Many plants that inhabit the arid regions of the world commonly survive by long-term storage of water in various parts of the plants. These plants are typically described as 'succulent'. Everyone knows a succulent plant when they see one, and their bizarre forms have made them popular with horticulturists and collectors. In spite of their ubiquity, scientists know very little about how extreme succulence has evolved in plants. In this project, the evolution of succulence will be investigated in a diverse lineage of ~2,000 flowering plants that include the cacti. Molecular data will be collected from ~500 species to reconstruct evolutionary relationships within the group. 'Succulence' will be characterized in quantitative terms using a newly devised measurement. Within this new framework a series of hypotheses will be tested regarding how and why succulence has evolved multiple times in plants.
The project will provide novel insights into the mechanisms that have promoted drought tolerance in plants, which may help predict responses of different plant lineages to future climate change. The project will provide extensive training opportunities for post-doctoral, graduate, and undergraduate scholars, including female and minority students. The project also includes a significant public outreach effort where the PI and students will create a series of short animated films about plant evolution. These films will be designed for individuals with a high-school level biology background, and will be broadcast on the extremely popular website 'creaturecast.org'.
Many plants inhabiting the arid regions of the world exhibit a common ecological strategy for survival that involves long-term storage of water in various tissues. These plants are typically described as ‘succulent’. Everyone knows a succulent plant when they see one, and their bizarre forms have made them popular with horticulturists and collectors. In spite of their ubiquity, scientists know very little about how extreme succulence has evolved in plants. In this project, the PI and her team investigated the evolution of the succulent syndrome in the Portulacineae (Caryophyllales), a diverse lineage of ~ 2,000 flowering plants that include the cacti. They built a large phylogenetic tree for the group, using DNA sequence data, which resulted in an improved understanding of evolutionary relationships among taxa the history of the group’s diversification. They discovered that the majority of cactus species are relatively ‘young’ in an evolutionary sense, with most diversification occurring during the last 5 million years and correlated with global patterns of increased cooling and aridification during that time. The team also characterized plant succulence in quantitative terms across a large number of species for the first time, using a newly devised metric. With this new framework, they made an important discovery: the degree of succulence of a plant leaf is constrained by the density of their vein systems, which are the important channels through which water, nutrients, and carbohydrates flow into and out of leaves. In the study group, species were able to overcome this limitation via the evolution of a new kind of venation system- what they term "3D venation". Species with 3D venation were able to store significantly more water in their leaves. Apparently, 3D venation may be a relatively simple innovation for plants, as the trait evolved 8-10 times within the single lineage investigated here. These results could aid other researchers who are working to genetically engineer a special form of water-efficient photosynthesis (CAM metabolism) into poplar; CAM requires moderate levels of succulence, and the genetic controls on succulence have so far been elusive- but a good deal is known regarding the genetic mechanisms controlling vein patterning. This grant has so far supported the publication of 12 scientific manuscripts, with several more still in preparation or review. These funds have also supported a PhD graduate student and two female post-doctoral research associates, and numerous Brown University undergraduates, including minorities, have been involved in this research through independent study as well as integration with one of the courses taught by the PI. The project also included a significant public outreach effort, where the PI has collaborated with a professional artist to create two short animated films about plant evolution. These are issued under a creative commons license and can be used freely for any educational purpose, and will soon be ready for broadcast on the extremely popular website ‘creaturecast.org’.
