Understanding how plants respond to pathogens is critical for maintaining a robust food supply. Key to this response is the movement of RNA, a type of genetic information, from one part of the plant to another. However, detailed understanding of how RNA moves in plants is not well understood due to the small amounts of transiting RNAs. In addition, nearly all of our knowledge comes from research on short-lived plants and very little is known about long-lived trees due to the time it takes to grow and study them. The discovery of a novel virus-like RNA (called CYVaV) that infects virtually all citrus varieties provides an unparalleled opportunity to explore the biology of trees and the movement of RNA in plants. CYVaV accumulates to high levels in tree veins and moves in the absence of specific virus-produced "movement" proteins. Since CYVaV RNA must use the trees molecular machinery to move its RNA, it is an ideal system to study RNA movement. This project uses CYVaV to develop technologies to understand the movement of RNAs in plants. In addition, CYVaV may enable researchers to introduce genetic elements in trees. An important broader impact is the possibility of using CYVaV as a vehicle to combat the bacterial disease Citrus Greening by targeting the bacteria and the insect depositing the bacteria. This could help save the US citrus industry from Citrus Greening.

Recent revelations that 50% of the companion cell (CC) transcriptome is moving long distances through sieve elements (SE) have elicited numerous questions concerning how and why this migration of mRNAs is occurring. Answering key questions on intracellular and intercellular movement of mRNAs requires a model system consisting of an abundant mobile RNA, whose location can be traced in living tissue under different cellular conditions. The recently discovered Citrus yellow vein associated virus (CYVaV) is a highly unusual virus-like RNA that traffics bi-directionally through the phloem of all commercial varieties of citrus and laboratory host Nicotiana benthamiana in the absence of encoded movement proteins (MPs), coat protein or silencing suppressors. This project develops CYVaV as a model for transiting mRNAs using MS2 CP and λN22 systems and as a VIGS vector to study effects of altered gene expression in citrus phloem. CYVaV causes few if any symptoms on citrus although it accumulates to levels detectable on ethidium-stained gels in infected plants. This should allow for high level expression of small RNAs, with the initial goal of down regulating CC callose synthase, which causes callose deposition in SE during stress or pathogen invasion. Since CYVaV should have no host-MP specific incompatibility, it will also be examined for an extended host range in other woody plants and vines, which currently have no useable vectors. By studying CYVaV movement and ability to serve as a VIGS vector, one broader impact of this work is using CYVaV to block the devastating effects of Citrus Greening.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

National Science Foundation (NSF)
Division of Integrative Organismal Systems (IOS)
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Gerald Schoenknecht
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University of Maryland College Park
College Park
United States
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