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. Cultivated varieties differed in average AZ morphology, revealing lack of convergence in how shattering reduction was achieved during domestication. In contrast, weedy rice populations typically converged on complete AZs, irrespective of origin. By examining AZ population-level morphology, our study reveals its evolutionary plasticity, and suggests that the genetic potential to modify the ecologically and agronomically important trait of shattering is plentiful in rice lineages.

Methods
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 intact vascular bundle and symmetric flanking AZ were selected for quantification. RGB color images were first adjusted to 8-bit grayscale in ImageJ. Under grayscale, darkly stained regions are converted to pixels with low gray values while lightly stained regions are converted to pixels with high gray values. The AZ was identified as the darkly stained cells that span from the edge of the floret toward the vascular bundle. For each image, a square region was selected in the non-AZ region near the pedicel side and its minimum gray value was set as a threshold. Line scans were drawn, with the plot profile tool in ImageJ, from the edge of the floret to the vascular bundle following the natural curve of the AZ. Pixels along the line scan with gray values greater than the threshold were classified as belonging to non-AZ tissue and pixels with lower values were classified as AZ tissue. The resulting profiles were used to produce three measures: relative length, discontinuity, and intensity. AZ relative length was calculated by dividing the number of pixels below the threshold by the total number of pixels in the line scan. AZ discontinuity was calculated by summing the number of AZ pixels clusters, where these refer to a series of contiguous pixels below the threshold bounded by non-AZ tissue on one or both sides. AZ intensity was calculated by dividing the threshold value by the mean grey value of the AZ tissue, which provided a scaled measure of the staining contrast between AZ and non-AZ tissues. The three measures were performed on the left and right side of each section and averaged for subsequent analyses.