Aspects of grain development in Brachypodium distachyon and discerning the roles of euAP2 genes in plant development

Cereals are very important for world food security. Existing knowledge on cereal grain development are mostly based on cultivated species. Consequently, little is known about grain development in undomesticated species. Understanding grain development in undomesticated species will offer comparative insight on cereal grain development and evolution. Such insight can inform future breeding and domestication efforts. In this thesis, we report studies on assimilate supply and programmed cell death (PCD) during grain development in Brachypodium distachyon, an undomesticated model for cereals. On nutrient supply to developing grains, we propose caryopsis endosperm assimilate acquisition route (CEAAR) models, that describe the post-phloem supply route of assimilate destined for storage in grains. We demonstrate that Brachypodium post-phloem assimilate delivery pathway is structurally similar to wheat and barley, but functionally identical to rice. Our studies on PCD in developing Brachypodium grains revealed the importance of rapid nucellar PCD for grain size and that the pattern of mesocarp PCD affects grain shape. We also identified candidate proteases that may be part of the molecular machinery for PCD execution in Brachypodium grains. Another aspect of this thesis investigated the roles of euAP2 genes in plant development, with an emphasis on grain development. We characterised barley HvAPETALA2 mutant lines and Brachypodium RNAi lines of Bd RICE STARCH REGULATOR 1. Although they are well-known for their roles in flowering, our results indicate that euAP2 genes influence other phenotypes such as plant height, tillering, ovule development, grain dimension, and starch granule distribution. Our co-expression analysis revealed a distinct co-expression profile for four out of six Arabidopsis euAP2 genes, indicating their functional diversity.