Plant Origins: What the Earliest Land Plants Looked Like Millions of Years Ago? [STUDY]

A team of scientists determined what the first land plants might have looked like millions of years ago by studying the mechanisms responsible for branching.

Despite fundamentally different growth patterns, their research discovered a common mechanism for branching in vascular plants.

Dr. Jill Harrison of Bristol's School of Biological Sciences explained that the dominant flowering plant group has a variety of shapes, and gardeners will be familiar with "pinching out" plants' shoot tips to stimulate side branch growth, resulting in a bushier overall form.

Earth's first plants likely to have been branched

(Photo : Anna Moneymaker/Getty Images)

Other vascular plants, unlike flowering plants, branch by splitting the shoot apex into two during growth, a process known as a dichotomy, as per ScienceDaily.

Lycophytes preserve the ancestral pattern of dichotomous branching as an ancient vascular plant lineage that formed coal seams during the Carboniferous era.

Researchers at the University of Bristol discovered that dichotomy is regulated by short-range auxin transport and coordinated in different parts of the plant by long-range auxin transport using surgical experiments in a lycophyte.

The discovery that both flowering plant and lycophyte branching are regulated by auxin transport, published in Development, suggests that similar mechanisms existed in the earliest vascular plants around 420 million years ago.

They inferred what the first land plants looked like around 480 million years ago by combining these findings with those made in the non-vascular, non-branching moss group.

Dr. Harrison's lab previously disrupted auxin transport in a moss, causing it to branch in a manner similar to the earliest branching fossils.

These findings suggested that the earliest land plants were branched, and that branching disappeared during the evolution of non-vascular mosses.

"The greening of the land by plants paved the way for all terrestrial life to evolve because it provided food for animals and oxygen to breathe, and branching was a key innovation in the radiation of land plants," Dr. Jill Harrison explained.

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Origin of plants

Plants (land plants, embryophytes) are monophyletic, descended from a freshwater ancestor that, if still alive, would be classified as charophycean green algae, as per PNAS.

Plants, but not charophyceans, have a life cycle that involves the alternation of two morphologically distinct developmentally associated bodies, sporophyte and gametophyte.

Body plan evolution in plants has resulted in fundamental changes in both gametophyte and sporophyte forms, as well as the evolutionary origin of regulatory systems, which generate different body plans in sporophytes and gametophytes of the same species.

Based on molecular phylogenetic information, the comparative analysis identifies fundamental body plan features that originated during charophycean algae radiation and were inherited by plants.

Cellulosic cell wall, multicellular body, cytokinetic phragmoplast, plasmodesmata, apical meristematic cell, apical cell proliferation (branching), three-dimensional tissues, asymmetric cell division, cell specialization capacity, zygote retention, and placenta are examples of these in probable evolutionary order.

The following body plan features have been linked to the origin of plants: multicellular sporophyte body, histogenetic apical meristem in the gametophyte body, and tissue differentiation capacity in both sporophyte and gametophyte.

The origin of a well-defined sporophytic apical stem cell and a system for its proliferation, which is linked to organ production and branching capacity, occurred sometime between the divergence of modern bryophytes and vascular plant lineages.

Later, roots and their meristems, as well as a multilayered tunica-corpus shoot apical meristem, emerged.

Regulatory genes affecting shoot meristems, which have been detected by analysis of higher plant mutants, may be relevant to understanding early plant body plan transitions.

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