Conventional and transgenic approaches to enhance crop performance are complementary and rely on many of the same molecular and informational tools.

Box 3-1 contains a list of plant traits of which variants are selected by breeders (sometimes inadvertently) as targets in the process of creating improved plants for various environments. Many of the traits are linked by biochemical interactions that influence the expression and control of gene products. Sometimes the genes controlling the different traits are ge-

BOX 3-1

Examples of Traits Targeted for Improvement

  • Architecture—height, number of leaves, tillers, branches, leaf angle, number of flowers and seeds, seed size, root structure, surface area.

  • Optimal planting density.

  • Flowering time and photoperiod responses.

  • Growth rates and regulation of hormones: brassinolide, auxins, gibberellins, cytokinins, ethylene.

  • Growth responses to light quality and quantity.

  • Photosynthesis rates and overall carbon fixing during growing season, chloroplast number and positioning, C3 vs. C4 metabolism, pathway regulation.

  • Heterosis (hybrid vigor) and male sterility for hybrid production.

  • Fertility, inbreeding and outbreeding.

  • Nitrogen and phosphorus uptake, use efficiency, translocation, storage, reduction, portioning between plant parts.

  • Water use efficiency: uptake, storage, transpiration rates, loss, tolerance of chronic drought and transient drought.

  • Heat or cold shock and sustained tolerance to heat, cold, freezing.

  • Seed germination in cold.

  • Flooding tolerance.

  • Oxidative stress tolerance.

  • Heavy-metal and salt tolerance.

  • Biosynthesis of key metabolites.

  • Nutritional composition—seeds, roots, leaves, fruits, stems.

  • Digestibility (by humans and animals).

  • Root endosymbionts.

  • Resistance to viral, fungal, and bacterial pathogens.

  • Resistance to weeds and to herbicides that control them.

  • Resistance to insects and other pests and predators.

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