Ploidy and size phenomena are observed to be correlated across several biological scales, from subcellular to organismal. Two kinds of ploidy change can affect plants. Whole-genome multiplication increases ploidy in whole plants and is broadly associated with increases in cell and organism size. Endoreduplication increases ploidy in individual cells. Ploidy increase is strongly correlated with increased cell size and nuclear volume. Here, we investigate scaling relationships between ploidy and size by simultaneously quantifying nuclear size, cell size, and organ size in sepals from an isogenic series of diploid, tetraploid, and octoploid Arabidopsis thaliana plants, each of which contains an internal endopolyploidy series. We find that pavement cell size and transcriptome size increase linearly with whole-organism ploidy, but organ area increases more modestly due to a compensatory decrease in cell number. We observe that cell size and nuclear size are maintained at a constant ratio; the value of this constant is similar in diploid and tetraploid plants and slightly lower in octoploid plants. However, cell size is maintained in a mutant with reduced nuclear size, indicating that cell size is scaled to cell ploidy rather than to nuclear size. These results shed light on how size is regulated in plants and how cells and organisms of differing sizes are generated by ploidy change.

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Highlights d Nitrate regulates shoot growth by modulating cell size and endoreplication d Nitrate upregulates LGO, increasing ploidy and cell size in cotyledons and leaves d Cell division can substitute for cell expansion to maintain organ size

In this issue of Developmental Cell, Skopelitis et al. (2017) demonstrate that sharp boundaries of gene expression can be created by threshold-based readout of mobile small RNA gradients. Support for this hypothesis comes from manipulation of small RNAs involved in top-bottom leaf patterning and from a novel synthetic biology approach.

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