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Hormone breakdown may restore balance
Abscisic acid (ABA) in seeds is involved in desiccation tolerance, in regulating storage product synthesis and in imposing dormancy, either transiently, to prevent precocious germination, or over the longer term. The ABA involved in these processes is probably synthesized in the embryo itself. However, the embryo and endosperm can also respond to ABA from exogenous sources, including maternal tissue. Indeed, the latter is, in many species, the source of high concentrations of ABA during environmental stress. However, ABA arriving at the wrong time can be disastrous. In maize, ABA inhibits cell division in the endosperm; if this happens early in seed development the kernels fail to develop properly. This may explain why kernels at the apex of the ear (the younger kernels) are more vulnerable to environmental stress than are basal kernels. The story is taken up by Wang et al. (Cornell University, USA; pp. 623-630). They withheld water from maize plants from 1 d after pollination up to 6 d after pollination, after which the plants were re-watered. Apical and basal kernels were analysed for ABA and its metabolites. In both positions, kernels accumulated more ABA in stressed plants than in control plants, but the increase was much greater in apical kernels (seven-fold, compared with three-fold). On re-watering, kernels in both positions lost ABA rapidly, reaching control levels within 24 h, but at all times during the experimental treatment apical kernels had more ABA than basal kernels. The loss of ABA was due both to oxidative catabolism and to efflux. The ABA lost by efflux was again oxidized, mostly in the placenta (where ovules are anchored in maternal tissue). Thus, the ABA concentration in these early kernels results from a dynamic balance of transport and metabolic processes. We must agree with the authors that this situation, so relevant to grain yield in stress conditions, ‘merits further investigation’.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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