Browsing by Subject "Brassica rapa"

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    The Adaptive Value and Genetic Basis of Maternal Effects in Competitive Environments
    (2008-10) Dechaine Berkas, Jennifer M.
    Plants rely heavily on environmental cues to direct life-history processes. In many species, the maternal environment is a reliable predictor of environmental conditions in the next generation, and several aspects of progeny phenotype are responsive to environmental cues during seed maturation. The ratio of red to far-red light (R:FR) is one environmental cue that has been widely shown to influence plant phenotypes across generations. Low R:FR are predictive of competitive conditions, because chlorophyll in neighboring leaves absorbs red light but allows far-red light to pass through. It has been suggested that effects of the maternal competitive environment on progeny phenotype are adaptive, but very few studies have convincingly tested this hypothesis. In addition, the genetic basis of environmental maternal effects is poorly understood. In this dissertation, I examine the adaptive value and genetic basis of maternal effects in competitive environments in the plant species Brassica rapa and Arabidopsis thaliana. In chapter 1, I investigate the adaptive value of maternal effects across two generations of competitive environments in B. rapa. Maternal environment effects did not enhance progeny fitness but did influence several other progeny traits, as well as selection gradients in the progeny generation. These results suggest that although environmental maternal effects are not adaptive in this study, they are genetically variable and may evolve or affect the evolution of progeny traits. I further investigate the effects of competition on B. rapa fitness traits in chapter 2, in which I use a quantitative trait loci (QTL) mapping approach to examine how the genetic architecture of multiple components of fitness differs across competitive environments. QTL expression varied across competitive treatments for total fruit production, but QTL were generally conserved in other fitness traits. In addition, I identify environment-specific QTL for seed mass and germination timing in seeds matured under a low R:FR. Lastly, in chapter 3, I investigate if and how phytochrome photoreceptor genes mediate the effects of maternal R:FR on progeny germination. My results suggest that all 5 phytochrome genes in A. thaliana partially mediate progeny germination response to maternal R:FR, and I identify novel roles for individual phytochrome loci in this response. As a whole, this research provides insight into the extent that maternal effects in competitive environments are adaptive and partially elucidates the genetic basis of maternal effects.
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    The effect of light and temperature on glucosinolate concentration in turnip (Brassica rapa)
    (2010-08) Justen, Veronica Lea
    Glucosinolates (GSLs) are thioglucosides produced by turnip (Brassica rapa subsp. rapa) and other cruciferous vegetables with important properties for plant defense and human health, particularly cancer prevention. We compared yield and GSL concentrations in the roots and shoots of turnip cultivars grown using plastic colored mulch and photoselective netting treatments in both a May and August planting. Turnip yield and GSL concentrations were dependent on tissue type, genotype and environmental factors. ‘Just Right’ roots and shoots consistently had the highest yield and gluconapin concentration of the cultivars examined in field experiments. In most instances, ‘Scarlet Queen’ roots had the highest total GSL concentrations while ‘Just Right’ roots had the least. Although colored plastic mulches significantly influenced both total and individual GSL concentrations, mulch-dependent increases in GSL concentrations were not consistent across tissue types, cultivars, planting dates and years of the study. Photoselective nettings did not consistently affect shoot or root yield or GSL concentration in root tissues. Netting was only a significant factor for glucobrassicanapin (GBN) concentration in shoots with the no netting treatment resulting in the highest GBN concentration. Planting date and year interactions were significant for total and individual GSL concentrations and proportions in both plastic mulch and photoselective netting experiments. These interactions are partially explained by differences in air temperatures and solar radiation prior to harvest. To assess the specific role of temperature on GSL concentration and biosynthetic regulation in roots and shoots, turnips were grown under three different temperature regimes in a controlled environment. Gene expression analyses indicate that some BrMYB transcription factor transcript levels are associated with temperature-dependent changes in GSL concentration, however this association varies between cultivar and tissue type. When compared to low temperature treatments, high temperature treatments increased total, aliphatic and indolic GSLs in a tissue and genotype specific manner. Gluconasturtiin (GNS), an aromatic GSL, concentration was inversely correlated with temperature with high temperature treatments resulting in 20% and 48% less GNS than low temperature treatments in JR and SQ roots, respectively. The indolic GSL, 1-methoxyglucobrassicin (1MGB) was the root GSL most elevated by increased temperature resulting in a nine-fold increase on average in both cultivars between the low and high temperature treatments. These results show promise for the use of temperature to enhance the health promoting properties of turnip as 1MGB has potent chemopreventive effects.
