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Figure 1. 

Control of grain size and number by the MAPK signaling in rice and a comparison to that in Arabidopsis. (a) OsMKKK10–OsMKK4–OsMPK6 and OsMKKK70–OsMKK4–OsMAPK6 ascades play key roles in grain-size control in rice. OsWRKY53 acts downstream of OsMAPK6 to regulate grain size by mainly promoting cell expansion and slightly promoting cell proliferation in the spikelet, whereas the major downstream targets of OsMAPK6 that facilitate cell proliferation in the spikelet hull remain elusive. OsER1 functions upstream of the OsMKKK10–OsMKK4–OsMPK6 module and regulates the morphology of panicles and the number of spikelets per panicle by influencing the metabolism of cytokinin. EPFL peptides act as ligands of OsER1 in this signaling pathway. GSK2, OsMKP1, and OsRac1 influence grain size by modulating the MAPK cascade, whereas FLR1 regulates grain size probably through the OsRac1-OsMAPK6 module. OsMKK3 is not included in the illustration as the upstream and downstream components are unknown. (b) In Arabidopsis, ER functions upstream of the MKK4/5–MPK3/6 cascade to control seed size by regulating DA1-UBP15 activity.