The Arabidopsis transcription factor TCP4 controls seed size by repressing the MINI3-SHB1-IKU pathway

As an important agronomic trait that directly determines grain yield, seed size is tightly controlled by the timing of endosperm cellularization and seed coat proliferation. However, the precise regulatory mechanisms that coordinate the two processes remain elusive. Here, we demonstrate that in Arabidopsis (Arabidopsis thaliana), CINCINNATA (CIN)-like TEOSINTE BRANCHED 1/CYCLOIDEA/PCF (TCP) transcription factors are crucial for seed size by controlling both endosperm cellularization and seed coat growth. Disruption of TCP function in the triple mutant tcp3/4/10 causes enlarged seeds due to the delayed endosperm cellularization and accelerated seed coat growth. TCP4 directly represses MINISEED3 (MINI3) and SHORT HYPOCOTYL UNDER BLUE1 (SHB1), key factors for endosperm cellularization, by recruiting the FERTILIZATION INDEPENDENT SEED-POLYCOMB REPRESSIVE COMPLEX 2 (FIS-PRC2) complex. TCP4 also interacts with MINI3 to repress MINI3-SHB1 complex activity, thereby suppressing the expression of MINI3 downstream genes. We further show that TCP4 represses the expression of AINTEGUMENTA (ANT), a key regulator of integument and seed coat growth, by directly binding to its promoter. Our findings demonstrate that CIN-like TCPs play critical roles in controlling seed size by promoting endosperm cellularization and concurrently inhibiting seed coat proliferation.