This is the ninth in a series of posts reviewing the 17 articles found in the October 2014 Special Issue of American Journal of Botany, Speaking of Food: Connecting Basic and Applied Science.
Transistions in Photoperiodic Flowering Are Common And Involve Few Loci in Wild Sunflowers (Helianthus; Asteraceae) by Lucas P. Henry, Ray H. B. Watson, and Benjamin K. Blackman
The seasonal timing of flowering is an important trait to consider in crop plants, because it dictates where geographically a particular crop can be grown and also plays a role in fitness and yield. Flowering time is determined by a combination of genetics and environmental factors. One of the major environmental factors is day length, a phenomenon known as photoperiod response (or photoperiodism). There are three main types of photoperiod response: short-day (plants flower when “grown in day lengths below a critical maximum threshold”), long-day (plants flower when “grown in day lengths above a critical minimum threshold”) and day-neutral (“plants flower at the same time under all day length conditions”). A plant’s response to day length can be obligate – restricted to a particular response – or facultative – capable but not restricted. Understanding the genetics of photoperiod response is important for breeding efforts, and can help in the development of crop varieties that have improved yields and that can be either grown in broader geographic areas or that are specifically selected for local regions.
Agricultural breeding programs often investigate wild relatives of crop plants for potential traits that could lead to improvements. There is “renewed interest” in these investigations “because genome-enabled methods [of identifying desirable genes] and international investment in germplasm resources have dramatically reduced the associated labor, time, and risk.” The authors of this study, recognizing extensive variation in flowering time in both common sunflower (Helianthus annuus) and its wild relatives, examined the genetic basis for this variation in an effort to support sunflower breeding programs.
Helianthus is a genus consisting of around 70 species, most of which are native to North America (a few occur in South America). Several species in this genus are cultivated as food crops and/or as ornamental plants. H. annuus is the most commonly cultivated species, valued for its edible seeds and the oil they produce as well as for various other things. Wild relatives of H. annuus have “been a frequent source of genetic raw material for agricultural innovation,” aided by the fact that “barriers to interspecies crosses are incomplete or can be overcome through embryo culture or chromosomal doubling.” Helianthus is a diverse genus, including generalist species occurring in “diverse environments over broad geographic regions” and specialist species occurring in “habitats characterized by high temperature, water, or salt stress.” For this reason, “wild sunflowers are prime sources to mine for alleles that confer higher yield in new or marginal” agricultural settings.
A relatively small subset of Helianthus species were involved in this study; however, the subset represented a “phylogenetically dispersed sample.” One interesting finding was that the evolution of an obligate short-day requirement for flowering has occurred in several species, “particularly those with ranges restricted to the southern United States.” The authors suggest that a reason for this finding could be that “long, hot, and humid summers” in this region “may be unfavorable for growth or reproduction.” Thus, while populations of H. annuus “likely escape these conditions by flowering in the long days of late spring,” other Helianthus species put off “flowering until the arrival of cooler, less humid falls.” Flowering during cooler times is beneficial because pollen fertility decreases and seed maturation slows at high temperatures. The risk of fungal pathogens attacking flowers and dispersed seeds is also reduced during periods of lower humidity.
Another important finding was that the diversity in photoperiod response in Helianthus appears to have a “relatively simple genetic architecture.” If this is the case, it could “greatly facilitate rapid crop improvement by marker-assisted selection.” Further studies are necessary, specifically those involving “intra- and interspecific crosses segregating for variation in photoperiod response,” in order to confirm the authors’ findings and justify “broader investment of resources into these applied efforts.”
While much was learned from this study, the authors acknowledge the need for “future investigations with greater taxonomic and environmental sampling.” Researchers recently produced a “draft genome” for sunflower. This additional resource will greatly aid breeding programs and further inform studies, like this one, that are interested in the “mechanistic factors and ecological agents that have promoted the emergence of the great diversity and lability in photoperiod response observed in wild sunflowers.”