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how do plants make seeds

How plants form their seeds

Around 80 to 85 percent of our calorie needs is covered through seeds, either directly as food or indirectly through use as feed. Seeds are the result of plant reproduction. During the flowering period, the male and female tissues interact with each other in a number of ways. When pollen lands on the flower’s stigma, it germinates and forms a pollen tube, which then quickly grows towards the plant’s ovary. Once it finds an ovule, the pollen tube bursts to release sperm cells, which fertilize the ovule and initiate seed formation.

Pollen tube interacts with female plant tissue

Led by Ueli Grossniklaus, professor at the Department of Plant and Microbial Biology at the University of Zurich, an international research team has now demonstrated how the pollen tube interacts with, and responds to, female plant tissue. The pollen tube does so by secreting extracellular signals (RALF peptides) which it uses to explore its cellular environment and regulate its growth. Two receptors on the cell’s surface enable it to perceive the secreted signals and transmit them to the inside of the cell.

Intracellular signals regulate growth

Working together with the teams of Christoph Ringli from UZH and Jorge Muschietti from the University of Buenos Aires, the team around Grossniklaus was able to determine that further proteins had to be active for the pollen tube to recognize the signals — LRX proteins. These proteins were identified at UZH 15 years ago by Beat Keller and his research group, but their function had previously not been clear. LRX proteins are localized in the cell wall surrounding plant cells, where the signals can dock. “We suspect that the pollen tube explores changes in the cell wall by sending out signals and responding accordingly, for example by realigning its growth,” says Ueli Grossniklaus. It is rare for plants to produce and perceive signals with the same cells. The researchers suspect that this allows the pollen tube, which grows extremely quickly, to faster respond to changes in its environment rather than being dependent on signals from other neighboring cells.

Molecular insights open up wide range of potential applications

The signaling pathways described by the researchers are involved in many other basic processes, and knowledge of how they work opens up numerous possible applications for plant breeding. “By better understanding how these proteins work, we can not only influence pollination and seed formation, but also the development and growth of plants or their defense against pests,” concludes Ueli Grossniklaus.

Vegetable, fruit, or grain — the majority of our food results from plant reproduction. Researchers have now discovered the key to how plants regulate pollen growth and seed formation. In addition to seed formation, knowledge about these signaling pathways can be used to influence plant growth or their defense against pests.

Plant reproduction

Scientists divide plants into two main groups depending on whether they reproduce by seeds or spores.

Plants that reproduce by seeds

Seed plants have special structures on them where male and female cells join together through a process called fertilisation. After fertilisation, a tiny plant called an embryo is formed inside a seed. The seed protects the embryo and stores food for it. The parent plant disperses or releases the seed. If the seed lands where the conditions are right, the embryo germinates and grows into a new plant.

Scientists divide seed plants into two main groups: plants with flowers and plants with cones. They also have special names for these groups. Plants that have flowers are called angiosperms. Plants that hold their seeds in cones are called gymnosperms.

Angiosperms – seed plants with flowers

Angiosperms have flowers. The flowers are special structures for reproduction. They contain male parts that make pollen and female parts that contain ovules. Some plants have these male and female parts in different flowers. Pollen is carried from a male part to a female part by wind, insects or other animals (a process called pollination), where it releases male gametes that fertilise the female gametes in the ovules. The ovules develop into seeds from which new plants will grow. In most angiosperms, part of the flower develops into fruit, which protects the seeds inside them. Fruit can be soft like oranges or hard like nuts.

Flowering plants form the biggest group of seed plants, with about 300,000 species around the world – that’s 90% of the whole plant kingdom. New Zealand has about 2,000 native angiosperms, and an amazing 25,000 introduced species found mainly in gardens, farms and orchards.

Flowering plants are all around us, even if sometimes we don’t recognise them as having flowers. We all know the showy flowers of native kōwhai, flax and pōhutakawa and all those lovely coloured flowers in our gardens, but the tall toetoe and the grasses in our lawns are also flowering plants.

Gymnosperms – seed plants with cones

Gymnosperms are seed plants but their seeds are held in cones. Next time you pick up a pine cone, look for loose seeds inside. Male cones make pollen, which is carried to female cones by the wind. After the female gametes are fertilised by male gametes from the pollen, the female cones produce seeds, which are then scattered away from the plant by wind or animals.

Most gymnosperms are trees. There are about 20 native gymnosperms in New Zealand, including our tallest tree, the kahikatea (Dacrycarpus dacrydioides, white pine). Others include mataī (Prumnopitys taxifolia, black pine), tōtara (Podocarpus totara), rimu (Dacrydium cupressinum, red pine) and kauri (Agathis australis). The main tree that makes up New Zealand’s plantation forests is the exotic gymnosperm Pinus radiata.

Plants that reproduce by spores

Ferns, mosses, liverworts and green algae are all plants that have spores. Spore plants have a different life cycle. A parent plant sends out tiny spores containing special sets of chromosomes. These spores do not contain an embryo or food stores. Fertilisation of the spores takes place away from the parent, usually in a damp place. An embryo is formed and a new plant grows from it. (For more information, view the Fern life cycle interactive.) New Zealand has about 200 species of ferns and over 500 species of moss.

Nature of science

Classification helps us put order into the world around us. Scientists start with very big categories like plants and animals and continue to divide the groups based on shared characteristics – like methods of reproduction.

Activity idea

Take a bag of (thawed) frozen broad beans to school and hand a few to each student. Students can remove the seed coat and split the bean to reveal the embryo inside. Use magnifying glasses to examine the embryos.

Scientists divide plants into two main groups depending on whether they reproduce by seeds or spores.