Plant chromosome polytenization contributes to suppression of root growth in high polyploids

Autopolyploidization, which refers to a polyploidization via genome duplication without hybridization, promotes growth in autotetraploids, but suppresses growth in high polyploids (autohexaploids or auto-octoploids). The mechanism underlying this growth suppression (i.e. 'high-ploidy syndrome') has not been comprehensively characterized. In this study, we conducted a kinematic analysis of the root apical meristem cells in Arabidopsis thaliana autopolyploids (diploid, tetraploid, hexaploid, and octoploid) to determine the effects of the progression of genome duplication on root growth. The results of the root growth analysis showed that tetraploidization increases the cell volume, but decreases cell proliferation. However, cell proliferation and volume growth are suppressed in high polyploids. Whole-mount fluorescence in situ hybridization analysis revealed extensive chromosome polytenization in the region where cell proliferation does not usually occur in the roots of high polyploids, which is likely to be at least partly correlated with the suppression of endoreduplication. The study findings indicate that chromosome polytenization is important for the suppressed growth of high polyploids.