Monday, November 15, 2010

HOW SOME PLANTS SPREAD THEIR SEEDS: READY, SET, CATAPULT

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Catapults are often associated with a medieval means of destruction, but for some plants, they are an effective way to launch new life. Dispersing seeds greater distances by catapulting can provide selective advantages, including the establishment of populations in new environments and escape from certain threats.

In new work published in the recent October issue of American Journal of Botany, Dr. Ellerby, students, and postdoctoral researcher Shannon Gerry at Wellesley College measured the mechanics involved in catapulting seeds for the ballistic disperser Cardamine parviflora.

"While plants are generally thought of as immobile organisms, many of them are capable of spectacularly rapid movements," stated Ellerby. For C. parviflora, the valves of the silique rapidly coil outward catapulting the seeds away from the parent plant. The entire coiling and launching process is completed in around 5 msec—faster than the blink of an eye.

Analysis of the launch showed that the catapulting mechanism is not very reliable in C. parviflora, with the majority of the seeds simply falling to the ground. For the seeds that were launched, however, the transference of stored energy to kinetic energy was ~20% efficient. An impressive number when compared to the 0.5% efficiency observed for a ballistic diplochore (Impatiens capensis) in a previous study of Ellerby and colleagues.

This incredible speed and high energy storage present a challenge for the researchers. "These seed pod catapults are on a hair trigger," said Ellerby. "Successfully positioning them in front of our high-speed camera without them exploding prematurely requires an incredibly steady hand."

Seed launching has evolved in a number of groups. Comparing the mechanics of seed dispersal and the morphology of fruits and seeds between plants utilizing ballistic methods and closely related plants that do not, can provide a deeper understanding of the evolution of ballistic mechanisms and the properties required for energy storage and transference.

Seed dispersal has been studied extensively in the model plant Arabidopsis thaliana, a close relation to Cardamine. Like most other members of the Brassicacae, A. thaliana does not disperse its seeds via catapulting. Instead, the seeds are dropped to the ground as the silique dehisces and splits. Despite these differences in seed dispersal mechanisms, the siliques of C. parviflora and A. thaliana are morphologically similar. One difference is the persistence of second layer on the inner surface of the valve in C. parviflora that degenerates in A. thaliana during maturation. This additional layer likely plays a role in valve coiling.

"Ultimately it will be important to analyze the spring-structures at a tissue and cellular level to determine precisely how they store such impressive amounts of energy," Ellerby said. "This could inform the design of human-engineered structures for absorbing or storing elastic energy."

(Photo: Dr. David J. Ellerby, Wellesley College, Wellesley, Massachusetts)

American Journal of Botany

STUDY SHOWS HOW ANCIENT PLANTS AND SOIL FUNGI TURNED THE EARTH GREEN

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A new breakthrough by scientists at the University of Sheffield has shed light on how the Earth's first plants began to colonise the land over 470 million years ago by forming a partnership with soil fungi.

The research, which was published (2 November 2010) in Nature Communications, has provided essential missing evidence showing that an ancient plant group worked together with soil-dwelling fungi to 'green' the Earth in the early Palaeozoic era, nearly half a billion years ago.

The research, which also involved experts from the Royal Botanic Gardens, Kew, Imperial College London and the University of Sydney, has provided new insights into our understanding of the evolving dynamic behaviour of the Earth's land plants and fungi.

Scientists have long-suspected that soil fungi formed mutually beneficial relationships with early land plants to play an essential role in assisting their initial colonisation of terrestrial environments. However, until now there has been a lack of evidence demonstrating if and how the earliest ancient land plants, from the early Palaeozoic era (over 470 million years ago), might have cooperated with fungi for mutual benefit.

The team studied a thalloid liverwort plant, which is a member of the most ancient group of land plants that still exists and still shares many of the original features of its ancestors. They used controlled-environment growth rooms to simulate a CO2-rich atmosphere, similar to that of the Palaeozoic era when these plants originated. This environment significantly amplified the benefits of the fungi for the plant's growth and so favoured the early formation of the association between the plant and its fungal partner.

The team found that when the thalloid liverwort was colonised by the fungi, it significantly enhanced photosynthetic carbon uptake, growth and asexual reproduction, factors that had a beneficial impact on plant fitness. The plants grow and reproduce better when colonised by symbiotic fungi because the fungi provide essential soil nutrients. In return, the fungi also benefit by receiving carbon from the plants. The research found that each plant was supporting fungi that had an area of 1-2 times that of a tennis court.

Professor David Beerling, from the Department of Animal and Plant Sciences at the University of Sheffield, said: "By studying these ancient plants we open a window on the past to investigate how the earliest land plants evolved. Our results support the idea that the 'greening' of the Earth was promoted by a symbiosis between plants and fungi. It shows that plants didn't get a toe-hold on land without teaming up with fungi – this has long been suspected, but until now not investigated. It will require us to think again about the crucial role of cooperation between organisms that drove fundamental changes in the ecology of our planet."

Martin Bidartondo from the Jodrell Laboratory at the Royal Botanic Gardens, Kew, said: "Fungi are present in every type of habitat throughout the world and are essential for many plants to grow. It is exciting that we are now beginning to discover the fungi associated with 'lower' plants, and that many more still remain to be investigated."

(Photo: U. Sheffield)

University of Sheffield

FOSSIL FINGER RECORDS KEY TO ANCESTORS' BEHAVIOR

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Scientists, in collaboration with researchers at the universities of Southampton and Calgary, used finger ratios from fossilised skeletal remains of early apes and extinct hominins, as indicators of the levels of exposure species had to prenatal androgens – a group of hormones that is important in the development of masculine characteristics such as aggression and promiscuity.

It is thought that androgens, such as testosterone, affect finger length during development in the womb. High levels of the hormones increase the length of the fourth finger in comparison to the second finger, resulting in a low index to ring finger ratio. Researchers analysed the fossil finger bone ratios of Neanderthals and early apes, as well as hominins, Ardipithecus ramidus and Australopithecus afarensis, to further understanding of their social behaviour.

The team found that the fossil finger ratios of Neanderthals, and early members of the human species, were lower than most living humans, which suggests that they had been exposed to high levels of prenatal androgens. This indicates that early humans were likely to be more competitive and promiscuous than people today.

The results also suggest that early hominin, Australopithecus - dating from approximately three to four million years ago - was likely to be monogamous, whereas the earlier Ardipithecus appears to have been highly promiscuous and more similar to living great apes. The research suggests that more fossils are needed to fully understand the social behaviour of these two groups.

Emma Nelson, from the University of Liverpool's School of Archaeology, Classics and Egyptology, explains: "It is believed that prenatal androgens affect the genes responsible for the development of fingers, toes and the reproductive system. We have recently shown that promiscuous primate species have low index to ring finger ratios, while monogamous species have high ratios. We used this information to estimate the social behaviour of extinct apes and hominins. Although the fossil record is limited for this period, and more fossils are needed to confirm our findings, this method could prove to be an exciting new way of understanding how our social behaviour has evolved."

Dr Susanne Shultz, from the Institute of Cognitive and Evolutionary Anthropology at the University of Oxford said: "Social behaviours are notoriously difficult to identify in the fossil record. Developing novel approaches, such as finger ratios, can help inform the current debate surrounding the social systems of the earliest human ancestors."

(Photo: E. Trinkaus and the Israel Antiquities Authority)

University of Liverpool

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