Louise Jackson, Paula Ellison, Irenee Ramirez, Tim Gookin, Maria Truco, Oswaldo Ochoa, Richard Michelmore

Wild plant species are often adapted to more stressful environments than their cultivated relatives. Roots are critical in exploiting soil resources that enable plants to withstand environmental stresses, but are difficult to study. Wild lettuce (Lactuca serriola L.) is more drought-adapted than its near relative, cultivated lettuce (L. sativa L.). The two species differ in root characteristics that govern the uptake of deep moisture in the soil profile (Jackson et al., 1995, Plant, Cell and Environment; Gallardo et al., 1996, Plant, Cell and Environment):

• Wild lettuce has a deeper taproot and more lateral roots at the tip of the taproot.
• Wild lettuce relies on deep moisture, and as long as deep moisture is available, it is not affected by drought in the top 15 cm of soil.
• The shallow root system of cultivated lettuce mainly uses moisture in the surface layer, and growth is reduced when surface drought occurs.

A genetic linkage map was created using a population from a cross between wild and cultivated lettuce. Genes controlling complexly inherited traits (i.e., quantitative trait loci or QTL) were identified using this map (Johnson et al., 2000, Theoretical and Applied Genetics). QTL for architectural traits, e.g., taproot length and number of lateral roots produced near the taproot tip, co-localize with QTL for the ability to extract water from deep in the soil. Wild lettuce is therefore a potential source of agriculturally important alleles to increase drought avoidance and higher productivity with lower water inputs.

The preliminary data from F₇ recombinant inbred lines (RILs) indicate that across the wide range of lettuce shoot sizes and morphologies represented by the 40 families (Ellison, 2002, MS Thesis, University of California, Davis):

• Highest shoot growth tended to occur in plants with molecular markers for the QTL for:

1)     Short taproot from cultivated lettuce, and

2)     More laterals at the tip of the taproot from wild lettuce.

• Plants with L. serriola markers for the QTL for more laterals at the tip of the taproot tended to extract more water from deeper in the profile.
• These data provide preliminary confirmation of the existence of these QTL.
• Under the mild water stress regime of this study, however, a deep, large taproot may be a carbon sink, and this may explain the lower shoot biomass of plants with this allele.
• Further work will emphasize the deep lateral trait as this field experiment showed that deep soil water extraction could be enhanced without a decrease in shoot biomass.

A marker-assisted breeding program is now underway to introgress L. serriola alleles for the two QTL regions into cultivated lettuce. One set of experiments used a BC₂ family that was derived from a cross with an F₃ family that was homozygous for L. serriola at both QTL regions. Plants that were homozygous for L. serriola markers for deep laterals had more laterals in the bottom 5 cm of the taproot than did individuals with L. sativa markers in this QTL region (QTL 4c-f).

Research is now aimed at using this BC₂ family to produce near isogenic lines (NILs) that are nearly identical to L. sativa, except for the deep rooting traits, and to conduct more comprehensive analysis of the trait for deep laterals at the tip of the taproot, and its potential for increasing deep exploitation of moisture from the soil profile, and thus reduce irrigation frequency and amount.