A mixed nitrate (NO₃^–) and ammonium (NH₄⁺) supply can promote root growth in maize (Zea mays), however, the changes in root morphology and the related physiological mechanism under different N forms are still unclear.  Here, maize seedlings were grown hydroponically with three N supplied in three different forms (NO₃^– only, 75/25 NO₃^–/NH₄⁺ and NH₄⁺ only).  Compared with sole NO₃^– or NH₄⁺, the mixed N supply increased the total root length of maize but did not affect the number of axial roots.  The main reason was the increased total lateral root length, while the average lateral root (LR) length in each axle was only slightly increased.  In addition, the average LR density of 2nd whorl crown root under mixed N was also increased.  Compared with sole nitrate, mixed N could improve the N metabolism of roots (such as the N influx rate, nitrate reductase (NR) and glutamine synthase (GS)enzyme activities and total amino content of the roots).  Experiments with exogenously added NR and GS inhibitors suggested that the increase in the average LR length under mixed N was related to the process of N assimilation, and whether the NR mediated NO synthesis participates in this process needs further exploration.  Meanwhile, an investigation of the changes in root-shoot ratio and carbon (C) concentration showed that C transportation from shoots to roots may not be the key factor in mediating lateral root elongation, and the changes in the sugar concentration in roots further proved this conclusion.  Furthermore, the synthesis and transportation of auxin in axial roots may play a key role in lateral root elongation, in which the expression of ZmPIN1B and ZmPIN9 may be involved in this pathway.  This study preliminarily clarified the changes in root morphology and explored the possible physiological mechanism under a mixed N supply in maize, which may provide some theoretical basis for the cultivation of crop varieties with high N efficiency.

Growth traits are among the most important economic traits in pigs and are regulated by polygenes with complex regulatory mechanisms.  As the major indicators of growth performance, the backfat thickness (BFT), loin eye area (LEA), and days to 100 kg (D100) traits are commonly used to the genetics improvement in pigs.  However, the available genetic markers for these traits are limited.  To uncover novel loci and candidate genes associated with growth performance, we collected the phenotypic information of BFT, LEA, and D100 in 1,186 pigs and genotyped all these individuals using the Neogen GGP porcine 80K BeadChip.  We performed a genome-wide association study (GWAS) using 4 statistical models, including mixed linear models (MLM), fixed and random model circulating probability unification (FarmCPU), settlement of MLM under progressively exclusive relationships (SUPER), Bayesian-information and linkage-disequilibrium Iteratively nested keyway (Blink), and identified 5, 3, and 6 high-confidence single nucleotide polymorphisms (SNPs) associated with BFT, LEA, and D100, respectively.  Variant annotation and quantitative trait locus (QTL) mapping analysis suggested that 6 genes (SKAP2, SATB1, PDE7B, PPP1R16B, WNT3, and WNT9B) were potentially associated with growth performance in pigs.  Transcriptome analysis suggested that the expression of Src Kinase Associated Phosphoprotein 2 (SKAP2) was higher in prenatal muscles than in postnatal muscles, and the expression of Phosphodiesterase 7B (PDE7B) continuously increased during the prenatal stages and gradually decreased after birth, implying their potential roles in prenatal skeletal muscle development.  Overall, this study provides new candidate loci and genes for the genetic improvement of pigs.

Genetic improvement of meat production traits has always been the primary goal of pig breeding. Geographical isolation, natural and artificial selection led to significant differences in the phenotypes of meat production traits between Chinese local pigs and Western commercial pigs. Comparative genomics and transcriptomics analysis provided powerful tools to identify genetic variants and genes associated with skeletal muscle growth. However, the number of available genetic variants and genes are still limited. In this study, a comprehensive comparison of transcriptomes showed that ribosomal protein S27-like (RPS27L) gene was highly expressed in skeletal muscle and up-regulated in Chinese local pigs when compared with Western commercial pigs. Functional analysis revealed that overexpression of RPS27L promoted myoblast proliferation and repressed differentiation in pig skeletal muscle cells. Conversely, the knockdown of RPS27L led to the inhibition of myoblast proliferation and the promotion of differentiation. Notably, a 13-bp insertion-deletion (InDel) mutation was identified within the RPS27L promoter, inserted in Chinese local breeds and predominantly deleted in Western commercial breeds. Luciferase reporter assay suggested this InDel modulated RPS27L expression by influencing transcription factor 3 (TCF3) and myogenic differentiation antigen (MYOD) binding to promoter. Furthermore, a positive correlation was observed between the expression of RPS27L expression and backfat thickness. Association studies demonstrated this InDel was significantly associated with the body weight of pigs at the age of 240 days. Together, our results suggested that RPS27L was a regulator of skeletal muscle development and growth, and was a candidate marker for improving meat production traits in pigs. This study not only provided a biomarker for animal breeding, but also was helpful for understanding skeletal muscle development and muscle-related disease in humans.