Pierangelo Landi

Classical Genetic and Quantitative Trait Loci Analyses of Heterosis in a Maize Hybrid between Two Elite Inbred Lines

The exploitation of heterosis is one of the most outstanding advancements in plant breeding, although its genetic basis is not well understood yet. This research was conducted on the materials arising from the maize single cross B73 × H99 to study heterosis by procedures of classical genetic and quantitative trait loci (QTL) analyses. Materials were the basic generations, the derived 142 recombinant inbred lines (RILs), and the three testcross populations obtained by crossing the 142 RILs to each parent and their F1. For seedling weight (SW), number of kernels per plant (NK), and grain yield (GY), heterosis was >100% and the average degree of dominance was >1. Epistasis was significant for SW and NK but not for GY. Several QTL were identified and in most cases they were in the additive–dominance range for traits with low heterosis and mostly in the dominance–overdominance range for plant height (PH), SW, NK, and GY. Only a few QTL with digenic epistasis were identified. The importance of dominance effects was confirmed by highly significant correlations between heterozygosity level and phenotypic performance, especially for GY. Some chromosome regions presented overlaps of overdominant QTL for SW, PH, NK, and GY, suggesting pleiotropic effects on overall plant vigor.

RFLP mapping of quantitative trait loci controlling abscisic acid concentration in leaves of drought-stressed maize (Zea mays L.)

Abstract Abscisic acid (ABA) concentration in leaves of drought-stressed plants is a quantitatively inherited trait. In order to identify quantitative trait loci (QTLs) controlling leaf ABA concentration (L-ABA) in maize, leaf samples were collected from 80 F3:4 families of the cross Os420 (high L-ABA)×IABO78 (low L-ABA) tested under drought conditions in field trials conducted over 2 years. In each year, leaf samples were collected at stem elongation and near anthesis. The genetic map obtained with 106 restriction fragment length polymorphism (RFLP) loci covered 1370 cM, which represented approximately 85% of the UMC maize map. Sixteen different QTLs with a LOD>2.0 were revealed in at least one sampling. Across samplings, only four QTLs significantly influenced L-ABA, accounting for 66% of the phenotypic variation and 76% of the genetic variation among families. At these QTLs, the alleles which increased L-ABA were contributed by Os420. The two most important QTLs were mapped on chromosome 2 near csu133 and csu109a. The effects associated with the QTL near csu133 were more pronounced near anthesis. The support intervals of the four primary QTLs for L-ABA did not overlap the presumed map position of mutants impaired in ABA biosynthesis.

Searching for quantitative trait loci controlling root traits in maize: a critical appraisal

The identification of quantitative trait loci (QTLs) for root traits can provide useful indications on their genetic basis and the associated effects on grain yield under different water regimes. Furthermore, the availability of molecular markers linked to QTLs controlling variation for root traits and grain yield will allow for the implementation of marker-assisted selection to improve productivity. In maize (Zea mays L.), four mapping populations have been investigated to locate QTLs for root traits under controlled conditions and/or in the field. A comparative analysis of the QTL results was carried out based on the availability of molecular markers common to the investigated populations and the UMC maize reference map. Several chromosome regions affected root traits in two or even three populations. A number of these regions also affected grain yield under well-watered and/or drought-stressed conditions. The most important QTL effects were detected on chromosome bins 1.03, 1.06, 1.08, 2.03, 2.04, 7.02, 8.06 and 10.04. Exploiting the syntenic information available for maize and rice, a number of QTLs for root traits described in rice were found to map in regions syntenic to a number of the listed maize chromosome bins (e.g., bin 2.04). The development of near isogenic lines (NILs) for the most important QTLs will allow to investigate whether the concomitant effects of the QTLs on root traits and grain yield are due to linkage and/or pleiotropy and, ultimately, may offer the opportunity to clone the genes underlying such QTLs. Although QTL analysis remains a resource-demanding undertaking, its integration with genomics and post-genomics approaches will play an increasingly important role for the identification of genes affecting root characteristics and grain yield in maize and for harnessing the favourable allelic variation at such loci.

