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The McCully Plant Biology Lecture 2022

April 8, 2022 at 2:30 AM to 3:30 AM

Location:210 Tory Building

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Title: Understanding Complex Trait Evolution: A Case Study with C4 photosynthesis.

Rowan F Sage, Department of Ecology and Evolutionary Biology, University of Toronto

Abstract: C4 photosynthesis independently evolved over 65 times in recent geological time, making it one of the most convergent of evolutionary phenomena in the living world. This is all the more remarkable considering C4 photosynthesis is a complex trait involving evolutionary modification of hundreds to thousands of genes, and large sectors of leaf physiology and anatomy. Understanding why and how C4 photosynthesis evolved could therefore provide insights into how complex traits evolve in the living world and do so repeatedly. In this presentation, I will discuss recent results from our group examining the C4 evolutionary process.  Using C3, C4 and C3-C4 intermediate species from over a dozen independent lineages, we demonstrate that C4 photosynthesis evolved in hot, typically dry and/or salinized environments where the inhibitory process of photorespiration is great in C3 plants. The initial steps in C4 evolution involved the formation of mechanisms to scavenge photorespired CO2 and ammonia in the leaves of C3 plants. This involved localizing the critical photorespiratory enzyme glycine decarboxylase into an inner tissue region of the leaf, typically the bundle sheath cells. By doing so, glycine produced in photorespiration must be transported into the bundle sheath for metabolism by GDC, releasing photorespired CO2 and ammonia. Recovery of photorespired ammonia appears to be the critical early driver of C4 evolution, in that initial upregulation of the C4 pathway enzymes facilitate the recycling of the nitrogen in the ammonia produced by GDC back to mesophyll tissues. In addition to the enhancement of C4 pathway enzymes, there are corresponding changes in plasmodesmatal density at the BS boundary to accelerate metabolite flux between the BS and mesophyll cells.  This initiates the anatomical changes that lead to the C4 Kranz anatomy. In short, complex trait evolution as demonstrated by C4 evolution occurs as a series of small steps, each one facilitating subsequent steps. In the case of C4 evolution, a need to scavenge photorespiratory products initiates the process, with the end result being a novel structure and function that leads to superior photosynthetic performance in higher plants.

McCully Lecture _Sage_2022_Poster