On centralizers of the graph automorphisms of niltriangular subalgebras of Chevalley algebras

Graph automorphisms of a Chevalley group correspond to each type of reduced indecomposable root system $\Phi$, which Coxeter graph has a non-trivial symmetry. It is well-known, that a Chevalley algebra and its niltriangular subalgebra $N$ has a graph automorphism $\theta$ exaclty when $\Phi$ is of type $A_n$, $D_n$ or $E_6$. We note connections with homomorphisms of root systems introduced in 1982.
The main theorem on the centralizers in $N$ of the automorphism $\theta$ gives new representations of niltriangular subalgebras, using also the unique series of unreduced indecomposable root system of type $BC_n$.