Comparative histology of abscission zones reveals the extent of convergence and divergence in seed shattering in weedy and cultivated rice

The modification of seed shattering has been a recurring theme in rice evolution. The wild ancestor of cultivated rice disperses its seeds, but reduced shattering was selected during multiple domestication events to facilitate harvesting. Conversely, selection for increased shattering occurred during the evolution of weedy rice, a weed invading cultivated rice fields that has originated multiple times from domesticated ancestors. Shattering requires formation of a tissue known as the abscission zone (AZ), but how the AZ has been modified throughout rice evolution is unclear. We quantitatively characterized the AZ characteristics of relative length, discontinuity, and intensity in 86 cultivated and weedy rice accessions. We reconstructed AZ evolutionary trajectories and determined the degree of convergence among different cultivated varieties and among independent weedy rice populations. AZ relative length emerged as the feature best distinguishing high and low shattering rice. Cultivate..., Rice florets were fixed in an FAA solution (37% formaldehyde: ethanol: water: acetic acid=10:50:35:5), and dehydrated in a graduated ethanol series (50%, 70%, 85%, 95%, and 2 changes of 100% ethanol), followed by clearing with an ethanol HistoClear solution (25%, 50%, 75%, and 3 changes of 100% Histoclear). The HistoClear was drained and paraplast was added and changed every 6 hours six times at 60℃. Florets were embedded in tissue cassettes with paraplast, serially sectioned into 10 𝜇m longitudinal slices with a Leica RM2125 microtome, mounted on slides, and placed on a slide warmer at 42℃ for at least 24 hours. The slides were stained with 1% toluidine blue solution for 30 minutes. Excess paraplast was cleared with two xylene washes. A glass cover slip was affixed to the slide with Permount. A Leica DM750 LED biological microscope with a 4X or 10x objective was used to image florets. Images were captured with an AmScope MU1000-HS camera and AmLite software.  
Images with a clear intac..., , # Comparative histology of abscission zones reveals the extent of convergence and divergence in seed shattering in weedy and cultivated rice

[https://doi.org/10.5061/dryad.dv41ns265](https://doi.org/10.5061/dryad.dv41ns265)

**Background introduction** 

Seed shattering requires the formation of one or more specialized cell layers, collectively called an abscission zone (AZ), where an organ separates from the plant. The cells that constitute the AZ are often distinct from surrounding cells, and are generally small in size, have dense cytoplasm, lack secondary cell wall, and have highly branched plasmodesmata. AZ location and anatomy vary among grass species. In rice, the abscission zone is formed in the rachilla, between the pedicel and floret, and consists of cells that lack lignin. 

Here, we take a population-level approach to characterize AZ morphology in different cultivated and weedy rice groups. We develop measures to quantify aspects of the AZ and use this comparative histology...