Cloning and validation of YELLOW STRIPE 3 (Ys3) and its implication on iron metabolism in maize (Zea mays L.)
In plants, iron (Fe) deficiency leads to chlorosis, reduced yield, and decreased nutritional quality. Graminaceous plants follow strategy II, in which chelating substances called “phytosiderophores” are produced and secreted into the rhizosphere. The Fe(III)-phytosiderophore complex is then taken up by the specific transporter, YELLOW STRIPE 1. The phytosiderophore in maize is 2’-deoxymugineic acid (DMA). In maize, the mutant ys3 is characterized by an intervenial chlorosis due to a defect in phytosiderophore secretion. Understanding genome-wide gene regulation upon Fe stress in ys3-background plants will provide important insides about Y s3 and its implication on Fe homeostasis. Map based cloning located the Ys3 gene in a 0.8 cM interval
on chromosome 3 spanning 13.59 Mbps and containing 207 high confidence gene models. However, only 50 genes were present in the maize, rice, and sorghum genomes. Within this subset of candidate genes, GRMZM2G063306 was predicted to encode a DMA efflux transporter orthologous to OsTOM1 and HvTOM1. The Ys3 gene then was sequenced in plants of the ys3 mutant
and wt showing 10 SNPs and 3 InDels in the coding sequence. However, only 2 InDels and 2 synonymous SNPs were unique for the ys3 mutant. Isolation of one novel ys3::Mu and four novel ys3::Ac novel ys3 mutations by direct transposon tagging confirmed the candidate gene GRMZM2G063306. An
additional F1S1 ys3::Mu individual showed a 6 bp insertion in exon 8, resulting in the insertion of two amino acids in the sequence. Increased expression levels of Y s3 T01, Y s3 T02, Dmas, and Ys1 was shown in root tissue of the ys3 mutant and wild type plants grown under Fe deficient conditions, in comparison with Fe sufficient conditions. Furthermore, a transcriptome profile of ys3×W22 F2 individuals grown under deficient and sufficient Fe
regimes revealed the immediate response of several Fe uptake and homeostasis genes along with bHLH transcription factors including GRMZM2G057413 (ZmIro2) and GRMZM2G350312 (ZmIro3) as well as novel candidate genes associated with transport, oxidation-reduction, and to the NAS family. In
addition, phenotypic and ionomic analyses were carried out to complement the transcriptome profile and thus, provide a complete and deep overview of gene response during Fe stress. This study revealed that Ys3 encodes a protein syntenic to rice and barley TOM1, which is in line with its predicted function as the specific transporter for phytosiderophore efflux in maize. Furthermore, it also provides important insides about Y s3 and its implication
on Fe homeostasis by investigating its response when grown under deficient and sufficient Fe regimes, which can later be used to improve Fe efficiency and thus, influence Fe content in grain to fight Fe deficiency in humans.