Seed size, a key factor affecting crop yield, is regulated by maternal and zygotic signals that control the growth of the seed coat, endosperm and embryo. One such signal are gibberellins, hormones that regulate plant growth and development. Here, Maria Dolores Gomez and colleagues show that DELLA proteins, repressors of gibberellin signalling, play a role in the maternal control of Arabidopsis seed size. First, the authors characterise seed phenotype of the gain-of-function DELLA mutants and observe larger seeds. They find evidence that DELLA proteins are expressed in developing ovules, and that DELLA acts maternally to increase ovule size by increasing cell division in ovule integuments, which will become the seed coat. Then, to investigate the molecular mechanisms of how DELLA proteins influence seed size, the authors focus their attention on AINTEGUMENTA (ANT), a transcriptional factor important for organ growth control. They find that DELLA proteins promote ANT transcriptional activation, and overexpressing ANT produces similar seed phenotypes as the gain-of-function DELLA mutant, suggesting a possible role of ANT in mediating DELLA function. Finally, the authors find that modulating DELLA activity only impacts seed size, but not seed structure or composition. Overall, the findings identify DELLA proteins as new players in controlling seed size.
