Pollen self-elimination CRISPR-Cas genome editing prevents transgenic pollen dispersal in maize.

This study reports the development of a programmed pollen self-elimination CRISPR-Cas (PSEC) system in which the pollen is infertile when PSEC is present in haploid pollen. PSEC can be inherited through the female gametophyte and retains genome editing activity in vivo across generations. This system could greatly alleviate serious concerns about the widespread diffusion of genetically modified (GM) elements into natural and agricultural environments via outcrossing.

Genome-wide association study dissects yield components associated with low-phosphorus stress tolerance in maize.

KEY MESSAGE: Phosphorus deficiency in soil is a worldwide constraint threatening maize production. Through a genome-wide association study, we identified molecular markers and associated candidate genes and molecular pathways for low-phosphorus stress tolerance. Phosphorus deficiency in soils will severely affect maize (Zea mays L.) growth and development, thus decreasing the final yield. Deciphering the genetic basis of yield-related traits can benefit our understanding of maize tolerance to low-phosphorus stress. However, considering that yield-related traits should be evaluated under field condition with large populations rather than under hydroponic condition at a single-plant level, searching for appropriate field experimental sites and target traits for low-phosphorus stress tolerance is still very challenging. In this study, a genome-wide association analysis using two natural populations was performed to detect candidate genes in response to low-phosphorus stress at two experimental sites representative of different climate and soil types. In total, 259 candidate genes were identified and these candidate genes are mainly involved in four major pathways: transcriptional regulation, reactive oxygen scavenging, hormone regulation, and remodeling of cell wall. Among these candidate genes, 98 showed differential expression by transcriptome data. Based on a haplotype analysis of grain number under phosphorus deficiency condition, the positive haplotypes with favorable alleles across five loci increased grain number by 42% than those without favorable alleles. For further verifying the feasibility of genomic selection for improving maize low-phosphorus tolerance, we also validated the predictive ability of five genomic selection methods and suggested that moderate-density SNPs were sufficient to make accurate predictions for low-phosphorus tolerance traits. All these results will facilitate elucidating genetic basis of maize tolerance to low-phosphorus stress and improving marker-assisted selection efficiency in breeding process.

Strand-specific RNA-Seq transcriptome analysis of genotypes with and without low-phosphorus tolerance provides novel insights into phosphorus-use efficiency in maize.

BACKGROUND: Phosphorus (P) stress is a global problem in maize production. Although macro/microarray technologies have greatly increased our general knowledge of maize responses to P stress, a greater understanding of the diversity of responses in maize genotypes is still needed. RESULTS: In this study, we first evaluated the tolerance to low P of 560 accessions under field conditions, and selected the low P-tolerant line CCM454 and the low P-sensitive line 31778 for further research. We then generated 24 strand-specific RNA libraries from shoots and roots of CCM454 and 31778 that had been subjected to P stress for 2 and 8 days. The P deficiency-responsive genes common to CCM454 and 31778 were involved in various metabolic processes, including acid phosphatase (APase) activity. Determination of root-secretory APase activities showed that the induction of APase by P stress occurred much earlier in CCM454 than that in 31778. Gene Ontology analysis of differentially expressed genes (DEGs) and CAT/POD activities between CCM454 and 31778 under P-sufficient and -deficient conditions demonstrated that CCM454 has a greater ability to eliminate reactive oxygen species (ROS) than 31778. In addition, 16 miRNAs in roots and 12 miRNAs in shoots, including miRNA399s, were identified as DEGs between CCM454 and 31778. CONCLUSIONS: The results indicate that the tolerance to low P of CCM454 is mainly due to the rapid responsiveness to P stress and efficient elimination of ROS. Our findings increase the understanding of the molecular events involved in the diversity of responses to P stress among maize accessions.

Large-scale evaluation of maize germplasm for low-phosphorus tolerance.

Low-phosphorus (LP) stress is a global problem for maize production and has been exacerbated by breeding activities that have reduced the genetic diversity of maize. Although LP tolerance in maize has been previously evaluated, the evaluations were generally performed with only a small number of accessions or with samples collected from a limited area. In this research, 826 maize accessions (including 580 tropical/subtropical accessions and 246 temperate accessions) were evaluated for LP tolerance under field conditions in 2011 and 2012. Plant height (PH) and leaf number were measured at three growth stages. The normalized difference vegetation index (NDVI) and fresh ear weight (FEW) were also measured. Genetic correlation analysis revealed that FEW and NDVI were strongly correlated with PH, especially at later stages. LP-tolerant and -sensitive accessions were selected based on the relative trait values of all traits using principal component analysis, and all the 14 traits of the tolerant maize accessions showed less reduction than the sensitive accessions under LP conditions. LP tolerance was strongly correlated with agronomic performance under LP stress conditions, and both criteria could be used for genetic analysis and breeding of LP tolerance. Temperate accessions showed slightly better LP tolerance than tropical/subtropical ones, although more tolerant accessions were identified from tropical/subtropical accessions, which could be contributed by their larger sample size. This large-scale evaluation provides useful information, LP-tolerant germplasm resources and evaluation protocol for genetic analysis and developing maize varieties for LP tolerance.

Meta-analysis and candidate gene mining of low-phosphorus tolerance in maize.

Plants with tolerance to low-phosphorus (P) can grow better under low-P conditions, and understanding of genetic mechanisms of low-P tolerance can not only facilitate identifying relevant genes but also help to develop low-P tolerant cultivars. QTL meta-analysis was conducted after a comprehensive review of the reports on QTL mapping for low-P tolerance-related traits in maize. Meta-analysis produced 23 consensus QTL (cQTL), 17 of which located in similar chromosome regions to those previously reported to influence root traits. Meanwhile, candidate gene mining yielded 215 genes, 22 of which located in the cQTL regions. These 22 genes are homologous to 14 functionally characterized genes that were found to participate in plant low-P tolerance, including genes encoding miR399s, Pi transporters and purple acid phosphatases. Four cQTL loci (cQTL2-1, cQTL5-3, cQTL6-2, and cQTL10-2) may play important roles for low-P tolerance because each contains more original QTL and has better consistency across previous reports.