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One of the most important traits for both wild and cultivated plants is the timing of the transition from vegetative to reproductive growth (flowering time), which needs to be controlled to avoid flowering under unfavorable conditions. This transition from vegetative to reproductive stage is affected by environmental cues and signals within the plant. Flowering time reflects the adaptation of a plant to its environment by tailoring vegetative and reproductive growth phases to local climatic effects. In an outcrossing species such as corn, Zea mays L., flowering time is a complex trait and controled by a large number of loci. The time required to mature varies widely in maize landraces, from 2 to 11 months. In addition, asynchrony of male and female flowering in maize may be adaptive in some cultivars, but can result in losses under drought conditions, especially in modern uniform varieties..

Maize is adapted to a range of environments, from the lowlands to the Andean highlands, and has been widely introduced worldwide into both temperate and tropical regions. Maize's genetic architecture for flowering time has evolved as its wild relatives adapted to distinct ecological zones in elevations differing by more than 3000 meters in Mexico; and then under both natural and artificial selection over the last 7,000 years, with especially intense selection over the past century. This genetic architecture has evolved under a predominantly outcrossing mating system in a species with little population differentiation.


Silking ear (female flower) and tassel (male flower) starting to shed pollen.

Flowering time has been extensively studied in the predominantly self-fertilizing species Arabidopsis. Like maize, Arabidopsis grows across a wide range of latitudes and has flowering time controlled by the interaction of the photoperiod (light sensing and circadian rhythm), vernalization, and autonomous flowering and gibberellic acid-response pathways. In grasses, which include maize, wheat, and rices, some of the same genes are involved in flowering, but they have different functions.

Our study has shown that large differences in flowering time among inbred maize lines are not causes by a few genes of large effects, but by the cumulative effects of numerous QTLs, each with only a small impact on the trait. Selection may have favored a genetic architecture of additive small-effect QTLs, so that most offspring are likely to have partially synchronous flowering time to ensure fitness. So far, only one major maize flowering time QTL has been positionally cloned, vgt1 (Salvi et al., 2007). Our group is in the process of fine mapping to identify QTLs further.

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