Seed size is one of the most diverse traits in flowering plants, ranging from minute grains to large nutrient-rich seeds, and diversity has contributed to the evolutionary adaptation of plants across diverse environments. In crops, however, variation in seed size has agricultural importance, as even small changes in seed size can significantly influence the yield.

Many genes influencing seed development have been identified, but how plants integrate hormonal information to fine-tune seed size remains poorly understood. In particular, for auxin, one of the earliest discovered and most pervasive plant hormones, the mechanisms linking its signaling to final seed size is unknown.

In a study published in Molecular Plant, a research team led by Prof. LI Yunhai from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences identified a direct molecular link between auxin signaling and seed size control in plants. They revealed how auxin signaling fine-tunes seed development in Arabidopsis, acting through a phosphorylation-based switch.

Researchers focused on a plasma membrane receptor kinase, TMK1, known to perceive the plant hormone auxin together with ABP1/ABLs. They found that the loss of TMK1 resulted in noticeably smaller seeds, while its overexpression produced larger seeds. Genetic analyses showed that TMK1 acted redundantly with its homolog TMK4, and that its effect was mainly mediated through maternal tissues by regulating cell proliferation and expansion.

Moreover, researchers traced TMK1's downstream targets, and identified a protein DA1, which has been recognized as a brake on organ growth. TMK1 chemically modified DA1 through phosphorylation, marking it for degradation by the cell's protein-recycling machinery. Once DA1 is degraded, its inhibitory effect is lifted, allowing the accumulation of UBP15, a growth-promoting factor that drives increased cell proliferation in the seed coat. Auxin itself strengthens this chain reaction, boosting DA1 phosphorylation and tipping the balance towards growth.

As TMK1, DA1 and UBP15 have conserved homologs in major crops such as rice, wheat and maize, the findings provide a practical way for future breeding. This study provides a more precise way to modulate seed size by fine-tuning the phosphorylation-based switch, with potential implications for improving agricultural productivity.