Tetratricopeptide Repeat 2 Is a Quantitative Trait Locus That Controls Seed Size

Seed size is a key trait affecting evolution and agronomic performance by influencing seedling establishment in natural populations and crop yields. The <i>Arabidopsis thaliana</i> Seed Size QTL1 (SSQ1) locus explains 10-15% of the variation in seed size. We report here that the causal gene for this locus is <i>Tetratricopeptide Repeat Protein 2</i> (<i>TPR2</i>), which encodes a co-chaperone. Expressing <i>TPR2</i> across ecotypes and genotypes showed consistent dosage effects. Each additional <i>TPR2</i>^Col-0 allele increased seed mass and volume by 10-14% with high reliability in Col-0, Sha, Tsu-1, and <i>tsu2</i> genetic backgrounds. Reciprocal genetic crosses indicated that this locus acts maternally, consistent with female sporophytic or female gametophytic mutations. To elucidate how <i>TPR2</i> regulates seed size, the biomass composition of seeds was measured. While oil content remained unchanged, sucrose levels were markedly elevated in <i>TPR2</i>^Col-0 transformant lines and reduced in <i>tpr2</i> mutants. Interestingly, heterologous expression of <i>TPR2</i>^Col-0 across genetic backgrounds increased seed protein accumulation by 18% on average. Based on these changes in sucrose and protein levels, potential modes of action for <i>TPR2</i> are discussed.