Iron (Fe) deficiency induces chlorosis (IDC) in plants and can result in reduced plant productivity. Therefore, development of Fe-efficient plants is of great interest. To gain a better understanding of the physiology of Fe-efficient plants, putative novel plant variants were regenerated from potato (Solanum tubersosum L. var. \'Iwa\') callus cultures selected under Fe deficient or low Fe supply (0-5 μM Fe). Based on visual chlorosis rating (VCR), 23% of callus-derived regenerants were classified as Fe-efficient (EF) and 77% as Fe-inefficient (IFN) plant lines when they were grown under Fe deficiency conditions. Stem height was found to be highly correlated with internodal distance, leaf and root lengths in the EF plant lines grown under Fe deficiency conditions. In addition, compared to the IFN plant lines and control parental biotype, the EF plants including the lines named A1, B2, and B9, exhibited enhanced formation of lateral roots and root hairs as well as increased expression of ferritin (fer3) in the leaf and iron-regulated transporter (irt1) in the root. These morphological adaptations and changes in expression the fer3 and irt1 genes of the selected EF potato lines suggest that they are associated with resistance to low Fe supply stress.

Chlorosis is one of the most common symptoms of plant diseases, including those caused by viruses and viroids. Recently, a study has shown that Peach latent mosaic viroid (PLMVd) exploits host RNA silencing machinery to modulate the virus disease symptoms through the silencing of chloroplast-targeted heat shock protein 90 (Hsp90C). To understand the molecular mechanisms of chlorosis in this viroid disease, we established an experimental system suitable for studying the mechanism underlying the chlorosis induced by the RNA silencing of Hsp90C in transgenic tobacco. Hairpin RNA of the Hsp90C-specific region was expressed under the control of a dexamethasone-inducible promoter, resulted in the silencing of Hsp90C gene in 2 days and the chlorosis along with growth suppression phenotypes. Time course study suggests that a sign of chlorosis can be monitored as early as 2 days, suggesting that this experimental model is suitable for studying the molecular events taken place before and after the onset of chlorosis. During the early phase of chlorosis development, the chloroplast- and photosynthesis-related genes were downregulated. It should be noted that some pathogenesis related genes were upregulated during the early phase of chlorosis in spite of the absence of any pathogen-derived molecules in this system.

Magnesium (Mg) is the second most abundant cation in living cells. Over 300 enzymes are known to be Mg-dependent, and changes in the Mg concentration significantly affects the membrane potential. As Mg becomes deficient, starch accumulation and chlorosis, bridged by the generation of reactive oxygen species, are commonly found in Mg-deficient young mature leaves. These defects further cause the inhibition of photosynthesis and finally decrease the biomass. Recently, transcriptome analysis has indicated the transcriptinal downregulation of chlorophyll apparatus at the earlier stages of Mg deficiency, and also the potential involvement of complicated networks relating to hormonal signaling and circadian oscillation. However, the processes of the common symptoms as well as the networks between Mg deficiency and signaling are not yet fully understood. Here, for the purpose of defining the missing pieces, several problems are considered and explained by providing an introduction to recent reports on physiological and transcriptional responses to Mg deficiency. In addition, it has long been unclear whether the Mg deficiency response involves the modulation of Mg2+ transport system. In this review, the current status of research on Mg2+ transport and the relating transporters are also summarized. Especially, the rapid progress in physiological characterization of the plant MRS2 gene family as well as the fundamental investigation about the molecular mechanism of the action of bacterial CorA proteins are described.