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    Insights into the hormonal regulation of nectar production and the biochemical characteristics of antimicrobial nectar proteins
    (2021-12) Schmitt, Anthony
    Nectar is a complex, carbohydrate rich solution that facilitates important plant-biotic relationships. Of the utmost importance is nectars ability to effectively attract and manipulate pollinators to maximize plant reproductive success while deterring unwanted consumers that could be deleterious to a plant’s fecundity. Therefore, the regulation of nectar secretion during times of peak pollinator activity, as well as other essential plant reproductive processes, is essential. Very little is known about the detailed mechanisms of nectar regulation and whether these mechanisms are conserved in the plant kingdom. Through the examination of null alleles of JA biosynthesis and response genes in Arabidopsis, this report finds that the octadecanoid pathway plays an important role in nectar production. The nectar-less JA synthesis mutant aos-2 showed no auxin response in nectaries, but both nectar production and the auxin response were restored upon exogenous JA and auxin treatment. Cumulatively, these observations strongly suggest an indispensable role for an octadecanoic acid- and auxin-dependent, but JA- and COI1-dispensible, pathway in regulating nectar production in Arabidopsis. Another important aspect of nectar biology is the chemical constituents of nectar droplets. While sugars are the predominant solutes of nectar, non-sugar solutes have been shown to provide additional levels of functionality to nectar. Some of these non-sugar solutes include amino acids, lipids, ions, secondary metabolites and proteins. Nectars generally secrete small arrays of proteins. Because nectar is a nutrient dense solution, it must be protected against non-mutualistic consumers such as deleterious communities of microorganisms. Nectar proteins generally exhibit direct and indirect antimicrobial activity and are thought to serve as a defense mechanism for nectar. To this end, I characterized non-specific lipid transfer proteins (nsLTPs) that are secreted into the nectar of Arabidopsis (AZI7), Cucurbita pepo (CpLTP1.1), and Brassica rapa (BrLTP2.1). Broadly, these nectar specific nsLTPs appear to have strong antimicrobial activity, particularly against pathogenic plant fungi, are very heat stable, and have some capacity to bind free fatty acids. Overall, these data improve our understanding of the hormonal regulation of nectar and takes the initial steps to gaining broad insight to the biological function of nectar proteins.
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    Look at the size of those pores! Variation in stomatal behavior in Brassica rapa suggests link to circadian clock
    (2021-07) Rquibi, Leila E.
    The circadian clock is an endogenous c. 24 hour timekeeping mechanism which regulates a number of plant processes. Stomata are leaf pores which open and close for water and gas exchange. In this study, I sought to examine whether stomatal behavior is influenced by the clock. To do so, I measured the stomatal aperture of 8 lines of Brassica rapa (B. rapa) with different circadian periods (the time it takes the clock to complete one full oscillation) 1 hour before dawn and 1 hour after dawn over three days. I hypothesized that plants with different periods would differ in mean stomatal aperture at each time point and the amount that aperture changed between the two timepoints. I found that the lines differed in their aperture and change in aperture, though those with similar period length did not necessarily demonstrate similar trends in aperture.
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    A Sucrose Transporter and Proper Hormone Response are Essential for Nectary Function in the Brassicaceae
    (2013-12) Klinkenberg, Peter
    Nectar is a reward presented by flowers to attract pollinators to facilitate fertilization. While much is known about the chemical make-up of nectar, little is known about the mechanisms of production and secretion of this pollinator attractant. SWEET9, a nectary enriched gene, was demonstrated to be vital for nectar production in two Brassicaceae species, Arabidopsis thaliana and Brassica rapa as determined by SWEET9pro::GUS histochemical staining and RT-PCR,. The Arabidopsis mutant atsweet9-3, produced no nectar and three independent mutants in B. rapa, (brsweet9-1, -2, and -3) similarly produced no nectar. All four mutants had normal nectary morphology. Transporter assays of SWEET9 expressed in Xenopus oocytes displayed sucrose uniport activity, suggesting a direct role in sugar export. To determine a potential mechanism for the regulation of SWEET9 expression, the plant hormone jasmonic acid (JA) was investigated because it was previously implicated in nectary function. Indeed, JA synthesis (aos-2 and dad1) and response (myb21-4) mutants displayed an absence of floral nectar, in addition to male-sterility. When treated with exogenous MeJA, aos-2 and dad1 mutants regained their nectar production and fertility, while the myb21-4 transcription factor mutant was insensitive to treatment. Significantly, SWEET9 expression was strongly decreased in the JA response mutant myb21-4, in addition to several other genes known to be important in nectary function. For example, all three JA mutants studied displayed decreased expression of PIN6, a nectary enriched gene required for proper auxin homeostasis in the nectaries of Arabidopsis. Additionally auxin response was lost in the JA synthesis mutant aos-2, suggesting an important hormonal crosstalk between JA and auxin. To further investigate the link between JA and the auxin response in nectaries, mutants with altered endogenous auxin levels were created. Mutants with decreased nectary auxin produced 50% less nectar than wild-type plants and had reduced auxin response. Cumulatively, these results identify SWEET9 as a sucrose transporter required for nectar production and that JA plays a major role in the regulation of nectary-specific genes and other hormonal pathways important for nectar production.