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  • Author or Editor: Kevin M. Folta x
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Vavilov’s Law of Homologous Series indicates that heritable variation for a given trait will occur in different species based on parallel selection. The conclusion comes from Vavilov’s study of extensive collections and careful attention to phenotypic variation across taxa. The same examination of variation can be applied to traits using the power of genetic and genomic resolution, because parallel traits would be expected to be governed by the same genetic loci, and perhaps even common mutations. In this review, these concepts are applied to two central traits—the control of “shattering” of kernels in cereals and in the control of photoperiodic flowering. One of the strengths of the law is that it can make predictions about traits and perhaps the genes or genomic regions that control them. With respect to genetic variation, the occurrence and physical location of genes associated with kernel retention may be predicted. Many grains share mutations, such as the Sh 1 gene, which were selected in parallel. Selection of the Sh1 gene led to higher yields due to better kernel retention. While the genes affected are often the same, the types of mutations are not, implying convergent selection. Flowering time is governed by multiple loci, so variation may be attributed only to a few candidates, yet because of the number of regulators the predictive power of the law is lower. The modern application of the Law of Homologous Series is that it allows basic researchers or plant breeders to make predictions about the genes controlling key traits, although the genetic basis of variation is likely not conserved.

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In the mission of plant husbandry, light is a critical yet passive entity. The potential to actively implement dynamic lighting strategies to control plant growth and development holds great promise in the future of plant cultivation. In other words, rather than simply using a single stable light condition to maintain photosynthesis, might it be possible to continually adjust fluence rate, wavelength combinations, and photoperiods to actively manipulate plant morphology and production? Research over the past 100 years suggests that it is so, and today's solid-state, narrow bandwidth lighting platforms offer a unique opportunity to test this hypothesis. The goal of this report is to describe the potential use of light as a growth regulator. Here light-emitting diode technology is well suited for the application, because light quantity, quality, photoperiod, and combinations thereof can be controlled with great precision. Specific light combinations may be adjusted throughout the life of a plant to potentially optimize traits of interest such as synchronization of flowering, maintenance of vegetative growth programs, control of plant stature, or acceleration of juvenility. This report describes the plant photosensory networks and how they sense and respond to light. The connection between light and internal hormone stasis is explored and then extended to questions of designing specific regimes to control plant growth and development. The concept of static signaling states is presented as a means to tightly control plant habits, in essence, using light to stabilize plant signal transduction pathways and their associated outcomes. Finally, the concepts presented are applied to the diploid strawberry Fragaria vesca to demonstrate the usefulness of the approach. These experiments provide proof-of-concept and lay a foundation for further studies.

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Plant productivity and product quality ultimately are dependent on an interaction between genetics and environment, and one of the most important environmental cues is light. Light quantity, quality, and duration provide critical information to plants that mediate growth and development. Light signal transduction is dependent on a series of photoreceptors and their associated signaling pathways that direct intracellular processes that lead to changes in gene expression that ultimately affect plant form, function, and content. For the last several decades, scientists have dissected these signaling pathways and understand how they connect the environment to a response. The advent of narrow-bandwidth illumination in commercial lighting invites the opportunity to manipulate plant behavior and productivity through precise alteration of the ambient spectrum. This review describes the biochemical links that convert incident light into predictable changes in plant growth and development. These sensors and pathways serve as biochemical switches that can be selectively toggled to control plant growth, development, physiology, or metabolite accumulation.

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The transition from vegetative growth to reproductive growth is carefully controlled by a number of independent signal transduction systems, one of which interprets photoperiod. Photoperiodic control of flowering time has been well-described in Arabidopsis and rice, revealing the presence of a generally common network of regulatory proteins. Timely and appropriate progression to flowering is critical to profitable production of cultivated strawberry (Fragaria ×ananassa), a species that includes long-day, short-day, and day-neutral cultivars. In an effort to characterize the photoperiodic flowering control mechanism in strawberry, the Fragaria orthologs of the photoperiod pathway genes were cloned and sequenced. Strawberry versions of Constans, Constans-like, Leafy, Flowering Locus T, and Suppressor of Constans Overexpression 1 were identified by screening cDNA libraries and through degenerate PCR approaches. Expression of these transcripts in short-day and day-neutral cultivars was tested under long and short photoperiods. Functional complementation of Arabidopsis mutants was performed where appropriate, alleles were identified, genetic linkage was determined where possible, and relationships between the strawberry genes and homologs from other species were studied. These trials define the mechanistic elements of an agriculturally important pathway in this valuable crop, and lays the foundation for transgenic studies in strawberry to manipulate the floral transition.

