Abstract
The normal developmental sequence in a grass grain entails the death of several maternal and filial tissues in a genetically regulated process termed programmed cell death (PCD). The progression and molecular aspects of PCD in developing grains have been reported for domesticated species such as barley, rice, maize and wheat. Here, we report a detailed investigation of PCD in the developing grain of the wild model species Brachypodium distachyon. We detected PCD in developing Brachypodium grains using molecular and histological approaches. We also identified in Brachypodium the orthologs of protease genes known to contribute to grain PCD and surveyed their expression. We found that, similar to cereals, PCD in the Brachypodium nucellus occurs in a centrifugal pattern following anthesis. However, compared to cereals, the rate of post-mortem clearance in the Brachypodium nucellus is slower. However, compared to wheat and barley, mesocarp PCD in Brachypodium proceeds more rapidly in lateral cells. Remarkably, Brachypodium mesocarp PCD is not coordinated with endosperm development. Phylogenetic analysis suggests that barley and wheat possess more vacuolar processing enzymes that drive nucellar PCD compared to Brachypodium and rice. Our expression analysis highlighted putative grain-specific PCD proteases in Brachypodium. Combined with existing knowledge on grain PCD, our study suggests that the rate of nucellar PCD moderates grain size and that the pattern of mesocarp PCD influences grain shape.
Highlights
Ordered and finely regulated elimination of cells in living organisms is called programmed cell death (PCD).PCD events that occur during grain development have been reported in cereals.Generally, the nucellus rapidly undergoes PCD in the first five days following fertilization, generating space and leaving behind growth materials for the incipient endosperm [1,2,3].In maize, nucellus PCD proceeds acropetally while in barley, rice, and wheat, nucellusPCD is centrifugal [4,5,6]
All that remains of the nucellar tissues at about 6 days post anthesis (DPA) are the nucellar epidermis, which is bordered on the outside by the inner integument; nucellar lysate, which is debris of lysed nucellus cells sandwiched between the nucellar epidermis and the expanding endosperm; and the nucellar projection, which is located at the chalazal region between the endosperm and pigment strand [2,3,7,8,9,10]
The nucellar epidermis of Brachypodium grains serves as an assimilate transport route during grain filling and is greatly enlarged by 5 DPA (Figure 2a) [40]
Summary
Ordered and finely regulated elimination of cells in living organisms is called programmed cell death (PCD).PCD events that occur during grain development have been reported in cereals.Generally, the nucellus rapidly undergoes PCD in the first five days following fertilization, generating space and leaving behind growth materials for the incipient endosperm [1,2,3].In maize, nucellus PCD proceeds acropetally while in barley, rice, and wheat, nucellusPCD is centrifugal [4,5,6]. The nucellus rapidly undergoes PCD in the first five days following fertilization, generating space and leaving behind growth materials for the incipient endosperm [1,2,3]. Nucellus PCD proceeds acropetally while in barley, rice, and wheat, nucellus. In rice the nucellar epidermis serves as a transport route for nutrients into the endosperm and eventually collapses by 21 DPA, terminating grain filling [9,12,13]. PCD in the nucellar projection region proceeds from cells proximal to the endosperm towards cells at the chalazal region [10,14,15]. Nucellar projection PCD has been detected at 3, 8 and 13 DPA in rice, barley and wheat, respectively [5,6,15,16]
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