α-Amylase and programmed cell death in aleurone of ripening wheat grains

Abstract

Late maturity alpha-amylase (LMA) in wheat is a genetic defect that may result in the accumulation of unacceptable levels of high pI alpha-amylase in grain in the absence of germination or weather damage. During germination, gibberellin produced in the embryo triggers expression of alpha-Amy genes, the synthesis of alpha-amylase and, subsequently, cell death in the aleurone. LMA also involves the aleurone and whilst LMA appears to be independent of the embryo there is nevertheless some evidence that gibberellin is involved. The aim of this investigation was to determine whether the increase in alpha-amylase activity in LMA-prone genotypes, like alpha-amylase synthesis by aleurone cells in germinating or GA-challenged grains, is followed by aleurone cell death. Programmed cell death was seen in aleurone layers from developing, ripe and germinated grains using confocal microscopy and fluorescent probes specific for dead or living cells. Small pockets of dying cells were observed distributed at random throughout the aleurone of ripening LMA-affected grains and by harvest-ripeness these cells were clearly dead. The first appearance of dying cells, 35 d post-anthesis, coincided with the later part of the 'window of sensitivity' in grain development in LMA-prone wheat cultivars. No dead or dying cells were present in ripening or fully ripe grains of control cultivars. In germinating grains, dying cells were observed in the aleurone adjacent to the scutellum and, as germination progressed, the number of dead cells increased and the affected area extended further towards the distal end of the grain. Aside from the obvious differences in spatial distribution, dying cells in 20-24 h germinated grains were similar to dying cells in developing LMA-affected grains, consistent with previous measurements of alpha-amylase activity. The increase in high pI alpha-amylase activity in developing grains of LMA-prone cultivars, like alpha-amylase synthesis in germinating grains, is associated with cell death, providing further evidence for the involvement of gibberellin in the LMA response.

Genetic, Hormonal, and Physiological Analysis of Late Maturity α-Amylase in Wheat

“…Previous studies have shown that highpI a-amylase is produced in small pockets of cells randomly distributed across the aleurone layer during a particular developmental window (Mrva et al, 2006). We failed to find any anatomical differences between aleurone cells from a constitutive +LMA genotype and a -LMA genotype, although this may have been due to a failure to section affected cells previously shown to be sparsely distributed in the aleurone (Mrva et al, 2006). However, what our anatomical studies show is that aleurone cells from +LMA and -LMA genotypes are at a similar developmental stage when LMA appears.…”

“…Using those cutoff criteria, only 37 probes were identified as differentially expressed at 20 and/or 23 DPA. Then, and thinking that only a very reduced proportion of aleurone cells in an LMA variety express LMA (about 2%; Mrva et al, 2006), we decided to use less stringent criteria to avoid the effects of possible gene expression dilution. So probes with 61.5-fold change between +LMA and -LMA groups and P , 0.01 were considered.…”

“…One explanation for that is the apparent simplicity of the aleurone tissue, formed by just a single layer of identical cell types, which may have a reduced number of genes being expressed. Another possible explanation is that only about 2% of the aleurone cells in the +LMA genotypes undergo LMA (Mrva et al, 2006), so gene expression changes associated with LMA may be diluted. Of the 56 probes up-regulated in LMA-prone aleurone (Table I), three have been previously related to PCD (Initiation factor2 family protein, Programmed cell death protein2, and NucleaseI; Young and Gallie, 2000), consistent with the involvement of GA in LMA, as GA response leads to PCD in mature aleurone (Fath et al, 2000).…”

“…However, while there are some points of similarity between a-amylase production in germination and LMA, there are a number of differences (Mrva et al, 2006;Mares and Mrva, 2008a). In germinated grain, a-amylase is synthesized by the scutellum and later by the adjacent aleurone, the latter under the influence of GA produced by the embryo.…”

Thermodynamics of Seed and Plant Growth

“…With the exception of stress situations, this phenomenon is connected with planned dismissing of individual parts of an organism, which lose functionality (programmed cell death), such as necrosis of aleurone and seed rest, after shifting to autonomic nutrition (Mrva et al, 2006). This approach represents the first step into a new area of quantitative biology.…”