The changes induced in the photosynthetic apparatus of spinach (Spinacia oleracea L.) seedlings exposed to
iron deficiency shortly after germination were characterized with two proteomic approaches coupled with
chlorophyll and xanthophyll analysis and in vivo measurements of photosynthesis. During the first 10 d of
iron deficiency the concentrations of
chlorophyll b and
violaxanthin were greatly reduced, but all
xanthophylls recovered after 13-17 d of
iron deficiency, when both
chlorophylls were negatively affected. No new
protein was formed in
iron-deficient leaves, and no
protein disappeared altogether.
Photosystem I (PSI)
proteins were largely reduced, but the stoichiometry of the antenna composition of PSI was not compromised. On the contrary, PSII
proteins were less affected by the stress, but the specific antennae Lhcb4 and Lhcb6, Lhcb2 and its
isoform Lhcb1.1 were all reduced, while the concentration of Lhcb3 increased. A strong reduction in thylakoid bending and an altered distribution pattern for the reduced PSI and PSII complexes were observed microscopically in
iron-deficient leaves. Supercomplex organization was also affected by the stress. The trimeric organization of Lhcb and the dimerization of Lhca were reduced, while monomerization of Lhcb increased. However, the trimerization of Lhcb was partially recovered after 13-17 d of
iron deficiency. In
iron-deficient leaves, photosynthesis was strongly inhibited at different light intensities, and a high de-epoxidation status of the
xanthophylls was observed, in association with a strong impairment of photochemical efficiency and an increase of heat dissipation as monitored by the non-photochemical quenching of fluorescence. All these negative effects of
iron deficiency were attenuated but not fully reversed after again supplying
iron to
iron-deficient leaves for 7-13 d. These results indicate that
iron deficiency has a strong impact on the proteomic structure of spinach photosystems and suggest that, in higher plants, adaptive mechanisms common in lower organisms, which allow rapid changes of the photosystem structure to cope with
iron stress, are absent. It is speculated that the observed changes in the monomer-trimer equilibrium of major PSII antennae, which is possibly the result of xanthophyll fluctuations, is a first adaptative adjustment to
iron deficiency, and may eventually play a role in light dissipation mechanisms.