No signals are present in the endosperm, integuments, nucellar projection, and vascular bundle

No signals are present in the endosperm, integuments, nucellar projection, and vascular bundle. == Fig. timely and spatially regulated programmed cell death (PCD) processes in maternal seed tissues. When the endosperm is coenocytic, PCD events are only observed within the nucellus. Thereby, remobilization of nucellar storage compounds by PCD could nourish the early developing endosperm when functional interconnections are absent between maternal and filial seed organs. Specific proteases promote PCD events. Characterization of the barley vacuolar PF-06282999 processing enzyme (VPE) gene family identified seven gene members specifically expressed in the developing grain. HvVPE2a (known as nucellain) together with closely similar HvVPE2b and HvVPE2d might be involved in nucellar PCD. HvVPE4 is strongly cell specific for pericarp parenchyma. Correlative evidence suggests that HvVPE4 plays a role in PCD events in the pericarp. Possible functions of PCD in the maternal tissues imply a potential nutritive role or the relief of a physical restraint for endosperm growth. PCD could also activate post-phloem transport functions. Keywords:Barley, cell expansion, grain development, maternal tissues, programmed cell death == Introduction == In angiosperms, the filial seed organs (embryo and endosperm) develop within maternal tissues. During early development of barley (Hordeum PF-06282999 vulgareL.) seeds, the diploid maternal tissues represent the bulk of the grain and consist of the nucellus, nucellar projection, inner and outer integuments, and pericarp. Vascular bundles are embedded in the pericarp. The nucellus degenerates shortly after flowering, except adjacent to the main vascular bundle, where it forms the nucellar projection, which functions as transfer tissue supplying assimilates to the endosperm. There is evidence that nucellar development and assimilate transfer involve programmed cell death (PCD;Radchuket PF-06282999 al., 2006;Thielet al., 2009). Likewise, the seed coat in dicotyledonous legumes functions as the maternal conduit for the flow of assimilates to the embryo (Weberet al., 1995;Patrick and Offler, 2001). The developmentally regulated degradation of the innermost cell layer, probably by PCD, initiates the storage phase inVicia fabaseeds and is accompanied by a switch from hexose to sucrose due to loss of ARF6 extracellular invertase expressed in this layer (Weberet PF-06282999 al., 1995). PCD is a genetically regulated process essential for development, integrity, and response to the environment. It is involved in deleting structures when unneeded, such as the embryo suspensor (Lombardiet al., 2007), thus controlling cell number and tissue size. In monocot seeds, PCD mediates degeneration of the PF-06282999 endosperm (Olsen, 2004), aleurone depletion in germinating seeds (Pennel and Lamb, 1999), and disintegration in the nucellar projection (Radchuket al., 2006). In dicot seeds, PCD is observed in the seed coat, suspensor, and endosperm (Lombardiet al., 2007). In apoptosis-like PCD, nuclei are the first targets of degradation, demonstrated by nuclear shrinkage and fragmentation, chromatin condensation, and DNA laddering. DNA fragments can be cytochemically determined by the terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) of the 3-OH groups (Gavireliet al., 1992). Fragmentation of DNA, detected by gel electrophoresis and TUNEL assay, indicated the presence of PCD in wheat (Triticum aestivumL.) pericarp and nucellus already at anthesis (Dominguezet al., 2001;Zhouet al., 2009). Several groups of cysteine proteases have been shown to be involved in plant PCD. Metacaspases are only distantly related to animal caspases, lack caspase activity, and cleave after arginine and lysine residues but not aspartate (Vercammenet al., 2007). However, at least some metacaspases can be functional homologues of caspases (Bozhkovet al., 2005). Other plant proteases possess caspase-like activity and are shown to be frequently involved in plant PCD. These include vacuolar processing enzyme (VPE), also called legumain (Hara-Nishimuraet al., 2005;Hatsugaiet al., 2006), subtilisin (Coffeen and Wolpert, 2004), and phytaspase (Chichkovaet al., 2010).Arabidopsis thalianahas four VPE genes, VPE, VPE, VPE, and VPE. Two are preferentially expressed in vegetative tissues and two are seed specific. Quadruple knockout mutants lack any caspase-1 activity, indicating the absence of other related proteases (Kuroyanagiet al.,.