Programmed cell death (PCD): an essential process of cereal seed development and germination

Abstract

The life cycle of cereal seeds can be divided into two phases, development and germination, separated by a quiescent period. Seed development and germination require the growth and differentiation of new tissues, but also the ordered disappearance of cells, which takes place by a process of programmed cell death (PCD). For this reason, cereal seeds have become excellent model systems for the study of developmental PCD in plants. At early stages of seed development, maternal tissues such as the nucellus, the pericarp, and the nucellar projections undergo a progressive degeneration by PCD, which allows the remobilization of their cellular contents for nourishing new filial tissues such as the embryo and the endosperm. At a later stage, during seed maturation, the endosperm undergoes PCD, but these cells remain intact in the mature grain and their contents will not be remobilized until germination. Thus, the only tissues that remain alive when seed development is completed are the embryo axis, the scutellum and the aleurone layer. In germinating seeds, both the scutellum and the aleurone layer play essential roles in producing the hydrolytic enzymes for the mobilization of the storage compounds of the starchy endosperm, which serve to support early seedling growth. Once this function is completed, scutellum and aleurone cells undergo PCD; their contents being used to support the growth of the germinated embryo. PCD occurs with tightly controlled spatial-temporal patterns allowing coordinated fluxes of nutrients between the different seed tissues. In this review, we will summarize the current knowledge of the tissues undergoing PCD in developing and germinating cereal seeds, focussing on the biochemical features of the process. The effect of hormones and redox regulation on PCD control will be discussed.

Endosperm and Nucellus Develop Antagonistically in Arabidopsis Seeds

“…The function of the nucellus in nonperispermic seeds is not yet clear. It was hypothesized that the nucellus degenerates in order to allocate its resources to embryo and endosperm development, which then become the primary nutrient storage compartments (Domínguez and Cejudo, 2014). Consistent with this hypothesis, we showed that Arabidopsis nucellus cells accumulate autophagosomes before undergoing cell death (Li and Vierstra, 2012).…”

“…This suggests that the nucellus plays a role at the boundary between the chalaza and the endosperm during seed development. In some monocotyledonous seeds, the nucellus cells positioned between the vascular bundle and the endosperm, termed the nucellar projection, survive longer than the other nucellus cells and mediate nutrient exchange (Domínguez and Cejudo, 2014). Furthermore, barley (Hordeum vulgare) grains have been shown to accumulate amyloplasts in the degenerating nucellus and nucellar projection and produce a-amylases for its degradation (Radchuk et al, 2009).…”

Histone Deacetylase Is Required for GA-Induced Programmed Cell Death in Maize Aleurone Layers

“…During seed germination, aleurone cells hydrolyze carbohydrate reserves and secrete hydrolases to the inner starchy endosperm, which is immediately followed by PCD (Fath et al, 2000). The PCD of aleurone cells is elicited by GA that was secreted by the embryo of the germinated seed so that nutrients in the aleurone layers can finally be completely utilized by the developing embryo (Fath et al, 2001a;Domínguez and Cejudo, 2014). Some putative GA-triggered PCD regulators have been identified in cereal aleurone cells (Fath et al, 2001b).…”

Developmental transitions: integrating environmental cues with hormonal signaling in the chromatin landscape in plants

“…The integuments undergo rapid cell division and expansion to form the seed coat [41], while the proximal region of the nucellus undergoes programmed cell death (PCD) [42]. Sporophyte PRC2 (EMBRYONIC FLOWER2 (EMF2)/VERNALIZATION2 (VRN2)-PRC2) exerts a block on seed coat development before fertilization, and lack of the core PRC2 subunits VRN2 and EMF2 results in dosage-dependent autonomous seed coat development [43].…”