Our group has several broad research interests, including plant-microbe and plant-animal interactions, as well as protein trafficking in plant cells. Each of these broad interests are currently being pursued through several integrated projects revolving around nectaries and nectar.
Flowering plants attract mutualistic animals by offering a reward of nectar. Specifically, floral nectar is produced to attract pollinators, whereas extrafloral nectar mediates indirect defenses (e.g. ant-plant interactions). There is a strong correlation between nectar quality (i.e. volume and composition) and the efficiency of the resulting plant-animal mutualism. Significantly, there is also a clear relationship between nectary form (the glands that produce nectar, Fig 1) and nectar quality. Remarkably, the molecular events involved in the development of nectaries, as well as the synthesis and secretion of the nectar itself, are poorly understood. Indeed, only a few genes have been reported to directly affect the de novo production or quality of floral nectar.
Comparative genomics of nectaries and nectar
According to our recent findings, the central components of active nectar secretion might be conserved across species and nectary type (floral vs. extrafloral). We hypothesize that nectar synthesis and secretion follows a central mechanism that is conserved among the core eudicots and applies to both floral and extrafloral nectar, and that this mechanism can be differentially regulated in different types of tissues, or species, in order to meet the physiological and ecological needs of the respective species. To test this hypothesis,the long-term goal of our group is to elucidate the genetic and physiological mechanisms that underlie nectary maturation and active nectar secretion, and to study the impact of select nectar components on specific plant-animal and plant-microbe interactions. In order to elucidate the conserved elements of these mechanisms we are applying a broad, comparative approach to the study of floral and extrafloral nectar/ies in a core group of dicotyledonous species.
Determining the molecular basis of nectar production can have broad implications, ranging from understanding the co-evolution of plant-animal interactions to increasing yields in multiple crop species, as well as targeted improvements in apiculture. For example, nearly 90% of flowering plant species produce nectar as a means to attract pollinators, including one-third of all crop species. U.S. pollinator-dependent crops have an estimated annual value of $25 billion.
Development of cultivars with increased nectar production
In a separate project we are developing crop plants with high levels of floral nectar production. An increase in nectar production results in a concomitant increase in pollinator visitation, pollination efficiency, and yield, even for highly selfing plant species. For example, pennycress is an emerging winter cover and seed crop and is one of the first cultivated plants to flower in Minnesota (mid-April to early May). Our hypothesis is that cultivated pennycress may serve as an important source of nutrition for honeybees and other pollinators in the spring when other plants are not yet flowering at an appreciable level in Minnesota. In particular, the development pennycress plants with enhanced nectar production would provide an important ecosystem service. Pollinator nutrition has become an area of intense research as a means to stem declining pollinator numbers, particularly for honey bees. Cultivars with stably enhanced levels of nectar production are now being developed and will be evaluated for their ability to impact pollinator visitation and yield.
Lin I, Chen L-Q, Sosso D, Gase K, Kim S-G, Kessler D, Klinkenberg PM, Gorder MK, Qu X-Q, Hou B-H, Carter CJ, Baldwin IT, Frommer WB (2014) Nectar secretion requires sucrose phosphate synthases and the sugar transporter SWEET9, Nature, 508: 546-549
Bender RL*, Fekete ML*, Klinkenberg PM, Hampton MW, Bauer B, Malecha M, Lindgren K, Maki J, Perera MADN, Nikolau BJ, Carter CJ (2013) PIN6 is required for nectary auxin response and short stamen development, Plant Journal, 74: 893-904
Bender R, Klinkenberg P, Jiang Z, Bauer B, Karypis G, Nguyen N, Perera MADN, Nikolau BJ, Carter CJ (2012) Functional genomics of nectar production in the Brassicaceae. FLORA, 7: 491-496
Xu WW, Carter C (2010) Parallel multiplicity and error discovery rate (EDR) in microarray experiments. BMC Bioinformatics, 11: 465.
Hampton M, Xu WW, Kram BW, Chambers E, Ehrnriter J, Gralewski JH, Joyal T, Carter CJ (2010) Identification of differential gene expression in Brassica rapa nectaries through expressed sequence tag analysis. PLoS ONE, 5: e8782.
Agee AE, Surpin M, Sohn EJ, Girke T, Rey AR, Kram BW, Carter C, Wentzell AM, Kliebenstein DJ, Jin HC, Park OK, Jin H, Hicks GR, Raikhel N (2010) MODIFIED VACUOLE PHENOTYPE1 is an Arabidopsis myrosinase-associated protein involved in endomembrane protein trafficking. Plant Physiology, 152:120-32
Ruhlmann JM, Kram BW, Carter CJ (2010) CELL WALL INVERTASE 4 is required for nectar production in Arabidopsis. Journal of Experimental Botany, 61: 395-404
Kram BW, Carter CJ (2009) Arabidopsis thaliana as a model for functional nectary analysis. Sexual Plant Reproduction, 22: 235-246
Kram BW*, Xu WW*, Carter CJ (2009) Uncovering the Arabidopsis thaliana nectary transcriptome: investigation of differential gene expression in floral nectariferous tissues. BMC Plant Biology, 9: 92 (*equal contribution)
Kram BW, Bainbridge EA, Perera A, Carter C (2008) Identification, cloning and characterization of a GDSL lipase secreted into the nectar of Jacaranda mimosifolia. Plant Molecular Biology, 68:173-183.
Sohn EJ, Rojas-Pierce M, Pan S, Carter C, Serrano-Mislata A, Madueño F, Rojo E, Surpin M, Raikhel NV (2007) The shoot meristem identity gene TFL1 is involved in flower development and trafficking to the protein storage vacuole. Proc Natl Acad Sci USA,104: 18801-18806.
Carter C, Healy R, O’Tool NM, Naqvi SM, Ren G, Park S, Beattie GA, Horner HT, Thornburg RW (2007) Tobacco nectaries express a novel NADPH oxidase that is implicated in the defense of floral reproductive tissues against microorganisms. Plant Physiology,143: 389-399.
Carter C, Shafir S, Vaknin L, Palmer RG, Thornburg R (2006) A novel role for proline in plant floral nectars. Naturwissenschaften,
Surpin M*, Rojas-Pierce M*, Carter C*, Hicks GR*,Vasquez J, Raikhel NV (2005) The power of chemical genomics to study the link between endomembrane system components and gravitropic response. Proc Natl Acad Sci USA, 102: 4902-4907. (*equal contribution)
Rojo E, Martin R, Carter C, Zouhar J, Pan S, Plotnikova J, Jin H, Paneque M, Sanchez-Serrano J-J, Ausubel FM, Baker B, Raikhel NV (2004) VPEg exhibits a caspase-like activity that contributes to defense against pathogens. Current Biology, 14: 1897-1906.
Carter CJ*, Pan S*, Zouhar J*, Avila EL, Girke T, Raikhel NV (2004) The vegetative vacuole proteome of Arabidopsis thaliana reveals predicted and unexpected proteins. Plant Cell 16: 3285-3303. (*equal contribution)
Carter CJ*, Bednarek SY*, Raikhel NV (2004) Membrane trafficking in plants: new discoveries and approaches. Current Opinion in Plant Biology, 7: 701-707. (*equal contribution)
Carter C, Thornburg RW (2004) Is the nectar redox cycle a floral defense against microbial attack? Trends in Plant Science, 9: 320-324.
Carter C, Thornburg RW (2004) Nectarin III is a bifunctional enzyme with monodehydroascorbate reductase and carbonic anhydrase activities. Plant Molecular Biology, 54: 415–425.