The most common symptom of
iron (Fe) deficiency in plants is leaf
chlorosis caused by impairment of
chlorophyll biosynthesis.
Magnesium (
Mg)-chelatase H subunit (CHLH) is a key component in both
chlorophyll biosynthesis and plastid signaling, but its role in Fe deficiency is poorly understood. Heterologous expression of the Arabidopsis thaliana
Mg-chelatase H subunit gene (AtCHLH) increased
Mg-chelatase activity by up to 6-fold and abundance of its product,
Mg-protoporphyrin IX (Mg-Proto IX), by 60-75% in transgenic rice (Oryza sativa) seedlings compared to wild-type (WT) controls. Noticeably, the transgenic seedlings showed alleviation of Fe deficiency symptoms, as evidenced by their less pronounced leaf
chlorosis and lower declines in shoot growth,
chlorophyll contents, and photosynthetic efficiency, as indicated by F v/F m and electron transport rate, compared to those in WT seedlings under Fe deficiency.
Porphyrin metabolism was differentially regulated by Fe deficiency between WT and transgenic seedlings, particularly with a higher level of Mg-Proto IX in transgenic lines, showing that overexpression of AtCHLH reprograms
porphyrin metabolism in transgenic rice. Leaves of Fe-deficient transgenic seedlings exhibited greater upregulation of deoxymugineic
acid biosynthesis-related genes (i.e.,
NAS, NAS2, and NAAT1), YSL2 transporter gene, and Fe-related
transcription factor genes IRO2 and IDEF2 than those of WT, which may also partly contribute to alleviating Fe deficiency. Although AtCHLH was postulated to act as a receptor for
abscisic acid (ABA), exogenous ABA did not alter the phenotypes of Fe-deficient WT or transgenic seedlings. Our study demonstrates that modulation of
porphyrin biosynthesis through expression of AtCHLH in transgenic rice alleviates Fe deficiency-induced stress, suggesting a possible role for CHLH in Fe deficiency responses.