Shoot apical meristem architecture in maize: Diversity, genetic control, and its relationship to adult plant morphology

Abstract

The maize shoot apical meristem (SAM) contains a pool of undifferentiated stem cells that produce all above-ground plant organs. Previous studies of the SAM have focused mainly on analysis of mutants conferring visible phenotypes. Though these approaches indicated the effect of genotype on SAM size, this represents the first work to characterize natural diversity in SAM architecture and its genetic control, as well as its relationship to adult plant traits. A time course of SAM growth throughout vegetative development was conducted to assess growth over time, resulting in the description of three phases of growth: initial proliferation, slower growth, and rapid expansion before transition. The results also indicated that 14 days after planting was the most appropriate time point for subsequent measurements. Quantitative trait loci for meristem traits (height, width, midpoint width, arc length, L1 arc cell number, plastochron internode, height:width ratio, volume, cell size) were mapped in the intermated B73 x Mo17 recombinant inbred line population. Distinct control of height- and width-related traits via several small-effect loci, as well as a lack of overlap with known meristem genes, was detected. One locus was validated using near-isogenic lines. Candidate genes were identified by correlating expression in the shoot apex with meristem traits across the population. To assess natural diversity and the impact of heterosis on maize SAM architecture, a set of 27 diverse inbreds and several of their F1 crosses were examined. This revealed that B73 and P39 were among the tallest outliers for SAM height and CML277 was the shortest. Heterosis was observed for meristem traits in several crosses of diverse inbreds to Mo17; some showed dominant effects. Only P39 exhibited heterosis with B73, demonstrating unique alleles contributing to the extreme heights observed in these inbreds. Recombinant inbred populations derived from crosses of P39 and CML277 inbreds to B73 were also used for mapping. Here, a few large-effect loci were identified that affected many meristem traits; most overlapped with known SAM-related genes. Genetic control was shown to be population-dependent. Architecture traits in the meristem were correlated with several adult plant traits, including flowering time, indicating potential shared regulatory mechanisms.