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Plant height is an important agronomic trait that is closely related to biomass yield and crop production. Despite legumes comprise one of the largest monophyletic families that are second only to grasses in terms of economic and nutritional values, due to an ancient genome duplication event, most legume plants have complex genomes, thus the molecular mechanisms that determine plant height are less known in legumes. Here, we report the identification and characterization of  MAIN STEM DWARF1  ( MSD1 ), which is required for the plant height in the model legume  Medicago truncatula . Loss of function of  MSD1  leads to severely reduced main stem height but normal lateral branch elongation in  M. truncatula.  Histological analysis revealed that the  msd1-1  main stem has shorter internodes with reduced cell size and number compared with the wild type, indicating that  MSD1  affects cell elongation and cell proliferation.  MSD1  encodes a putative GA 20-oxidase that is expressed at significantly higher levels in the main shoot apex than in the lateral shoot apices, suggesting that  MSD1  expression is associated with its effect on the main stem elongation. UPLC-MS/MS analysis showed that GA 9  and GA 4 , two identified products of the GA 20-oxidase, were severely reduced in  msd1-1 , and the dwarf phenotype of  msd1-1  could be rescued by supplementation with gibberellic acid GA 3 , confirming that  MSD1  functions as a biologically active GA 20-oxidase. Moreover, we found that disruption of either  MtGA20ox7  or  MtGA20ox8 , homologs of  MSD1 , has little effects on the elongation of the main stem, while the  msd1-1 mtga20ox7-1 mtga20ox8  triple mutants exhibits a severe short main shoot and lateral branches, as well as reduced leaf size, suggesting that  MSD1  and its homologs  MtGA20ox7  and  MtGA20ox8 , redundantly regulate  M. truncatula  shoot elongation and leaf development. Taken together, our findings demonstrate the molecular mechanism of  MSD1 -mediated regulation of main stem elongation in  M. truncatula  and provide insights into understanding the functional diversity of GA 20-oxidases in optimizing plant architecture in legumes.

The GA 20-Oxidase Encoding Gene MSD1 Controls the Main Stem Elongation in Medicago truncatula