Tight closure/is not plus closure/graded dimension two/Fakt/Beweisverweis

Consider

$R={𝔽}_2(t)[x,y,z]/(g).$

In this ring  $y^3{z}^3\in I^{\ast }$,  but it can not belong to the plus closure. Else there would be a curve morphism $Y\to C_{{𝔽}_2(t)}$ which annihilates the cohomology class $c$ and this would extend to a morphism of relative curves almost everywhere.