Researchers from the Chinese Academy of Sciences (CAS) and the University of
Chinese Academy of Sciences reported that a MAPK cascade signalling
regulates grain size in rice. The results are published in Molecular Plant.

MAPK signaling pathways have been known to have significant functions in
multiple processes of plant growth and development and defense responses.
The MAPK cascade contains at least three kinases: a MAPK kinase kinase
(MKKK), a MAPK kinase (MKK) and a MAPK.

A previous study showed that loss-of-function mutant of OsMKK4 (smg1) forms
small grains in rice. In the latest study, they identified a
gain-of-function mutant of OsMKK4, which forms large grains. They also
identified the smg2 mutant, which exhibits similar small-grain phenotype as
smg1. SMG2 encodes OsMKKK10.

Overexpression of constitutively active OsMKKK10 leads to large grains.
Biochemical analysis showed that OsMKKK10 could sequentially phosphorylate
and activate OsMKK4 and OsMAPK6. Further analyses showed that enhanced
activity of OsMAPK6 results in large grains, while decreased activity of
OsMAPK6 leads to small grains. Genetic analysis showed that OsMKKK10,
OsMKK4, and OsMAPK6 function in a common pathway to control grain size.

Their analyses revealed that OsMKKK10, OsMKK4 and OsMAPK6 act as a cascade
to promote grain growth.

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