Abstract
The regenerative capacity of the embryonic callus, a complex quantitative trait, is one of the main limiting factors for maize transformation. This trait was decomposed into five traits, namely, green callus rate (GCR), callus differentiating rate (CDR), callus plantlet number (CPN), callus rooting rate (CRR), and callus browning rate (CBR). To dissect the genetic foundation of maize transformation, in this study multi-locus genome-wide association studies (GWAS) for the five traits were performed in a population of 144 inbred lines genotyped with 43,427 SNPs. Using the phenotypic values in three environments and best linear unbiased prediction (BLUP) values, as a result, a total of 127, 56, 160, and 130 significant quantitative trait nucleotides (QTNs) were identified by mrMLM, FASTmrEMMA, ISIS EM-BLASSO, and pLARmEB, respectively. Of these QTNs, 63 QTNs were commonly detected, including 15 across multiple environments and 58 across multiple methods. Allele distribution analysis showed that the proportion of superior alleles for 36 QTNs was <50% in 31 elite inbred lines. Meanwhile, these superior alleles had obviously additive effect on the regenerative capacity. This indicates that the regenerative capacity-related traits can be improved by proper integration of the superior alleles using marker-assisted selection. Moreover, a total of 40 candidate genes were found based on these common QTNs. Some annotated genes were previously reported to relate with auxin transport, cell fate, seed germination, or embryo development, especially, GRMZM2G108933 (WOX2) was found to promote maize transgenic embryonic callus regeneration. These identified candidate genes will contribute to a further understanding of the genetic foundation of maize embryonic callus regeneration.
Highlights
As one of the main crops for animals and humans, maize (Zea mays L.) is an important target for genetic manipulation (Zhang et al, 2014; Li et al, 2016)
The phenotypes for callus browning rate (CBR), callus differentiating rate (CDR), callus plantlet number (CPN), callus rooting rate (CRR), and green callus rate (GCR) have been described by Zhang et al (2017b), readers are encouraged to refer to the original study (Zhang et al, 2017b)
A significantly positive correlation was observed between CDR and CPN, while a significantly negative correlation was found between CBR and GCR (P = 0.01)
Summary
As one of the main crops for animals and humans, maize (Zea mays L.) is an important target for genetic manipulation (Zhang et al, 2014; Li et al, 2016). Genetic research has suggested that embryonic callus induction is controlled by nuclear genes in maize (Schlappi and Hohn, 1992). Research exploring callus regenerative capacity has mainly focused on Arabidopsis, rice, wheat, maize, and other plants. WUSCHEL-related homeobox 5 (WOX5) expression in the quiescent center (QC) is considered as a marker of the root stem cell niche in Arabidopsis (Sarkar et al, 2007). QTL mapping showed that two QTLs on chromosomes 1 and 9 control green shoot re-differentiation in rice, with the latter considered to be a major locus (Ping et al, 1998). More systematic studies are required to reveal the genetic basis of maize embryonic callus regenerative capacity
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