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Fragaria ×ananassa, the cultivated octoploid strawberry, is an intensively cultivated fruit crop in which relatively small variations in disease susceptibility and flowering habit can have significant economic impacts. In order to facilitate future studies of the molecular mechansisms governing these characters, we have initiated studies to identify and sequence the strawberry homologs of a number of important genes known to be critical to pathogenesis response and photoperiodism in model systems such as arabidopsis, rice, and tomato. Using the primary Florida cultivar Strawberry Festival, we have employed a variety of techniques to identify such genes, including EST sequencing of a salicylate-induced cDNA library, PCR with degenerate primers, and colony hybridization. Possible homologs of the targeted genes and their relationships to similar genes in other species are presented. These results will form the basis of future studies of gene expression and evolutionary relationships among the Rosaceae and other species.

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The past year has brought substantial progress in the development of functional and structural genomic tools for strawberry. Sequencing of cDNA library clones from the cultivated strawberry Fragaria × ananassa and the diploid model species Fragaria vesca has provided more than 3000 new EST sequences. We have also constructed a large (∼40 kb) insert genomic (fosmid) library from F. vesca. About 33,000 fosmid clones have been picked and spotted onto hybridization filters. Filters have been successfully probed with three single copy gene probes, one gene family probe, and chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA) probe sets. The combined cpDNA and mtDNA clone content of the library is about 11%. After correction for organelle insert content, the nuclear genome coverage of the library is about 6×. Complete sequencing of two fosmid clones identified 12 putative protein-encoding genes, four of which were organized in colinearity with the corresponding chromosomal region of Arabidopsis thaliana. We will sequence an additional 50 fosmid clones, and use the resulting sequence data as the basis for developing a novel marker technology, to be described. These genomic tools will provide a basis for connecting specific genes to specific traits in the octoploid, cultivated strawberry, paving the way for implementation of gene-based, marker assisted selection as a tool for strawberry breeders. Opportunity for cross-species comparisons of gene sequence and composition, as well as genome organization and linkage group structure, between Fragaria and other members of the economically important Rosaceae family has been significantly enhanced, thus expanding the relevance of the project results to peach, cherry, apple, rose, brambles, and many other Rosaceous species.

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Coleus (Plectranthus scutellarioides) is an attractive and popular ornamental plant with propagation mainly achieved through vegetative cuttings. For commercial purposes, it is of interest to enhance the speed of establishment while maintaining high quality. Light quality has been shown to influence adventitious root development, so these experiments examined the effect of narrow-bandwidth light treatments on root growth and overall plant quality for seven coleus cultivars with vegetative cuttings in potting soil and one cultivar with shoot tip in vitro cultures onto Murashige and Skoog (MS) agar medium. During the 28 days of the propagation period, the cuttings grown under narrow-bandwidth red light (R; 663.4 nm at peak) more than doubled in the adventitious root number compared with those under blue light (B; 445.7 nm at peak) and green light (G; 530.0 nm at peak) in five cultivars. R light also increased fresh weight of the cuttings by 55.6% more than G light. In comparison, the cuttings grown under G light yielded significantly lower root and shoot dry mass than other light treatments. R light cuttings showed more dry mass content (9.63%) than those under white light (W; 437.4 nm and 559.5 nm at peak) and G light (7.85% and 5.86%, respectively). A positive correlation (R 2 = 0.598, P < 0.001) was found between the formation of adventitious roots and gained fresh weight of cuttings. R light made the reddish color of leaves significantly stronger in most cultivars, whereas the cuttings exposed to G light became less vivid compared with other light conditions. When the shoot tips were propagated in vitro onto MS medium, R light treatment initiated the root development more rapidly than other lights, with significantly greater rooting rate (20.0% and 63.6%, respectively) at day 5 and 10. The shoot tips under R light also formed significantly more roots (12.3 per cutting) than those grown under narrow-bandwidth B light (5.8 per cutting). The shoot tips showed browning at an early stage and newly emerged leaves grew very compactly under B light. The combination of red and green light (R+G) increased more than twice as much roots and dry mass compared with W light. In addition, the R+G light led to morphological changes, including larger leaves and longer petioles and internodes than those in other light treatments. The exposure to R+G+B and B light made the shoots very compact for the 28 days of in vitro culture period and significantly increased the chlorophyll contents resulting in dark green leaves.

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