Modeling the root protophloem unloading network

Grayson Pierce Ostermeyer

doi:10.7273/000004221

Phloem is the vascular system that carries out long-distance transport of important biological compounds in all higher plants. During photosynthesis, light-exposed green tissues synthesize sugar molecules that can be broken down in other regions of the plant to drive processes like functional metabolism, organ growth, and nutrient storage. The structural conduit that forms a channel linking photosynthetic source tissues to recipient sink tissues is composed of interconnected sieve tube elements. Flow of energy-rich, carbon-based molecules through this tube is a prominent research focus of plant scientists because sink organs include many economically important agricultural products, such as fruits and tubers. The terminal segment of phloem in plant roots, the protophloem, acts as a platform through which assimilates unload from the phloem vasculature to supply the actively growing root tip with biochemical fuel. Cellular organization of the protophloem tissue exhibits predictable patterns in plant roots that are characteristic in each species. Outflow of vascular cargo into adjacent cells of the root protophloem occurs through sub-microscopic pores called plasmodesmata that link the internal contents of nearly every cell in the plant body. Plasmodesmal morphology has a considerable effect on the symplasmic flow rate and regulates the unloading capacity of roots. Previous research efforts have described variations in the shape of plasmodesmata as simple or branched based on the number of intercellular connections formed by the pore structure. Recently, a novel plasmodesma type was discovered at the interface between root protophloem sieve elements and phloem pole pericycle cells that is shaped like a funnel, possibly endowing this pore type with enhanced assimilate unloading potential. Our research efforts began by refining a sample preservation protocol for angiosperm roots and illuminating species-specific variations in the tissue structure of protophloem using transmission electron microscopy. We then proceeded to model flow dynamics in plasmodesmata using electron tomography and finite element analysis, producing the first 3D reconstructions of plasmodesmata in the process. Our imaging workflow and flow analysis approach may be useful in experiments aimed at manipulating plasmodesma geometry to accommodate greater flux of assimilates into sink crops, thereby increasing food availability for a growing world population.

Modeling the root protophloem unloading network

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