The version 0.19 of JuMP based on MathOptInterface (MOI) was designed to be extended in a natural way so that new classes of optimization problems can be modeled with JuMP. The packages PolyJuMP.jl and SumOfSquares.jl extend JuMP to enable the creation of polynomial variables as well as equality or nonnegative constraints on polynomials. The bridge mechanism of MOI is exploited to express the cone of Sum-of-Squares polynomials as a linear image of the cone of positive semidefinite matrices.