Automorphisms of the endomorphism semigroup of a free commutative algebra

We describe the automorphism group of the endomorphism semigroup $\End(K[x_1,...,x_n])$ of ring $K[x_1,...,x_n]$ of polynomials over an {\it arbitrary} field $K$. A similar result is obtained for automorphism group of the category of finitely generated free commutative-associative algebras of the variety $\mathcal{CA}$ commutative algebras. This solves two problems posed by B. Plotkin (\cite{24}, Problems 12 and 15). More precisely, we prove that if $\varphi\in \Aut\End(K[x_1,...,x_n])$ then there exists a semi-linear automorphism $s:K[x_1,...,x_n]\to K[x_1,...,x_n]$ such that $\varphi(g)=s\circ g\circ s^{-1}$ for any $g\in\End(K[x_1,...,x_n])$. This extends the result by A. Berzins obtained for an infinite field $K$.