Overexpression of microRNA OsmiR397 improves rice yield by increasing grain size.pdf

? 20 13 N at u re A m er ic a, In c. A ll ri g h ts r es er ve d . nature biotechnology? advance online publication  l e t t e r s Increasing grain yields is a major focus of crop breeders around the world. Here we report that overexpression of the rice microRNA (miRNA) OsmiR397, which is naturally highly expressed in young panicles and grains, enlarges grain size and promotes panicle branching, leading to an increase in overall grain yield of up to 25% in a field trial. To our knowledge, no previous report has shown a positive regulatory role of miRNA in the control of plant seed size and grain yield. We determined that OsmiR397 increases grain yield by downregulating its target, OsLAC, whose product is a laccase- like protein that we found to be involved in the sensitivity of plants to brassinosteroids. As miR397 is highly conserved across different species, our results suggest that manipulating miR397 may be useful for increasing grain yield not only in rice but also in other cereal crops. Rice yield is a complex trait that is directly associated with grain size and with panicle number, the number of grains per panicle1. Although several genes have been found to regulate these traits2–8, our knowledge of the gene networks that control rice yield is still limited. Exploring new genes that modulate these traits would help us better understand the relevant molecular mechanisms and would also facili- tate the breeding of new varieties with higher yields. miRNAs, a class of abundant small noncoding RNAs, have been identified as impor- tant regulators of gene expression in both plants and animals and are involved in many aspects of plant development, including the modu- lation of plant agricultural traits9. Two recent studies reported that OsmiR156 could negatively control the number of panicle branches and grain yield10,11. These observations suggest a role for miRNAs in controlling rice yield. However, miRNAs that positively regulate g