\usepackage {pstcol,pst-node,pst-tree}

\usepackage {pstcol,pst-node,pst-tree}
\psset{arrows=->,framearc=.2}
\newcommand{\Treebox}[1]{%
\Tr{\psframebox{#1}}}
\pstree[treemode=R]
{\Treebox{A$\rightarrow$B}}{
\pstree{
\Treebox{B$\rightarrow$C}
}{
\Treebox{A$\rightarrow$D}
\pstree[treemode=L]
{\Treebox{B$\rightarrow$E}}
{\Tn\TC[arrows=<-]}
}
}