Characterization of root-yield-1.06, a major constitutive QTL for root and agronomic traits in maize across water regimes

A previous study on maize F(2:3) families derived from Lo964xLo1016 highlighted one QTL in bin 1.06 (hereafter named root-yield-1.06) affecting root and agronomic traits of plants grown in well-watered (WW) and water-stressed (WS) conditions. Starting from different F(4) families, two pairs of near isogenic lines (NILs) were developed at root-yield-1.06. The objective of this study was to evaluate root-yield-1.06 effects across different water regimes, genetic backgrounds, and inbreeding levels. The NILs per se and their crosses with Lo1016 and Lo964 were tested in 2008 and 2009 near to Bologna, with the well-watered (WW) and water-stressed (WS) treatments providing, on average, 70 mm and 35 mm of water, respectively. For NILs per se, the interactions QTL x water regime and QTL x family were negligible in most cases; the QTL additive effects across families were significant for several traits, especially root clump weight. For NILs crosses, analogously to NILs per se, the interactions were generally negligible and the additive effects across water regimes and families were significant for most traits, especially grain yield. A meta-analysis carried out considering the QTLs described in this and previous studies inferred one single locus as responsible for the effects on roots and agronomic traits. Our results show that root-yield-1.06 has a major constitutive effect on root traits, plant vigour and productivity across water regimes, genetic backgrounds, and inbreeding levels. These features suggest that root-yield-1.06 is a valuable candidate for cloning the sequence underlying its effects and for marker-assisted selection to improve yield stability in maize.

Comparison between responses to gametophytic and sporophytic recurrent selection in maize (Zea mays L.).

SummaryIn order to evaluate the response at both the gametophytic and sporophytic level of a selection based on the pollen competitive ability and to compare its effects with those obtainable from a conventional sporophytic procedure, three recurrent selection plans were developed in maize starting from the same F2 population. Two gametophytic recurrent selection procedures at high (GSH) and low (GSL) selection intensity were performed by utilizing, to advance the populations, kernels taken from the base (GSH) or apex (GSL) of ears obtained from pair-crosses of randomly chosen plants. The third scheme was a sporophytic full-sib recurrent selection procedure (SS); the only selection criterion was the machine-harvestable grain yield of the families. In a sixyear period of selection, six cycles of both GSH and GSL and three cycles of SS were performed. The source and the selected populations (16 entries) were tested for pollen performance and for sporophytic traits. The selection cycles advanced through GSH showed a progressive increase, as compared to GSL, in pollen tube length measured at 4 h of in vitro culture. The SS cycles response was intermediate at 4 h whereas at 2 h it exceeded both GSH and GSL. A slight decrease in pollen diameter was evidenced in populations advanced with GSL procedure. The SS selection caused a marked increase for grain yield, lateness, leaves per plant and plant height. No response was shown by gametophytic selection for grain yield. The GSH procedure, however, led to an increase in kernel weight and to a decrease in kernel moisture, leaf number and plant height, as compared to GSL. Though gametophytic selection showed limited effects on sporophytic traits, it can be considered an efficient tool to supplement conventional sporophytic selection.

Chlorsulfuron Tolerance and Acetolactate Synthase Activity in Corn (Zea mays L.) Inbred Lines1

Seven corn inbred lines previously shown to differ in response to soil residues of chlorsulfuron were characterized as to the target-enzyme acetolactate syn- thase (ALS) specific activity and to its susceptibility to the herbicide. ALS from plantlets at the five-leaf stage of growth was similarly susceptible to chlorsulfuron in all lines and its specific activity in the shoots was not significantly correlated with in vivo tolerance to the herbicide. By contrast, differences in ALS specific activity in roots of plants both at the five- and three-leaf stages of growth were significantly correlated (r = 0.96** and r = 0.93**, respectively) with in vivo tolerance. Correlation was also noted in extracts from cultured excised root tips (r = 0.94**). Callus tissue of a chlorsulfuron-tolerant line was less affected by the herbicide and had a significantly higher ALS specific activity than callus from a chlorsul- furon susceptible line, whereas inhibition of ALS by the herbicide was similar in both lines. These results indicate that the naturally occurring differences in ALS levels in the roots of the investigated inbred lines contribute largely to the differential in vivo response observed to chlorsulfuron. Nomenclature: Chlorsulfuron, 2-chloro-N- (((4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino) carbonyl)

