Monday, November 5, 2007 - 2:30 PM
70-3
The Role of Ethylene in Cereals: From Developmental Programmed Cell Death to the Abiotic Stress Response.
Programmed cell death (PCD) in plants is used to modify existing organs or to discard organs that are no longer required as part of a normal developmental plan or in order to adapt to environmental conditions. Examples of this in maize are the death of root cortical cells to create air spaces (i.e., lysigenous aerenchyma) in response to flooding and kernel abortion in response to insufficient light as a means to conserve resources. In many cases of PCD, ethylene serves to induce PCD or regulate the timing of its induction. In cereal seed, the endosperm at maturity consists of non-living reserve cells and living aleurone cells. Cell death initiates within the central and upper crown regions of the endosperm at approximately 16 days after pollination and proceeds towards the base of the kernel during its subsequent development. The death of the endosperm likely facilitates access of those hydrolases produced during germination to support growth of the seedling. Ethylene and abscisic acid (ABA) are involved in regulating endosperm PCD where ethylene promotes and ABA delays the program. Starch biosynthetic mutants exhibit higher levels of ethylene production as well as ectopic PCD in the scutellum, resulting in inviable embryos which could be phenocopied in wild-type kernels by treatment with exogenous ethylene. The onset and progression of cell death during wheat starchy endosperm development was controlled by ethylene in a manner similar to that observed for starchy maize endosperm. Ethylene receptor expression in the endosperm was substantially lower than in the embryo during kernel development, suggesting that PCD is induced selectively in the starchy endosperm because it is more sensitive to the ethylene generated during kernel development. Maize mutants impaired in ethylene biosynthesis exhibited delayed leaf senescence in aging leaves, increased chlorophyll content in all leaves, and increased photosynthetic activity under normal and water-stress conditions, suggesting that ethylene negatively regulates photosynthetic activity. Root growth in such mutants was also affected in response to mechanical impedance, suggesting that ethylene is important in regulating root growth in response to soil conditions.
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