources. We believe the carotenoid-deficient mutants vp5, vp7(=ps), w3 and the persistent herbicide fluridone completely inhibit ABA synthesis in the kernel if applied prior to 10 days after pollination (DAP). Due to the rapid metabolism of ABA, "maternal ABA" that may have been present in kernel blocks harvested at 5 DAP and cultured in vitro for 10 days should be completely degraded, or nearly so. These assumptions provide the logical basis for our partitioning methodology shown in Table 1.
Preliminary data for this partitioning model are shown in Table 2. The field grown vs. cultured values are not directly comparable. The kernel blocks were cultured at a constant 30C which seemed to accelerate development, and their 15 DAP ABA levels are comparable to 18 DAP ABA in field grown embryos. However, comparisons within culture systems suggest that over 50% of the ABA in 15 DAP field grown embryos was of maternal origin. They also show that the cob contribution is appreciably less than that of the maternal plant.
Table 1. Sources of ABA found in maize kernels.
Table 2. Abscisic acid concentration [pMol ABA embryo-1 ± SE in 15 DAP embryos from field grown and in vitro cultured wild typea (WT), fluridone treatedb (WT-F) and homozygous carotenoid-deficientc (MUT) kernels of maize.
J.D. Smith, B.G. Cobb, C.W Magill, D.J. Hole and C.A. Blakey
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