\psset{arrows=->,framearc=.2}
\def\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=<-]}
      }
}