Vacuolar processing enzyme 4 contributes to maternal control of grain size in barley by executing programmed cell death in the pericarp

The angiosperm embryo and endosperm are limited in space because they grow inside maternal seed tissues. The elimination of cell layers of the maternal seed coat by programmed cell death (PCD) could provide space and nutrition to the filial organs. Using the barley (Hordeum vulgare L.) seed as a model, we elucidated the role of vacuolar processing enzyme 4 (VPE4) in cereals by using an RNAi approach and targeting the enzymatic properties of the recombinant protein. A comparative characterization of transgenic versus wild-type plants included transcriptional and metabolic profiling, flow cytometry, histology and nuclear magnetic imaging of grains. The recombinant VPE4 protein exhibited legumain and caspase-1 properties in vitro. Pericarp disintegration was delayed in the transgenic grains. Although the VPE4 gene and enzymatic activity was decreased in the early developing pericarp, storage capacity and the size of the endosperm and embryo were reduced in the mature VPE4-repressed grains. The persistence of the pericarp in the VPE4-affected grains constrains endosperm and embryo growth and leads to transcriptional reprogramming, perturbations in signalling and adjustments in metabolism. We conclude that VPE4 expression executes PCD in the pericarp, which is required for later endosperm filling, and argue for a role of PCD in maternal control of seed size in cereals.

INHIBITION OF CASPASE-LIKE ACTIVITIES PREVENTS THE APPEARANCE OF REACTIVE OXYGEN SPECIES AND DARK-INDUCED APOPTOSIS IN THE UNICELLULAR CHLOROPHYTE DUNALIELLA TERTIOLECTA(1)

When the chlorophyte alga Dunaliella tertiolecta Butcher is placed in darkness, a form of programmed cell death with many similarities to apoptosis is induced, including the induction of caspase-like proteases. Many uncertainties about the regulation and mediators that participate in the process remain. To examine the relationship between caspase-like activities and different apoptotic events (i.e., phosphatidylserine [PS] translocation), increases in membrane permeability and numbers of dead cells revealed by SYTOX-green staining, and the generation of reactive oxygen species (ROS), we used the broad-range caspase inhibitor Boc-D-FMK to block the activity of the whole class of caspase-like proteins simultaneously. In the presence of the inhibitor, ROS were not produced, and cells did not die. Loss of membrane asymmetry, indicated by external labeling of PS by annexin V, was apparent at midstages of light deprivation, although it did not conform to the typical pattern for PS exposure observed in metazoans or vascular plants, which occurs at early stages of the apoptotic event. Thus, we have evidence for a link between ROS and cell death involving caspase-like enzymes in an alga. The fact that caspase-like inhibitors prevent not only cell death, but also ROS and loss of cell membrane integrity and asymmetry, suggests that caspase-like proteases might have regulatory roles early in cell death, in addition to dismantling functions.