The Rhizosphere Bacterial Communities Differ Among Domesticated Maize Landraces – An Experimental Confirmation

The plant-associated microbiome has been shown to vary considerably between species and across environmental gradients. The effects of genomic variation on the microbiome within single species are less clearly understood, with results often confounded by the larger effects of climatic and edaphic variation. In this study, the effect of genomic variation on the rhizosphere bacterial communities of maize was investigated by comparing different genotypes grown within controlled environments. Rhizosphere bacterial communities were profiled by metabarcoding the universal bacterial 16S rRNA v3-v4 region. Initially, plants from the inbred B73 line and the Ancho - More 10 landrace were grown for 12-weeks and compared. The experiment was then repeated with an additional four Mexican landraces (Apachito - Chih 172, Tehua - Chis 204, Serrano - Pueb 180 and Hairnoso de Ocho) that were grown alongside additional B73 and Ancho - More 10 genotypes. In both experiments there were significant genotypic differences in the rhizosphere bacteria. Additionally, the bacterial communities were significantly correlated with genomic distance between genotypes, with the more closely related landraces being more similar in rhizosphere bacterial communities. Despite limited sampling numbers, here we confirm that genomic variation in maize landraces is associated with differences in the rhizosphere bacterial communities. Further studies that go beyond correlations to identify the mechanisms that determine the genotypic variation of the rhizosphere microbiome are required.

The Nutritional Content of Five Southwestern US Indigenous Maize (Zea Mays L.) Landraces of Varying Endosperm Type

Any relative nutritional differences among the diverse maize (Zea mays L.) landraces traditionally maintained in the Greater Southwest are little understood. In this article, we investigate a range of nutritional traits of five indigenous maize landraces in the US Southwest based on different kernel endosperm types: pop, flour, flint, dent, and sweet. We present macronutrient and micronutrient values for accessions of each landrace grown in the same environmental grow-out experiment. Macronutrient values vary considerably across these endosperm accessions. Sweet and flour maize had higher values of fat and protein, whereas dent had the highest carbohydrate content. Sweet and flour maize were comparatively the best sources of micronutrients. Sweet maize yielded the highest values of potassium, thiamin, and magnesium, and flour kernels had the highest riboflavin and niacin content. These results indicate that the maintenance of diverse maize landraces had nutritional as well as ecological, symbolic, and culinary value in both the past and today. Compared to modern commercial maize standards, traditional southwestern maize landraces had a somewhat higher caloric value, many had higher vitamin and mineral content, and all accessions but dent displayed higher protein values. This suggests that southwestern maize-focused diets that included diverse landraces may have been more nutritious than previously understood.

Bacterial Communities in the Embryo of Maize Landraces: Relation with Susceptibility to Fusarium Ear Rot

Locally adapted maize accessions (landraces) represent an untapped resource of nutritional and resistance traits for breeding, including the shaping of distinct microbiota. Our study focused on five different maize landraces and a reference commercial hybrid, showing different susceptibility to fusarium ear rot, and whether this trait could be related to particular compositions of the bacterial microbiota in the embryo, using different approaches. Our cultivation-independent approach utilized the metabarcoding of a portion of the 16S rRNA gene to study bacterial populations in these samples. Multivariate statistical analyses indicated that the microbiota of the embryos of the accessions grouped in two different clusters: one comprising three landraces and the hybrid, one including the remaining two landraces, which showed a lower susceptibility to fusarium ear rot in field. The main discriminant between these clusters was the frequency of Firmicutes, higher in the second cluster, and this abundance was confirmed by quantification through digital PCR. The cultivation-dependent approach allowed the isolation of 70 bacterial strains, mostly Firmicutes. In vivo assays allowed the identification of five candidate biocontrol strains against fusarium ear rot. Our data revealed novel insights into the role of the maize embryo microbiota and set the stage for further studies aimed at integrating this knowledge into plant breeding programs.

