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United States Department of Agriculture

Agricultural Research Service

Research Project: The Phloem As An Inter-Organ Communication System: a Role in Epigenetic Regulation of Nutrient Acquisition and Utilization

Location: Plant, Soil and Nutrition Research

2013 Annual Report


1a.Objectives (from AD-416):
Available nutrient levels within many soils impose limits on overall plant productivity and yield. For food security, future agricultural practices will need to incorporate important traits for nutrient use efficiency to generate elite high-yielding crops.

Although it has long been known that nutrient acquisition by the roots is controlled by root-to-shoot and shoot-to-root signaling systems, currently little is known regarding the molecular components that function in these pathways. Recently, analysis of phloem sap collected from plants subjected to nutrient deprivation, identified a number of small RNA species that may mediate in the epigenetic regulation of ion acquisition by the roots.


1b.Approach (from AD-416):
To further explore this possibility, we plan to utilize the cucumber genome, in combination with our ability to collect analytical quantities of phloem sap from this species, to identify and characterize potential siRNA, miRNA and mRNA molecules involved in regulating nutrient acquisition by the root system. Next, we will identify the phloem proteins that deliver such candidate RNA species to the root, as well as the cells targeted for regulation. Finally, we plan to explore the gene regulatory network(s) that operates within these target tissues to allow plants to adapt to the ever changing nutrient availability within the soil. These studies will provide insights into the mechanisms that evolved to allow plants to coordinate nutrient acquisition by the root with developmental and physiological processes occurring within the above-ground regions of the plant. As an important outcome, this knowledge would contribute towards the engineering of crops having enhanced nutrient use efficiency.


3.Progress Report:

In 2013, our collaborators at UC Davis were able to collect the phloem sap (phloem is the system that transports sugars from photosynthesizing leaves to growing roots and shoots) from cucumber plants that were subjected to phosphorous deficiency. We isolated mRNAs (nucleic acids made from gene sequences that are the template for the proteins encoded by the genes) and microRNAs (small RNA molecules that do not code for proteins and regulate expression of genes) during the onset (first 12 hours) of plant P deficiency. Our results indicated the presence of highly tissue-specific responses to P deficiency. Interestingly, we identified more than 100 mRNAs and 28 miRNAs that were upregulated in phloem sap within 12 h of imposing a Pi deficiency condition at the root level. The majority of these RNAs are important regulators of stress response, development, signal transduction, sucrose and phosphate metabolism, with some encoding transporters and transcription factors. These RNA molecules are candidate P signaling molecules transported from the shoot to the roots indicating the plant is experiencing P deficiency. This in turn triggers a cascade of events (changes in root architecture, increased expression of P uptake genes, etc) that the plant uses to increase P acquisition. The ultimate goal of this research is to identify and understand the operation of the gene regulatory network that operates to control P acquisition and homeostasis which will offer important insights into the mechanisms plants use to adapt to the ever changing Pi availability in the soil.


Last Modified: 4/18/2014
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