Gene Mapping And Breeding Application Of Albino Green-revertible Mutant Bm58 In Maize

Maize（Zea mays L.）is the largest food crop in China.Photosynthesis provides material energy for plant growth and development,which can be used to increase maize yield by improving photosynthesis.Chloroplasts are important organelles of plant leaves for photosynthesis.Chlorophyll in chloroplasts converts light energy into chemical energy through the photosynthetic electron transport chain for the growth of maize.When chlorophyll synthesis is blocked or degradation is accelerated,leaf color changes and leaf color mutants are formed.Leaf color mutants can not only be used as ideal materials to study the regulation of chlorophyll anabolic metabolism,chloroplast development mechanism and photosynthesis mechanism of plants,but also can be used as markers to distinguish true hybrids from pseudohybrids in the process of maize hybridization breeding,so as to detect hybrid seed purity.The maize albino green transmutation mutant bm58 was a natural mutant found in the fields.We identified its phenotype,chlorophyll content,photosynthetic parameters,chlorophyll fluorescence kinetic parameters,chloroplast ultrastructure observation,genetic analysis and gene mapping.The main research results are as follows:1.Phenotypic characteristics of bm58:The mutant showed normal green color from seedling emergence to the first leaf development.Bleaching began at the second leaf stage and gradually expanded until it began to turn normal green at the fifth leaf stage.2.Determination of physiological and biochemical indexes:Chlorophyll a,chlorophyll b and total chlorophyll contents of the mutant bm58 were very low at the four-leaf stage,accounting for 7.3%,6.1%and 13.6%,respectively.With the growth of the plant,the contents of chlorophyll a,chlorophyll b and total chlorophyll increased rapidly.After the eight-leaf stage,the content of chlorophyll a,chlorophyll b and total chlorophyll increased rapidly.The chlorophyll content of bm58 and wild-type plants was basically same,and their relative content reached 97.8%,94.4%and 94.6%,respectively,which was consistent with the change of leaf color.The net photosynthetic rate（P_n）,initial fluorescence（F₀）,maximum fluorescence（F_m）and PSⅡprimary light conversion efficiency（F_v/F_m）of bm58 were all lower than those of the wild type when the mutant and wild type were albinized,and the values increased with the leaf color turning green.The photosynthetic capacity of bm58 was lower than that of wild type.3.Ultrastructure observation of leaves:According to ultrastructure observation of bm58and wild-type leaves,mesophyll cells of albino mutant were not developed perfectly,garland structure was damaged,chloroplast number was significantly reduced compared with wild-type mutant in V4 stage,and there was no obvious thylakoid lamellae in leaf green.After regreening,chloroplast development returned to normal,which was no different from that of the wild type,indicating that the chloroplast development of the mutant was abnormal in the albino stage,and gradually recovered as the leaf color turned to green.4.Genetic analysis of the mutant bm58:The mutant bm58 was crossed with inbred lines B73,Chang7-2 and HZS respectively,and F₁plants showed normal green color,indicating that the mutant was controlled by recessive gene.The separation ratio of normal green plants and leaf color mutant plants in F₂population of B73 and Chang7-2 by Chi-square test was 3:1,and 15:1 in the F₂population matched by HZS,indicating that the mutant gene was controlled by two pair of recessive nuclear genes.5.Location of mutant bm58 gene:The mutant bm58 was hybridized with B73,Chang7-2and HZS inbred lines respectively,and F₂generation isolation population was constructed.Two hundred and eight pairs of SSR primers evenly distributed on 10 chromosomes of maize were used in the laboratory.The bm58 mutant was initially located between two markers,p-umc1520 and p-umc1779,on chromosome 6 of maize with a physical distance of 6.3 Mb.In the process of fine mapping of bm58,the F₂mapping population was further expanded,and196 pairs of new markers were designed and developed within two markers,p-umc1520 and p-umc1779.Using these three F₂populations,the gene was finally mapped between two markers,M9 and M10,with a physical distance of 83 kb.There was one candidate gene in this region,named Zm BM58,and the functional annotation was chloroplast chaperone protein Dna JA7A.6.In the process of mapping the bm58 mutant gene,a pair of SSR marker M13 was screened,which had polymorphism loci in the isolated populations under different backgrounds of B73 and Chang7-2.The presence of the albino transgreen gene could be rapidly detected by using this marker,and the rapid utilization of this gene could be realized in combination with conventional breeding.