Decreased stomatal density in poplar leads to improved water-use efficiency and drought tolerance

Abstract

Regulation of stomatal density is a topic that has recently drawn a lot of attention worldwide. Stomata serve to control CO2 influx and water efflux from leaves and play a direct role in the global water cycle. This project researched genes that are able to increase or decrease leaf stomatal numbers when mutated or manipulated in a number of model species. The focus of this research was to generate highly drought tolerant trees with increased water conservation abilities in order to alleviate pressures placed on current water supplies. The goal of this project was to decrease stomatal numbers in poplar as a method of altering water usage and to achieve greater water-use efficiency. In Arabidopsis thaliana, epidermal patterning factor1 (EPF1) is involved in stomatal complex development and serves to control leaf stomatal number and spatial distribution. The most prominent role of EPF1 involves the enforcement of the one-cell-spacing rule that prevents multiple stomata from developing adjacent to each other without an epidermal or pavement cell for separation. Overexpression of AtEPF1 in Arabidopsis and poplar resulted in reduced stomatal density, a characteristic recently shown to be directly related to water-use efficiency and indirectly to drought tolerance. These experiments provided the background necessary to identify and isolate the poplar homolog of AtEPF1; PtaEPF1. PtaEPF1 was overexpressed in Arabidopsis and poplar and the resulting data indicated that both genes are able to decrease stomatal density and limited leaf water exodus from the leaves when overexpressed; however the transgenic plants generated in this study showed several abnormal leaf phenotypes in addition to decreased stomatal densities. Although no significant decreases in overall leaf biomass were detected in these plants, further decreases in stomatal density were likely to negatively influence CO2 assimilation rates. This study compiled currently available data regarding responses of plants to climate change, reforestation efforts, breeding technologies, and unconventional methods of generating abiotic tolerance to ensure this research was unique and provided valuable information to the research community. Leaf stomatal density data were collected from a selection of several fine hardwood tree species, Arabidopsis stomatal density mutants, and transgenics poplar generated during this analysis. This project began with a generalized idea regarding climate change and proceeded to narrow in focus from the analysis of numerous tree species to a single species outlook. Although often found in riparian areas, most Populus spp. are capable of tolerating a wide range of environments. Therefore, based on the data generated, the proposed improvements in abiotic tolerance would be most beneficial for a species unaccustomed to variable environmental conditions. Manipulation of stomatal density may allow endangered tree populations to tolerate the highly variable future climate events predicted to occur.^

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