Intragenomic Conflict between Knob Heterochromatin and B Chromosomes Is the Key to Understand Genome Size Variation along Altitudinal Clines in Maize

In maize, we studied the causes of genome size variation and their correlates with cultivation altitude that suggests the existence of adaptive clines. To discuss the biological role of the genome size variation, we focused on Bolivian maize landraces growing along a broad altitudinal range. These were analyzed together with previously studied populations from altitudinal clines of Northwestern Argentina (NWA). Bolivian populations exhibited numerical polymorphism for B chromosomes (Bs) (from 1 to 5), with frequencies varying from 16.6 to 81.8 and being positively correlated with cultivation altitude. The 2C values of individuals 0B (A-DNA) ranged between 4.73 and 7.71 pg, with 58.33% of variation. The heterochromatic knobs, detected by DAPI staining, were more numerous and larger in individuals 0B than in those with higher doses of Bs. Bolivian and NWA landraces exhibited the same pattern of A-DNA downsizing and fewer and smaller knobs with increasing cultivation altitude, suggesting a mechanistic link among heterochromatin, genome size and phenology. The negative association between the two types of supernumerary DNA (knob heterochromatin and Bs), mainly responsible for the genome size variation, may be considered as an example of intragenomic conflict. It could be postulated that the optimal nucleotype is the result of such conflict, where genome adjustment may lead to an appropriate length of the vegetative cycle for maize landraces growing across altitudinal clines.

Diversity Assessment of the Montenegrin Maize Landrace Gene Pool Maintained in Two Gene Banks

Due to the loss of agro-biodiversity, there is a strong effort to find apparent and efficient mechanisms for the conservation and sustainable use of genetic diversity. A joint monitoring of the diversity and collections structure of the Montenegrin maize landraces conserved in the Serbian (MRIZPGB) and Montenegrin (MGB) gene banks has been conducted in order to improve the composition of the collections and to identify and eliminate possible redundancy. Based on a separate analysis of white- and yellow-orange maize landraces, it can be concluded that the diversity and evolution of distinct maize landraces grown and collected in Montenegro have been simultaneously shaped by both environmental (i.e., natural selection) and socially driven factors (farmers’ selection, migration and colonization processes of the human population). Although it has been determined that the authenticity and variability of the Montenegrin maize landraces gene pool have largely been preserved in the MRIZPGB collection, a significant amount of redundancy was observed. The obtained results will contribute to the cost-efficient conservation of the maize gene pool in the Montenegrin and Serbian gene banks. The recognized and well-preserved original variability of the MRIZPGB and MGB Montenegrin gene pool represents a valuable source for pre-breeding activities on broadening the white and flint maize breeding programmes under temperate conditions.

Race diversity in dryland maize (Zea mays L.) landraces from southern Nuevo León, Mexico

Objective: To analyze the race diversity and geographic distribution of the native maize landraces currently cropped at southern Nuevo León, México. Design/Methodology/Approach: Data was obtained from 41 accessions which represent the commercial production in the dry land area, where fertilization and pest control are scarcely used. Landraces were classified according to the CONABIO guidelines for ear traits. Results: The measured accessions correspond to seven maize races and to seven interracial crosses. The two most frequent maize races were Ratón and Cónico Norteño, mostly located in the dry areas with less rain. Study Limitations/Implications: Three races, Celaya, Tablilla de Ocho and Elotes Cónicos, had not been previously reported; while two formerly reported races Tabloncillo and Olotillo, were no longer found. This study did not include the grain-colored accessions. Findings/Conclusions: Three collections stood out for producing large ears with large kernels, thus showing a high yield potential. The maize landraces harvested in dryland areas might offer advantages to be grown under harsh environments or be used as gene donors for drought tolerance.

Morphological and eco-geographic diversity analysis of maize germplasm in the high altitude Andes region of Ecuador

The Andean region of Ecuador is the place of origin of many maize landraces grouped into 24 races. Definition of priorities for maize diversity conservation in this region can be supported by the spatial identification of areas with a high eco-geographical and phenotypic diversity. Six hundred thirty-six maize samples were morphologically characterized using 14 descriptors and assigned to a distinctive race. Additionally, sampled farms were characterized by 12 environmental variables. From these data, maps of morphological and eco-geographical diversity were obtained by using techniques to determine eco-geographical and phenotypic distances and applying them to each geographical neighbourhood. The races Patillo Ecuatoriano, Racimo de Uva and Uchima exhibited high intra-racial morphological variation, particularly in the shape of the ear, kernel row layout, cob diameter and total kernel number. The highest number of different races was observed in Imbabura, Azuay and Chimborazo provinces. The highest levels of morphological diversity were found in three cells (10 × 10 km), located in Pichincha, Chimborazo and Loja provinces. Two ecological niches, located in Loja province, showed high levels of eco-geographical diversity. A comparison between diversity maps revealed shared hotspots of morphological and eco-geographical diversity in the central and southwest areas of Imbabura province. The Andean highlands of Ecuador are an optimal refuge for the conservation of maize diversity, and the criteria of eco-geographical and morphological diversity and race richness should be considered when defining priority in situ conservation areas.