IspH is a key enzyme in the last step of the methyl-D-erythritol-4-phosphate (MEP) pathway. Loss of function of IspH can often result in complete yellow or albino phenotype in many plants. Here, we report the characterization of a recessive mutant of maize, zebra7 (zb7), showing transverse green/yellow striped leaves in young plants. The yellow bands of the mutant have decreased levels of chlorophylls and carotenoids with delayed chloroplast development. Low temperature suppressed mutant phenotype, while alternate light/dark cycle or high temperature enlarged the yellow section. Map-based cloning demonstrated that zb7 encodes the IspH protein with a mis-sense mutation in a conserved region. Transgenic silencing of Zb7 in maize resulted in complete albino plantlets that are aborted in a few weeks, confirming that Zb7 is important in the early stages of maize chloroplast development. Zb7 is constitutively expressed and its expression subject to a 16-h light/8-h dark cycle regulation. Our results suggest that the less effective or unstable IspH in zb7 mutant, together with its diurnal expression, are mechanistically accounted for the zebra phenotype. The increased IspH mRNA in the leaves of zb7 at the late development stage may explain the restoration of mutant phenotype in mature stages.
Plant height (PH) is one of the most important traits in maize breeding programs. In popcorn, inferior plant traits can be improved with the dent/flint corn germplasm. In the current study, a total of 259 F2:3 families, developed from a cross between a dent corn inbred and a popcorn inbred, were evaluated for 4 PH traits. Quantitative trait loci (QTLs) for each trait were detected using composite interval mapping methods. In addition, genetic interrelationships were investigated using multiple-trait joint analysis for PH with ear height (EH), and for PH with top height (TH). In total, 6, 5, 2, and 6 QTLs were identified for PH, EH, TH, and TH/PH in single-trait analysis, respectively. Joint-analysis data suggest a strong and complex genetic relationship between PH and EH, and between PH and EH, with no QTLs controlling any single trait independently. In addition, 4 kinds of QTLs detected were classified as closely linked QTLs, pleiotropic QTLs, QTLs with opposite effects, and additional QTLs. It was, consequently, difficult to improve lodge resistance through selection on any individual PH trait. The current study demonstrates that multiple-trait joint analysis not only identified additional QTLs, but also revealed the genetic relationship among different highly correlated traits at the molecular level.
BackgroundAs a newly identified category of DNA transposon, helitrons have been found in a large number of eukaryotes genomes. Helitrons have contributed significantly to the intra-specific genome diversity in maize. Although many characteristics of helitrons in the maize genome have been well documented, the sequence of an intact autonomous helitrons has not been identified in maize. In addition, the process of gene fragment capturing during the transposition of helitrons has not been characterized.ResultsThe whole genome sequences of maize inbred line B73 were analyzed, 1,649 helitron-like transposons including 1,515 helAs and 134 helBs were identified. ZmhelA1, ZmhelB1 and ZmhelB2 all encode an open reading frame (ORF) with intact replication initiator (Rep) motif and a DNA helicase (Hel) domain, which are similar to previously reported autonomous helitrons in other organisms. The putative autonomous ZmhelB1 and ZmhelB2 contain an extra replication factor-a protein1 (RPA1) transposase (RPA-TPase) including three single strand DNA-binding domains (DBD)-A/-B/-C in the ORF. Over ninety percent of maize helitrons identified have captured gene fragments. HelAs and helBs carry 4,645 and 249 gene fragments, which yield 2,507 and 187 different genes respectively. Many helitrons contain mutilple terminal sequences, but only one 3'-terminal sequence had an intact "CTAG" motif. There were no significant differences in the 5'-termini sequence between the veritas terminal sequence and the pseudo sequence. Helitrons not only can capture fragments, but were also shown to lose internal sequences during the course of transposing.ConclusionsThree putative autonomous elements were identified, which encoded an intact Rep motif and a DNA helicase domain, suggesting that autonomous helitrons may exist in modern maize. The results indicate that gene fragments captured during the transposition of many helitrons happen in a stepwise way, with multiple gene fragments within one helitron resulting from several sequential transpositions. In addition, we have proposed a potential mechanism regarding how helitrons with multiple termini are generated.
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