Inflorescence Abscission Zones in Grasses: Diversity and Genetic Regulation

Abstract

Abscission of the fruit and associated floral structures, also called shattering, is a crucial survival strategy for propagation of wild grasses, and the loss of shattering in cereal crops is one of the most important domestication traits. Abscission occurs in specialised cell layers called the abscission zone (AZ). The fruit and floral AZs in different grass species are morphologically and anatomically diverse. In this article, we take a comparative approach and summarise our knowledge of the AZs in grasses at morphological, anatomical, and genetic levels. We show that the AZs in different grass species differ in terms of the position along the inflorescence, cell shapes of the AZ, and cell wall properties in the AZ, indicating distinctive mechanisms of abscission in these species. We further review the progress in illuminating the genetic regulation of AZ development in a few cereal crops. Current data suggest both conserved regulatory molecular mechanisms within grasses and between grass and dicot models, as well as unique gene functions currently found only in certain grass species. The rapid growth of genomic sequencing and molecular technologies will further advance our understanding of the molecular regulatory network and evolution of the AZ development.

TheSetaria viridisgenome and diversity panel enables discovery of a novel domestication gene

“…The precise mechanism of shattering is not known in any cereal, and recent data suggest that each species may have a unique mechanism 12 . Unlike other grasses, the AZ in Setaria viridis is not histologically distinct, with only subtle differences between wild type and domesticated plants 7,8 . As expected, we found that the anatomy and histology of SvLes1-INS and SvLes1-WT spikelets were indistinguishable ( Figure S4).…”

“…As in most wild species, seeds of S. viridis fall off the plant at maturity, a process known as shattering 7 . A zone of specialized cells, the abscission zone (AZ), forms below the spikelet during seed maturation, and loss of connections between the cells of the AZ result in cell separation and seed drop.…”

“…Setaria italica (foxtail millet) is the domesticated form of S. viridis; like most domesticates it fails to form an AZ and thus is non-shattering 8 . While loss of shattering has been an important early step in the domestication of all cultivated grasses, the genes involved generally differ among species 7,[9][10][11][12] . In particular, genes and QTL identified in rice domestication are, for the most part, not conserved among other species 7,11,12 .…”

“…While loss of shattering has been an important early step in the domestication of all cultivated grasses, the genes involved generally differ among species 7,[9][10][11][12] . In particular, genes and QTL identified in rice domestication are, for the most part, not conserved among other species 7,11,12 . However, most of these studies 5 have compared wild species to domesticates and none have considered the standing variation for reduced shattering in wild accessions.…”

Divergent gene expression networks underlie morphological diversity of abscission zones in grasses

“…The AZ may form below the flower in the rachilla, below sets of flowers in the pedicel or rachis, or even below the entire inflorescence ( Fig. 1a) (Doust et al, 2014b;Yu & Kellogg, 2018).…”