American Society of Naturalists

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“Belowground competition can influence the evolution of root traits”

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Sara M. Colom and Regina S. Baucom (Apr 2020)

Experimental evidence that belowground competition can influence the evolution of root traits!

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Excavating the story on how belowground competition influences root evolution

<i>Ipomoea purpurea</i> (left) and <i>I.&nbsp;hederacea</i> growing in competition in the field (summer 2017). Plants were trained on bamboo stakes at an angle to prevent aboveground competition.<br />(Credit: Regina Baucom)
Ipomoea purpurea (left) and I. hederacea growing in competition in the field (summer 2017). Plants were trained on bamboo stakes at an angle to prevent aboveground competition.
(Credit: Regina Baucom)

How does belowground root-root competition influence plant diversity and evolution? Roots provide structural support and likewise play a key role in nutrient and water acquisition from the soil. Roots are also key in mediating belowground plant-plant interactions. Despite these important ecological and functional roles, however, research on if and how belowground competition may influence the evolution of root traits remains relatively uncharted territory.

Research from Sara Colom and Regina Baucom in the Ecology and Evolutionary Biology Department at the University of Michigan fills this gap by addressing the potential that belowground competition acts as an agent of selection on root traits. They performed a series of greenhouse and field experiments at the Matthaei Botanical Gardens in Ann Arbor, Michigan, using two sister species of morning glories (Ipomoea purpurea and Ipomoea hederacea) as their model system.

Root sample from an <i>Ipomoea</i> plant excavated from the field for imaging and phenotyping (summer 2018).<br />(Credit: Sara Colom)
Root sample from an Ipomoea plant excavated from the field for imaging and phenotyping (summer 2018).
(Credit: Sara Colom)

They found that belowground root phenotypes varied between species—with I. hederacea being wider than I. purpurea, and I. purpurea exhibiting lateral roots that are more angled toward the soil surface than I. hederacea—but that there was still significant phenotypic overlap between them, such that they likely compete for the same resources when growing in close proximity. They also found evidence for genetic variation underlying root traits, and that competition between the two species negatively influenced fitness in field conditions. Importantly, they found that belowground competitive interactions between the two species altered the pattern of selection on root traits differently in each: competition with I. purpurea changed the pattern of selection on root angle in I. hederacea, and competitive interactions with I. hederacea changed the pattern of selection on root size in I. purpurea.

Overall, this research shows that belowground competition can have important implications on plant diversity and evolution, and highlights that research on the evolutionary ecology of root traits has long been overlooked.


Sara Colom (researcher) preparing for belowground root excavation, and untangling the aboveground biomass of <i>Ipomoea</i> plants growing in the field (summer 2017).<br />(Credit: Malia Santos)
Sara Colom (researcher) preparing for belowground root excavation, and untangling the aboveground biomass of Ipomoea plants growing in the field (summer 2017).
(Credit: Malia Santos)

Abstract

Although root traits play a critical role in mediating plant-plant interactions and resource acquisition from the soil environment, research examining if and how belowground competition can influence the evolution of root traits remains largely unexplored. Here we examine the potential that root traits may evolve as a target of selection from interspecific competition using Ipomoea purpurea and I hederacea, two closely related morning glory species that commonly co-occur in the United States as a model system. We show that belowground competitive interactions between the two species can alter the pattern of selection on root traits in each species. Specifically, competition with I purpurea changes the pattern of selection on root angle in I hederacea, and competitive interactions with I hederacea changes the pattern of selection on root size in I purpurea. However, we did not uncover evidence that intraspecific competition altered the pattern of selection on any root traits within I hederacea. Overall, our results suggest that belowground competition between closely related species can influence the phenotypic evolution of root traits in natural populations. Our findings provide a microevolutionary perspective of how competitive belowground interactions may impact plant fitness, potentially leading to patterns of plant community structure.