In plants, both water and pathogens move through the same transport tissue (xylem) that consists of hollow, pipe-like cells (conduits). Because water flow along this complex network of conduits occurs under tension, bubbles can be produced that eliminate water transport. The xylem conduits are of fixed length and are to some extent interconnected, but the extent of the interconnectedness has not been evaluated. The fixed length and interconnections may be important for limiting bubble expansion or pathogen transport. The investigators will use High Resolution Computed Tomography (a kind of CAT scan) to reveal the three dimensional structure and connections of the xylem conduits, and from these data resolve how the network design may facilitate water transport and limit pathogen movement. The investigators will use grapevine (Vitis species) and the xylem-limited bacterial pathogen Xylella fastidiosa as a model system to examine the influence of xylem structure on the ability of pathogens and embolisms to move through the network. Reconstructions of the vessel network will be used to mathematically model the 3D arrangement of the hydraulic network and its transport properties. Graphical user interfaces will greatly simplify both the analysis of imaging data and the use of network simulation software. The user interface will be web-based to make it readily accessible to students, teachers, or plant scientists anywhere in the world for use in developing their own three-dimensional xylem structural models, or analyzing characteristics of xylem networks particular to their interests. Model predictions of system hydraulics and pathogen movement will be compared with experimental data on grapevines. The results will significantly enhance our understanding of how system level properties of the xylem network influence plant water use in the face of biotic and abiotic stresses, and may reveal novel design features related to disease resistance. The project brings together a novel combination of physics and plant biology to apply the advanced technology of CAT scans, used for example to evaluate blood flow in brains, to the perplexing problems of drought and disease effects on plant and crop growth. This approach may be transformational in producing detailed three-dimensional pictures that have been increasingly exploited in medicine but not in plant biology, and in training scientists in the application of CAT scan technology to plants.
Both water and disease-causing pathogens traffic the xylem, a system of conduits in plants that connects virtually all tissues. The structure of the xylem vessel is widely considered to be a compromise between the efficiency of water transport and safety from dysfunction via embolism. A few large and well-connected vessels would move water easier, but would also allow pathogens better access to systemic infection. Because water is transported through the xylem conduits under tension (negative pressure), it is subject to cavitation and embolism, filling the conduit with gas and eliminating the capacity to transport water. The tension is increased under drought conditions, and so too is the extent of cavitation, reducing water flow and intensifying the effects of drought on plant growth. Much greater attention has been focused on design features related to safety from cavitation and embolism rather than those related to xylem-limited pathogens. Almost no studies have incorporated information about the three-dimensional structure of the xylem network and its effect on xylem function. In fact, understanding of the three dimensional network design is limited to interpretations from a few sets of serial sections of plant stems. Our understanding of the structure of the conduit network has been limited by its inaccessibility to study. High Resolution Computed Tomography (HRCT), a higher energy CAT scan, was used to produce images of the internal structure of grapevine (Vitis spp.) stems that revealed new details and structures. We discovered that some large vessels are connected by multicellular 'bridges' that increase the opportunities for both pathogen and embolism spread; and, that vessels are connected with each other when they are within a fixed distance, 13 microns, and not connected when adjacent but separated by more than 13 microns. The most striking discovery was that a consequence of the variation in conduit size and connectivity is water flow is reversed in some conduits despite the fact that most water is moving up from roots to leaves. This physical consequence of the 3D organization may be important in helping pathogens move around the plant to establish systemic infections. An example of the flows in individual xylem vessels as calculated in our simulations is given in Figure 1. Red is upward flow and blue is downward flow. The second important discovery was that we could do HRCT experiments with intact living plants as they dried down or recovered from dry conditions. In these experiments, we were able to visualize the spread of embolisms (Fig. 2) and watch the plant repair the embolisms by refilling the vessels with water (Fig. 3). The development of the ability to visualize embolism spread and the details of refilling should enable rapid progress in understanding how both of these important processes work in plants.
