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The genetic dissection of root architecture and functions allows for a more effective and informed design of novel root ideotypes and paves the way to evaluate their effects on crop resilience to a number of abiotic stresses. In maize, limited attention has been devoted to the genetic analysis of root architecture diversity at the early stage. The difference in embryonic (including seminal and primary) root architecture between the maize reference line B73 (which mostly develops three seminal roots) and the landrace Gaspé Flint (with virtually no seminal roots) was genetically dissected using a collection of introgression lines grown in paper rolls and pots. Quantitative trait locus (QTL) analysis identified three QTLs controlling seminal root number (SRN) on chromosome bins 1.02, 3.07, and 8.04-8.05, which collectively explained 66% of the phenotypic variation. In all three cases, Gaspé Flint contributed the allele for lower SRN. Primary root dry weight was negatively correlated with SRN (r= -0.52), and QTLs for primary root size co-mapped with SRN QTLs, suggesting a pleiotropic effect of SRN QTLs on the primary root, most probably caused by competition for seed resources. Interestingly, two out of three SRN QTLs co-mapped with the only two known maize genes (rtcs and rum1) affecting the number of seminal roots. The strong additive effect of the three QTLs and the development of near isogenic lines for each QTL in the elite B73 background provide unique opportunities to characterize functionally the genes involved in root development and to evaluate how root architecture affects seedling establishment, early development, and eventually yield in maize., (© The Author 2016. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

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.

Near-isogenic hybrids (NIHs), developed from crossing maize (Zea mays L.) backcross-derived lines (BDLs) differing for the parental alleles at a major QTL for leaf ABA concentration (L-ABA), were field-tested for 2 years under well-watered and water-stressed conditions. Differences among NIHs for L-ABA and other morpho-physiological traits were not affected by water regimes. On average, the QTL allele for high L-ABA markedly reduced stomatal conductance and root lodging. To elucidate the effects of the QTL on root architecture and L-ABA, root traits of two pairs of BDLs were measured in plants grown in soil columns at three water regimes. Differences among BDLs were not affected by water regimes. Across water regimes, the QTL confirmed its effect on L-ABA and showed a concurrent effect on root angle, branching, number, diameter, and dry weight. Based on these results, it is concluded that the QTL affects root lodging through a constitutive effect on root architecture. In addition, there is speculation that the QTL effects on root traits and L-ABA are probably due to pleiotropy rather than linkage and a model is proposed in which the QTL has a direct effect on root architecture, while indirectly affecting L-ABA.