| Photosynthesis is a vital component of carbon cycle in nature,which can use light energy to convert carbon dioxide and water into organic matters that store stable chemical energy and release oxygen.It plays an irreplaceable role in maintaining the growth and development in photosynthetic organisms.Chlorophyll molecules are one of the most abundant tetrapyrrole molecules and non-covalently associated with chlorophyll-binding proteins to harvest light energy and free electrons.The change of chlorophyll content can regulate the light use efficiency of crops,avoid the damage of photosystems caused by excessive light energy absorption,maximize the photosynthetic efficiency of crops,and ultimately affect the crop yield.Maize(Zea mays L.)is one of the most successful crops using heterosis.In the process of hybrid breeding,the effects of chlorophyll content from parents are often ignored on photosynthetic capacity and photosynthetic matters accumulation of hybrids.Therefore,as ideal materials used for exploring molecular and regulatory mechanisms of chloroplast development and photosynthesis,chlorophyll-deficient mutants are studied to identify new photosynthetic genes and functions,which has significant value in maize.Meanwhile,there is an important practical significance to research relationship among chlorophyll content,photosynthesis and yield by chlorophyll-deficient mutants to maximize photosynthetic efficiency and increase yield.To analyze the molecular regulation mechanisms of photosynthesis and explore the effects of chlorophyll contents on photosynthetic capacity and yield in maize,in this study,pale-green leaf(pgl)mutant from artificial mutations and yellowish-green leaf(ygl)mutant from natural variations were used as research materials to study the function of photosynthesis-related genes,and the relationship between chlorophyll content of parents and photosynthesis and yield of hybrids were preliminarily studied.The results are as follows:1.The functional study of a key photosynthetic gene ZmCRD1 in maize(1)Through EMS mutagenesis of maize inbred line B73 pollens,we constructed a maize leaf color mutant library and screened a pale green chlorophyll-deficient mutant pgl.The pgl mutant exhibited a reduced chlorophyll content phenotype throughout the all growth and development stages.The genetics analysis showed that the mutated phenotype was controlled by a single recessive nuclear gene.(2)The ZmCRD1 gene encoding magnesium-protoporphyrin IX monomethyl ester cyclase with two missense mutations,p.A44 T and p.T326 M,at the first exon and the fifth exon on chromosome 8 was identified as the causal gene through gene mapping in pgl.Allelic test analysis showed that the mutation of ZmCRD1 gene was a primary cause for chlorophyll deficiency.Phylogenetic analysis indicated that CRD1 protein was highly conserved in higher plants and p.T326 M was more likely the causal mutation causing functional change of ZmCRD1 protein.(3)In terms of functions,the gene expression pattern analysis and subcellular localization showed that ZmCRD1 gene mainly functioned in chloroplasts of photosynthetic tissues.And the expression level of ZmCRD1 gene was significantly reduced in pgl mutant and regulated by light.The mutation of ZmCRD1 gene could casue morphological changes of chloroplast,decreased starch accumulation,perturbed photosystem,growth retardation and production reduction.Transcriptomic analysis and q RT-PCR analysis showed that the mutation of ZmCRD1 gene could alter the expression level of many gene involved in chlorophyll biosynthesis,chlorophyll degradation,chloroplast development,photosynthesis and glycometabolism pathway.Meanwhile,the mutation of ZmCRD1 gene also had the potential to improve resistance of corn northern leaf blight,and biotic and abiotic stress response related genes were up-regulated expression in pgl mutant.The above results confirmed that ZmCRD1 gene plays a crucial role in chlorophyll biosynthesis,chloroplast development,photosynthesis and disease-resistant.2.Gene mapping and transcriptomic analysis of yellow mutant ygl in maize(1)A spontaneous yellow mutant ygl controlled by a single recessive nuclear gene displayed a chlorophyll-deficient phenotype throughout the whole life period.The ygl mutant had reduced chlorophyll content,abnormal chloroplast morphology and structure,less starch accumulation and production reduction compared to wild type Z52.Chlorophyll fluorescence parameters analysis showed that photosynthesis was perturbed and particularly the activity of PS Ⅰ was significantly inhibited in ygl mutant.(2)Through positional mapping,YGL locus was mapped into a 301 kb region on chromosome 2 including 11 protein coding genes,but no variation points were identified at the promoters and coding regions of 11 candidate genes by Singer sequencing.Zm00001d003007 gene in mapping region encoding a putative pentatricopeptide repeat superfamily protein displayed lower expression level at the different growth stages,therefore,Zm00001d003007 gene as a key candidate gene might control yellow phenotype of ygl mutant.(3)Transcriptomic analysis showed that the expression levels of genes involved in chloroplast development,photosynthesis,pigment metabolism and starch synthesis were significantly decreased in ygl mutant.DAB and NBT histochemical stain showed that leaves of ygl mutant had more catalase active sites and reactive oxygen(ROS)accumulation.The YGL locus could trigger ROS and hormone responses by regulating the gene expression of the ROS signaling pathway and hormone metabolism pathways.These results indicated that the YGL locus serves as a crucial regulatory factor to maintain a dynamic balance among chloroplast development,photosynthesis and ROS and hormone homeostasis.3.The study on correlation among chlorophyll content,photosynthetic capacity and yield in maize(1)The above studies found that chlorophyll biosynthesis pathway was changed due to mutation of ZmCRD1 gene in pgl mutant from artificial mutagenesis but it had complete chloroplast structure,while the chloroplast structure was severely damaged in spontaneous ygl mutant,and its decline of the proportion of chlorophyll content was higher than pgl mutant.In order to explore the effects of different types of chlorophyll-deficient mutants on the photosynthetic capacity and yield of maize hybridized combinations,we compared the photosynthetic capacity and yield of hybridized combinations produced by pgl mutant and ygl mutant.The results showed that compared to control,the photosynthetic capacity of most of cross combinations of pgl and ygl were significantly decreased indicating that the mutation of PGL locus and YGL locus could affect the photosynthetic capacity of hybrids,and these two loci played an important role in maintaining the stability of photosynthesis.(2)The yield of pgl mutant related hybridized combinations displayed an increased trend or remained unchanged,and hundred-grain weight and yield per hectare of ygl mutant related hybridized combinations had a decreased trend or unchanged,which indicated that the mutation of PGL locus had a potential to increase the yield of maize hybrid and the mutation of YGL locus affected the yield of some maize hybrids,especially the hundred-grain weight.In conclusion,using the hybrid parents with relatively lower chlorophyll content and complete chloroplast structure to breed maize hybrid combinations is helpful to improve the yield of hybrid combinations.This research provides a new idea for the breeding of new maize germplasm with high yield and high light efficiency,which has important significance for guiding the improvement of photosynthetic capacity and yield in maize. |
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