Donald Boyd Fisher - US grants

Phloem is the conducting system for distribution of photosynthetic assimilates in vascular plants and also a pathway by which systemic viral infection can be spread. The movement of solutes out of phloem conducting cells is a crucial regulation point for the distribution of phloem-mobile substances in the plant. Little is known, however, about the mechanisms or controls involved. An experimental approach to understanding these mechanisms would be to introduce labelled compounds that have a range of solute sizes and chemical structures into the phloem and follow the movement of these compounds. However, this has been precluded because the phloem cells are pressurized and present means of introducing material into the phloem also produces significant artifacts. An aphid naturally removes sugars from phloem by selectively inserting a stylet into the conducting cell. The exploratory work proposed here would attempt to overcome the problem of introduced artifacts by developing a method for solute injection directly into the phloem conducting cells via severed aphid stylets, an approach suggested by earlier measurements of turgor pressure in phloem conducting cells. An injection system will be constructed that will utilize thermal expansion to generate pressure. Precise pressure control in this injection system should be possible with a precisely-regulated thermoelectric device that has a pressure transducer incorporated into the injector. Development of this system should allow injection of tracer material into phloem cells by a slight and precisely controlled over pressurization.

Plant growth depends on the production of organic materials, mostly sugars and amino acids, by photosynthesis. At first consideration, it seems reasonable to suppose that the rate of plant growth should depend directly on the concentration of these substrates: the higher their concentration, the faster the rate of growth. However, this supposition is not supported by available evidence. Young seeds, especially, seem to maintain preset growth rates that show little relation to sugar and amino acid concentrations. Instead, growth rates appear to be controlled by the transport of these substances from conducting cells into the growing embryo, a distance of less than a millimeter. Developing wheat grains offer advantages for studying this transport, since the pathway is well- defined and growth rates and concentrations can be measured at key points along the path. These investigations will develop a better understanding of transport along this pathway and, consequently, of how the growth rate of seeds might be controlled.//

Despite the central role of sinks in determining photosynthetic assimilate partitioning in plant growth, very little is known about the control of assimilate import into a sink tissue. The overal goal of this research is to understand how import by a sink is controlled. Because the developing wheat grain offers significant experimental advantages, Dr. Fisher's work has focused on that system. However, findings from this research have important implications for all sinks in which unloading of the sieve element/companion cell complex follows a symplastic pathway. All of the transport steps, from within the sieve tubes to the endosperm cavity, are passive, reversible and relatively nonspecific. Assimilate movement out of the grain phloem into surrounding parenchyma cells is accompanied by the largest turgor and concentration differences over any step of the source-to-sink pathway. Because solute movement out of the sieve tubes occurs via pressure-driven flow, while grain growth rate is constant, flow rate must be inversely related to the pressure and concentration gradients across this part of the pathway. This, and the high resistance involved, implicate this step as an important control point for assimilate import into the grain. This research will focus more closely on this step of assimilate import. Important questions remain to be addressed in developing wheat grains, and they will continue to be the main object of the work. However, to test whether import might be controlled at this step in other sinks, some work with growing root tips will also be initiated. The main questions to be addressed in growing roots are whether sieve tube unloadiong is reversible, the pressure and concentraiton gradients involved, and their relationship to root growth rate.