Roots are important plant organs for the absorption of water and nutrients. To date, there have been few genome-wide association studies of maize root system architecture (RSA) in the field. The genetic basis of maize RSA is poorly understood, and the maize RSA-related genes that have been cloned are very limited. Here, 421 maize inbred lines of an association panel were planted to measure the root systems at the maturity stage, and a genome-wide association study was performed. There was a strong correlation among eight RSA traits, and the RSA traits were highly correlated with the aboveground plant architecture traits (e.g., plant height and ear leaf length, r = 0.13–0.25, p < 0.05). The RSA traits of the stiff stalk subgroup (SS) showed lower values than those of the non-stiff stalk subgroup (NSS) and tropical/subtropical subgroup (TST). Using the RSA traits, the genome-wide association study identified 63 SNPs and 189 candidate genes. Among them, nine candidate genes co-localized between RSA and aboveground architecture traits. A further co-expression analysis identified 88 candidate genes having high confidence levels. Furthermore, we identified four highly reliable RSA candidate genes, GRMZM2G099797, GRMZM2G354338, GRMZM2G085042, and GRMZM5G812926. This research provides theoretical support for the genetic improvement of maize root systems, and it identified candidate genes that may act as genetic resources for breeding.

Maize (Zea mays L.) has undergone profound changes in root anatomy for environmental adaptation during domestication. However, the genetic mechanism of plasticity of maize root anatomy during the domestication process remains unclear. In this study, high-resolution mapping was performed for nine root anatomical traits using a maize-teosinte population (mexicana × Mo17) across three environments. Large genetic variations were detected for different root anatomical traits. The cortex, stele, aerenchyma areas, xylem vessel number, and cortical cell number had large variations across three environments, indicating high plasticity. Sixteen quantitative trait loci (QTL) were identified, including seven QTL with QTL × environment interaction (EIQTL) for high plasticity traits and nine QTL without QTL × environment interaction (SQTL). Most of the root loci were consistent with shoot QTL depicting domestication signals. Combining transcriptome and genome-wide association studies revealed that AUXIN EFFLUX CARRIER PIN–FORMED LIKE 4 (ZmPILS4) serves as a candidate gene underlying a major QTL of xylem traits. The near-isogenic lines (NILs) with lower expression of ZmPILS4 had 18–24% more auxin concentration in the root tips and 8–15% more xylem vessels. Nucleotide diversity values analysis in the promoter region suggested that ZmPILS4 was involved in maize domestication and adaptation. These results revealed the potential genetic basis of root anatomical plasticity during domestication.

In plants, nitrogen remobilization from source to sink organs is an important process regulated by complex transcriptional regulatory networks. However, the relationship between nitrogen remobilization and leaf senescence and the molecular regulatory network that controls them are unknown in maize. Here, using 15N labeling and a transcriptome approach, a dynamic analysis of the nitrogen remobilization process was conducted in two elite maize inbred lines (PH4CV and PH6WC) with contrasting leaf senescence. PH4CV showed higher nitrogen remobilization efficiency (NRE) than PH6WC, mainly in the middle and lower leaves from 15 d to 35 d after silking. The co-expression network analysis revealed that ethylene and cytokinin metabolism-related genes triggered the onset of nitrogen remobilization, while abscisic acid and jasmonic acid biosynthesis-related genes controlled the progression of nitrogen remobilization. By integrating genetic analysis, functional annotation, and gene expression, two candidate genes underlying a major quantitative trait locus of NRE were identified, namely an early senescence acting gene (ZmASR6) and an ATP-dependent Clp protease gene (GRMZM2G172230). Hormone-coupled transcription factors and downstream target genes reveal a gene regulatory network for the nitrogen remobilization process after silking in maize. These results uncovered a sophisticated regulatory mechanism for nitrogen remobilization, and further provided characterization of valuable genes for genetic improvement of nitrogen use efficiency in maize.

Effective in-season crop nitrogen (N) status diagnosis is important for precision crop N management, and remote sensing using an unmanned aerial vehicle (UAV) is one efficient means of conducting crop N nutrient diagnosis. Here, field experiments were conducted with six N levels and six maize hybrids to determine the nitrogen nutrition index (NNI) and yield, and to diagnose the N status of the hybrids combined with multi-spectral data. The NNI threshold values varied with hybrids and years, ranging from 0.99 to 1.17 in 2018 and 0.60 to 0.71 in 2019. A proper agronomic optimal N rate (AONR) was constructed and confirmed based on the measured NNI and yield. The NNI (R² = 0.64–0.79) and grain yield (R² = 0.70–0.73) were predicted well across hybrids using a random forest model with spectral, structural, and textural data (UAV). The AONRs calculated using the predicted NNI and yield were significantly correlated with the measured NNI (R² = 0.70 and 0.71 in 2018 and 2019, respectively) and yield (R² = 0.68 and 0.54 in 2018 and 2019, respectively). It is concluded that data fusion can improve in-season N status diagnosis for different maize hybrids compared to using only spectral data.