Genomics of Root Architecture and Functions in Maize

Genomics approaches and platforms offer novel opportunities to identify and clone the loci that control root architecture in maize. Mutants for root features have been described and in some cases the relevant genes have been cloned. A number of studies have also described quantitative trait loci (QTLs) that provide access to valuable genetic diversity for the morpho-physiological features that characterize root functionality. Nonetheless, although a number of major QTLs have been identified, none of these QTLs has been cloned so far, mainly due to the difficulty to accurately phenotype the target traits in a sufficiently large number of plants. It is foreseeable that QTL cloning will be facilitated by the adoption of high-throughput phenomics platforms as well as by the information made available through the sequencing of the maize genome and the profiling of the transcriptome, proteome, and metabolome, all of which will contribute to the identification of plausible candidate genes. Allele mining in germplasm and mutant collections through forward- and reverse-genetics approaches, coupled with marker-assisted backcrossing and/or genetic engineering, will further facilitate the introgression of novel genetic variation in elite materials. New QTL-based modeling approaches will improve our capacity to understand genotype × environment interaction at varying water and nutrient regimes, thus contributing to define the most promising “molecular” ideotypes. This notwithstanding, a sizeable impact of marker-assisted selection and other genomics approaches in tailoring the ideal root architecture in maize will only be possible through a deeper knowledge of root functions under a broad range of growing conditions and an integration with conventional breeding methodologies.

Sporophytic response to pollen selection for Alachlor tolerance in maize.

In order to assess the efficiency of male gametophytic selection (MGS) for crop improvement, pollen selection for tolerance to herbicide was applied in maize. The experiment was designed to test the parallel reactivity to Alachlor of pollen and plants grown in controlled conditions or in the field, the response to pollen selection in the sporophytic progeny, the response to a second cycle of MGS, and the transmission of the selected trait to the following generations. The results demonstrated that pollen assay can be used to predict Alachlor tolerance under field conditions and to monitor the response to selection. A positive response to selection applied to pollen in the sporophytic progeny was obtained in diverse genetic backgrounds, indicating that the technique can be generally included in standard breeding programs; the analysis of the data produced in a second selection cycle indicated that the selected trait is maintained in the next generation.

Recombinant near-isogenic lines: a resource for the mendelization of heterotic QTL in maize

Although heterosis is widely exploited in agriculture, a clear understanding of its genetic bases is still elusive. This work describes the development of maize recombinant near-isogenic lines (NILs) for the mendelization of six heterotic QTL previously identified based on a maize (Zea mays L.) RIL population. The efficient and inexpensive strategy adopted to generate sets of NILs starting from QTL-specific residual heterozygous lines (RHLs) is described and validated. In particular, we produced nine pairs of recombinant NILs for all six QTL starting from RHLs F4:5 originally obtained during the production of the RIL population mentioned above. Whenever possible, two different NIL pairs were generated for each QTL. The efficiency of this procedure was tested by analyzing two segregating populations for two of the selected heterotic QTL for plant height, yield per plant and ears per plant. Both additive and dominant effects were observed, consistently with the presence of the QTL within the introgressed regions. Refinement of QTL detection was consistent with previous observations in terms of effects and position of the considered QTL. The genetic material developed in this work represents the starting point for QTL fine mapping aimed at understanding the genetic bases of hybrid vigor in maize.

Variation among Andean races of maize for cold tolerance during heterotrophic and early autotrophic growth.

Cold in the initial growth stages is an important stressfactor for maize grown in regions with a temperate climate,particularly in case of early sowing. Sources of tolerancehave been identified in adapted genotypes, but promisinggenes for cold tolerance should also be found in materialdeveloped under the lower-temperature margins of the cropdistribution. This research was conducted in order to testAndean maize accessions for cold tolerance expressed duringboth the heterotrophic and early autotrophic growth stages.Experiments were conducted in controlled environments tostudy cold tolerance traits (germination %, germinationindex and plant growth rate) at continuous 10°C (heterotrophic growth) and at varying 10–16°C (autotrophic growth). An experiment was also performed inthe field with early sowing (both heterotrophic and autotrophic growth). In each experiment, a control trialwas conducted in more favourable conditions (i.e. continuous25°C in a controlled environment or late planting inthe field) so that cold tolerance traits could also beexamined as the ratio between the stress and the controltrial. None of the accessions was superior for all coldtolerance traits. However, several Andean maize accessionsoutperformed the US Corn-belt hybrid checks for one or moretraits, both in heterotrophic and autotrophic growth. Overall, BOZM 855, PMS 636, Poblacion D, Poblacion E andBOZM 696 were the best accessions, suggesting that they canbe a promising source of genes for improving cold toleranceof adapted maize genotypes.