The anatomy of abscission zones is diverse among grass species

Premise: Abscission zones (AZ) are specialized cell layers that separate plant parts at the organ junction upon developmental or environmental signals. Fruit or seed abscission has been well studied in model species because of its crucial role for seed dispersal. Previous work showed that AZ localization differs among species of Poaceae and that AZ formation is histologically and genetically distinct in three distantly related grass species, refuting the idea of a broadly conserved module. However, whether AZ structure is consistent within subfamilies is unknown. Methods: Eleven species were selected from six subfamilies of Poaceae, and their AZ was investigated using paraffin-embedded, stained material. Observations were added from the literature for an additional six species. Data were recorded on AZ location and whether cells in the AZ were distinguishable by size or lignification. Characteristics of the AZ were mapped on the phylogeny using maximum likelihood. Results: Abscission zone anatomy and histology vary among species, and characteristics of the AZ do not correlate with phylogeny. Twelve of the seventeen studied species have an AZ in which the cells are significantly smaller than surrounding cells. Of these, eight have differential lignification. Differential lignification is often associated with differential cell size, but not vice versa. Conclusions: Neither smaller cells in the AZ nor differential lignification between the AZ and surrounding cells is required for abscission, although differential cell size and lignification are often correlated. Abscission zone anatomy does not correlate with phylogeny, suggesting its rapid change over evolutionary time.