Diverse branching forms have evolved multiple times across the tree of life to facilitate resource acquisition and exchange with the environment. As an ancient vascular plant lineage that formed coal seams during the Carboniferous period, lycophytes preserve the ancestral pattern of dichotomous branching. Using surgical experiments in a lycophyte, University of Bristol scientists have discovered that dichotomy is regulated by short range auxin transport and coordinated in different parts of the plant by long range auxin transport.
Selaginella bryopteris in Royal Botanical Garden of Madrid, Spain. Image credit: Luis Fernández García / CC BY-SA 2.1 ES.
Lycophytes are a key group to resolve questions about the evolution of sporophyte branching in land plants.
They originated over 420 million years ago, and some species exhibit ancestral branching architectures, closely resembling their fossil relatives.
Lycophytes are the sister group to euphyllophytes and are hence ideally placed in the plant tree of life to identify vascular plant homologies.
“Diverse shapes abound in the dominant flowering plant group, and gardeners will be familiar with ‘pinching out’ plants’ shoot tips to stimulate side branch growth, leading to a bushier overall form,” said Dr. Jill Harrison, a researcher at the University of Bristol.
“However, unlike flowering plants, other vascular plants branch by splitting the shoot apex into two during growth, a process known as dichotomy.”
Through analyses of dichotomous branching in a lycophyte species called Selaginella kraussiana, Dr. Harrison and colleagues discovered that dichotomy is regulated by short range auxin transport and coordinated in different parts of the plant by long range auxin transport.
The findings that both flowering plant and lycophyte branching are regulated by auxin transport imply that similar mechanisms were present in the earliest vascular plants around 420 million years ago.
By combining these findings with discoveries made in the non-vascular, non-branching moss group, we can infer what the first land plants looked like around 480 million years ago: they were likely branched, and that branching was lost during the evolution of non-vascular mosses.
“The greening of the land by plants paved the way for all terrestrial life to evolve as it provided food for animals and oxygen to breathe, and branching was a key innovation in the radiation of land plants,” Dr. Harrison said.
“Our work implies that branching evolved earlier than thought, which is an important evolutionary conclusion.”
“Aside from that, the fact that we have shown that plants that are so distantly related use the same genetic mechanisms to regulate branching brings great potential to transfer knowledge in engineering plant shape to improve future productivity and yield.”
The team’s paper was published in the journal Development.
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Victoria M.R. Spencer et al. 2023. Diverse branching forms regulated by a core auxin transport mechanism in plants. Development 150 (6): dev201209; doi: 10.1242/dev.201209
Source : Breaking Science News