Why many different plant seedlings look like salad cress

The first time my husband met my research subject the legume Medicago truncatula he was presented with a row of seedlings growing on an agar plate (see photos below). His response to this was “Ahh, you work on cress!”

Medicago truncatula seedlings (left) and Lepidium sativum seedlings (right) both have two cotyledons that are similar in shape. Image of Lepidium sativum by Malte taken from wikimedia commons.

Medicago truncatula seedlings (left) and Lepidium sativum seedlings (right) both have two cotyledons (first leaves) that are similar in shape. Image of Lepidium sativum by Optograph distributed under a CC BY-SA 3.0 licence.

 My husband is not a biologist or gardener so his mistake is understandable. Although M. truncatula seedlings look very similar to garden cress (Lepidium sativum) often found in salads they are not closely related. Cress is not a legume and instead belongs to the Brassicaceae family alongside oil seed rape, cabbage and the model plant Arabidopsis thaliana (also known as thale cress). The similarities in appearence between M. truncatula and garden cress are short lived. Older plants of the two species look like this:

Older M. truncatula and L. sativum plants look rather different. From these images it is possible to see that their leaves are a different shape with M. truncatula being trifoliate like clover. The flowers are also different in structure and colour. Images by Ninjatacoshell (left)  and Malte (right) from Wikimedia Commons

Older M. truncatula (left) and L. sativum (right) plants look rather different. From these images it is possible to see that their leaves are a different shape with M. truncatula having trifoliate leaves like clover. The flowers are also different in structure and colour. Images by Ninjatacoshell (left)
and Malte (right) distributed under CC BY-SA 3.0 licences.

Both species have grown additional leaves and stems to the single stem and two small leaves (the cotyledons) found on the young seedlings. The additional leaves look rather different to the cotyledons. The cotyledons are present in the embryo and are the store of nutrients within the seed. The nutrients are required for early development of the embryo until the plant emerges above the soil and can start harnessing the sun’s energy using photosynthesis. The cotyledons often emerge and provide the plant with energy through photosynthesis until the “true” leaves develop later. To find out more about the development of a typical dicot embryo into a young plant click this link.

The number of cotyledons has been used to classify the flowering plants (angiosperms) into two groups: the dicots (two cotyledons) and the monocots (one cotyledon). Although the monocots are a monopyletic group (monophyletic meaning a group that encompasses a species and all it descendants), the dicots are not so it is not the most useful term to use. The stereotypical dicots belong to the monophyletic eudicot clade. This clade is by far the largest of all the flowering plants accounting for 75% of the species including M. truncatula, salad cress and even trees such as oak.

Plant evolutionary tree. The flowering plant (angiosperms) diverged from the gymnosperms (e.g. pine, fir) over 200 million years ago. The "basal angiosperms" Amborellaceae, Nymphaeaceae and Austrobasileyales  diverged first followed by the Magnoliid clade before the Eudicots and the Monocots diverged about 140 million years ago. The dicots are highlighted in yellow illustrating that the dicots are not a monopyletic group.

Plant evolutionary tree. The flowering plants (angiosperms) diverged from the gymnosperms (e.g. pine, fir) over 200 million years ago. The “basal angiosperms” Amborellaceae, Nymphaeaceae and Austrobasileyales
diverged first followed by the magnoliid clade before the eudicots and the monocots diverged about 140 million years ago. The dicots are a polyphyletic group highlighted in yellow.

The monocot clade diverged from the eudicots about 140 Million years ago (1) and consists of the grasses and cereals including wheat and rice. Monocot embryos differ from the eudicots in several ways (click link for for more information). Other differences include pollen and flower structure and the monocots have leaves with parallel veins instead of diverging from a central vein. In eudicots the vascular bundles (xylem and phloem) are arranged in a ring in the stem, whereas in monocots the vascular bundles are scattered. In woody dicots (e.g. oak trees) each tree ring represents a year of growth where a new ring of vascular bundles develops. For more information about the differences between eudicots and monocots click this link  (N.B. the article uses the term dicot, but the differences listed refer to the eudicots the “stereotypical” dicots.)

The other groups of dicots consist of the magnoliid clade (e.g. avocado, black pepper) and the so called “basal angiosperms”. These dicots diverged from the common ancestor of the eudicots and the monocots with the basal angiosperms diverging earliest.

I find it pretty impressive that eudicot plants ranging from tiny weeds like A. thaliana to large trees like oak can all develop from a cress-like seedling. I would stick to using garden cress (L. sativum) in salads though!

References:

1. Chaw SM (2004) Dating the monocot-dicot divergence and the origin of core eudicots using whole chloroplast genomes

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5 thoughts on “Why many different plant seedlings look like salad cress

  1. Pingback: Garden Notes Mid June ’13

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