Transcriptome analysis reveals the molecular mechanisms of the defense response to gray leaf spot disease in maize

Abstract Background Gray leaf spot (GLS), which is caused by the necrotrophic fungi Cercospora zeae-maydis and Cercospora zeina, is one of the most impactful diseases in maize worldwide. The aim of the present study is to identify the resistance genes and understand the molecular mechanisms for GLS resistance. Results Two cultivars, ‘Yayu889’ and ‘Zhenghong532,’ which are distinguished as resistant and susceptible cultivars, respectively, were challenged with the GLS disease and a RNA-seq experiment was conducted on infected plants at 81, 89, 91, and 93 days post planting (dap). Compared with the beginning stage at 81 dap, 4666, 1733, and 1166 differentially expressed genes (DEGs) were identified at 89, 91, and 93 dap, respectively, in ‘Yayu889,’ while relatively fewer, i.e., 4713, 881, and 722 DEGs, were identified in ‘Zhenghong532.’ Multiple pathways involved in the response of maize to GLS, including ‘response to salicylic acid,’ ‘protein phosphorylation,’ ‘oxidation-reduction process,’ and ‘carotenoid biosynthetic process,’ were enriched by combining differential expression analysis and Weighted Gene Co-expression Network Analysis (WGCNA). The expression of 12 candidate resistance proteins in these pathways were quantified by the multiple reaction monitoring (MRM) method. This approach identified two candidate resistance proteins, a calmodulin-like protein and a leucine-rich repeat receptor-like protein kinase with SNPs that were located in QTL regions for GLS resistance. Metabolic analysis showed that, compared with ‘Zhenghong532,’ the amount of salicylic acid (SA) and total carotenoids in ‘Yayu889’ increased, while peroxidase activity decreased during the early infection stages, suggesting that increased levels of SA, carotenoids, and reactive oxygen species (ROS) may enhance the defense response of ‘Yayu889’ to GLS. Conclusion By combining transcriptome and proteome analyses with comparisons of resistance QTL regions, calmodulin-like protein and leucine-rich repeat receptor-like protein kinase were identified as candidate GLS resistance proteins. Moreover, we found that the metabolic pathways for ROS, SA, and carotenoids are especially active in the resistant cultivar. These findings could lead to a better understanding of the GLS resistance mechanisms and facilitate the breeding of GLS-resistant maize cultivars

Cultivar Differences in Root Nitrogen Uptake Ability of Maize Hybrids

Although, considerable differences in root size in response to nitrogen (N) application among crop species and cultivars have been widely reported, there has been limited focus on the differences in root N uptake ability. In this study, two maize (Zea mays L.) hybrids, Zhenghong 311 (ZH 311, N-efficient) and Xianyu 508 (XY 508, N-inefficient), were used to compare differences in root N uptake ability. The two cultivars were grown in field pots Experiment I (Exp. I) and hydroponic cultures Experiment II (Exp. II) supplemented with different concentrations of N fertilizer. In both experiments, the levels of accumulated N were higher in ZH 311 than in XY 508 under low- and high-N supply, and the increment in accumulated N was greater under N deficiency. The maximum N uptake rate (Vm) and average N uptake rate (Va) in Exp. I, the root N kinetic parameter maximum uptake rate (Vmax) per fresh weight (FW) and Vmax per plant in Exp. II, and the root N uptake rate in both experiments were significantly higher for ZH 311 than for XY 508. In contrast, the root-to-shoot N ratio in both experiments and the root N kinetic parameter Michaelis constant (Km) in in Exp. II were markedly higher in XY 508 than in ZH 311, particularly under N-deficient conditions. Higher root N kinetic parameters Vmax per FW and Vmax per plant and lower Km values contributed to higher N affinity and uptake potential, more coordinated N distribution in the root and shoot, and higher root N uptake rates throughout the growth stages, thus enhancing the N accumulation and yield of the N-efficient maize cultivar. We conclude that the N uptake ability of roots in the N-efficient cultivar ZH 311 is significantly greater than that in the N-inefficient cultivar XY 508, and that this advantage is more pronounced under N-deficient conditions. The efficient N acquisition in ZH 311 is due to higher N uptake rate per root FW under optimal N conditions and the comprehensive effects of root size and N uptake rate per root FW under N deficiency

Non-structural carbohydrates in maize with different nitrogen tolerance are affected by nitrogen addition.

Non-structural carbohydrates (NSCs) are an important energy source for plant growth and metabolism. Analysis of NSC changes can provide important clues to reveal the adaptation mechanisms of plants to a specific environment. Although considerable differences have been reported in NSCs in response to nitrogen (N) application among crop species and cultivars, previous studies have mostly focused on the differences in leaves and stems. However, the effects of N on the characteristics of accumulation and translocation of NSC in maize with different levels of N tolerance remain unclear. To determine differences in the N levels, two cultivars (N-efficient ZH311 and N-inefficient XY508) were grown in field pots (Experiment I) and as hydroponic cultures (Experiment II) and were supplemented with different concentrations of N fertilizer. In both experiments, low-N stress significantly increased the accumulation of NSCs in maize vegetative organs and increased the translocation rate of NSCs in the stems and their apparent contribution to yield, thereby reducing the yield loss caused by low-N stress. N application had a greater effect on starch content in the vegetative organs of ZH311, but had less effect on soluble sugar (SS) and NSC content in the whole plant and starch content in the ears. ZH311 could convert more starch into SS under low N conditions to adapt to low N environments than XY508, while ensuring that grain yield and starch quantity were not affected. This is evidently an important physiological mechanism involved in this cultivar's tolerance to low N conditions

Characterization and genetic mapping of a novel recessive genic male sterile gene <i>ms305</i> in maize (<i>Zea mays</i> L.)

<div><p>The male sterility system shows tremendous value for the long-term utilization of hybrid crop breeding. In this study, a male sterile mutant, <i>K305ms</i>, was derived from the M₃ progeny of maize (<i>Zea mays</i> L.) inbred line K305 exposed to ⁶⁰Co-γ irradiation. Male sterile <i>K305ms</i> plants did not show any obvious differences from their sibling male fertile plants (K305F) during the vegetative stage, but failed to produce functional pollen at the reproductive stage. Microscopic observations determined that the dyads and tetrads from the pollen of <i>K305ms</i> plants developed abnormally, and subsequently the microspores were shriveled. Genetic analysis indicated that the male sterility of <i>K305ms</i> was controlled by a single recessive genic gene. Gene mapping showed that the responsible gene was located between two simple sequence repeat markers on chromosome 2L in a region of 10.3 cM, bnlg469b and bnlg1940, with genetic distances of 2.9 and 7.4 cM, respectively. According to the microscopic and mapping characteristics, our results showed that this gene was distinguishable from all other reported male sterile genes in maize, and it is temporarily designated as <i>ms305</i>. The linkage map in this study will provide a useful fundamental basis for molecular marker-assisted selection as well as for further map-based cloning of <i>ms305</i>.</p></div