This organelle houses several important steps
in the synthesis of phytohormones involved in defence, such
as SA, JA, and ABA. As the ultimate source of photoassimilate,
the chloroplast also contributes to sugar status,
which can influence the SA pathway and interact with
signalling through other phytohormones such as ABA that
are involved in biotic challenge (Finkelstein and Gibson,
2002; Roitsch et al., 2003; Asselbergh et al., 2008).
Moreover, several recent studies have shown that chloroplast-
located proteins are involved in cross-talk with the
cytosol and nucleus to govern the outcome of defence
signalling. Further important information in this area is
likely to be generated by the continued use of genetic
studies in amenable species such as Arabidopsis.