Iron (Fe) is an essential metal cofactor. Since the operation of the photosynthetic
electron transport chain of plants relies on Fe containing cofactors, chloroplasts
are primary sites of Fe allocation in the mesophyll cells performing photosynthesis.
Although the Fe uptake into chloroplasts and the incorporation of Fe into cofactors
of photosynthesis has been extensively studied in the past decades, pieces of information
how essential metals, among others Fe is removed from these incorporation sites. Among
the few pieces of information on the removal of plastidial Fe, results of Pottier
et al. (2019; DOI: 10.1093/jxb/ery388) indicated that autophagosome dependent chloroplast
degradation is crucial in the process. Autophagy related Gene 7 protein (ATG7) is
an E1-like enzyme that takes part both in the first and second conjugation system,
conjugating phosphatidylethanolamine to ATG8 and ATG5 to ATG12, respectively that
are fundamental steps in autophagosome formation. ATG7 is encoded by a single gene
(At5g45900) in the Arabidopsis genome. Since cargo recruitment is based on receptors
interact with ATG8, ATG7 is an upstream element that also impacts the size of the
autophagosome. Two Col-0 based T-DNA insertion lines, atg7.1 and atg7.2 were involved
to the current analysis. Mature rosette leaves at flowering initiation were samples.
By physiological performance and chloroplast Fe content, the leaves of ATG7 affected
lines segregated from that of the Col-0 plants, were Fe content in the chloroplasts
of ATG7 affected lines was significantly higher. In contrast, Fe content in the chloroplasts
of Col-0 leaves negatively correlated to the senescence status. Accumulation of Fe
was also confirmed by X-ray fluorescence emission (PIXE) mapping, and low energy X-ray
fluorescence (LEXRF) on cryosectioned samples. In ATG7 affected lines the relative
transcript abundance of Atg7 drastically increased whereas in Col-0, it correlated
positively to the senescence status. In ATG7 affected lines, relative transcript abundance
of Atg5 also increased, in function of the senescence status. Thus we confirm that
the pathway in ATG7 function as upstream element is a key mechanism in the removal
of Fe from chloroplasts.
This work was supported by the grant K-146865 of NKFIH, Hungary. Á.S. was supported
by the János Bolyai Scholarship of the Hungarian Academy of Sciences (BO-00113-23-8).
Instrument center access was financed under ReMade@ARI PIDs 27548 & 34653 (financed
as part of HORIZON-INFRA-2021-SERV-01, 101058414, 10039728 and 22.0018). We acknowledge
Elettra-Sincrotrone, Trieste, Italy for the beam time accesses (20235332 & 20245567).
