Without eyes or ears, plants must rely on the interaction of molecules to determine appropriate mating partners and avoid inbreeding.  In a new study, University of Missouri researchers have identified pollen proteins that may contribute to the signaling processes that determine if a plant accepts or rejects individual pollen grains for reproduction.

Like humans, the mating game isn’t always easy for plants.  Plants rely on external factors such as wind and animals to bring them potential mates in the form of pollen grains.  When pollen grains arrive, an introduction occurs through a “conversation” between the pollen (the male part of the flower) and the pistil (the female part of the flower).  In this conversation, molecules take the place of words and allow the pollen to identify itself to the pistil.  Listening in on this molecular conversation may provide ways to control the spread of transgenes from genetically-modified crops to wild relatives, offer better ways to control fertilization between cross species, and lead to a more efficient way of growing fruit trees.

“Unlike an animal’s visual cues about mate selection, a plant’s mate recognition takes place on a molecular level,” said Bruce McClure, associate director of the Christopher S. Bond Life Sciences Center and researcher in the MU Interdisciplinary Plant Group and Division of Biochemistry.  “The pollen must, in some way, announce to the pistil its identity, and the pistil must interpret this identity.  To do this, proteins from the pollen and proteins from the pistil interact; this determines the acceptance or rejection of individual pollen grains.”

In the study, researchers used two specific pistil proteins, NaTTS and 120K, as “bait” to see what pollen proteins would bind to them.  These two pistil proteins were used because they directly influence the growth of pollen down the pistil to the ovary where fertilization takes place.

Three proteins, S-RNase-binding protein (SBP1), the protein NaPCCP and an enzyme, bound to the pistil proteins.  This action suggests that these proteins likely contribute to the signaling processes that affect the success of pollen growth.

“Our experiment was like putting one side of a Velcro strip on two pistil proteins and then screening a collection of pollen proteins to see which of the pollen proteins have the complementary Velcro strip for binding,” McClure said.  “If it sticks, it’s a good indication that the pollen proteins work with the pistil proteins to determine the success of reproduction.”

In previous studies, McClure showed that S-RNase, a protein on the pistil side, caused rejection of pollen from close relatives by acting as a cytotoxin, or a toxic substance, in the pollen tube.

For their study, the MU team used Nicotiana alata, a relative of tobacco commonly grown in home gardens as “flowering tobacco.” The study, “Pollen Proteins Bind to the C-Terminal Domain of Nicotiana Alata Pistil Arabinogalactan Proteins,” was published in the Journal of Biological Chemistry and was co-authored by McClure; Kirby N. Swatek, biochemistry graduate student; and Christopher B. Lee, post-doctoral researcher at the Bond Life Sciences Center.

Faculty from six of MU’s colleges and schools perform interdisciplinary research in the Christopher S. Bond Life Sciences Center with a vision to become a recognized world-wide center of scientific excellence and leadership in life sciences research, innovation and education.  The Center integrates the strengths of multiple, often disparate, disciplines to promote discovery that boosts the production and quality of food, improves human and animal health and enhances environmental quality.  The Center enriches the state of Missouri and its people by generating new businesses and jobs, fueling the economy through the creation and dissemination of new knowledge, and training young people to solve complex interdisciplinary problems.

VIB researchers at Ghent University have discovered the substance that governs the formation of root offshoots in plants, and how it works. Root offshoots are vitally important for plants – and for farmers. Plants draw the necessary nutrients from the soil through their roots. Because they do this best with a well-branched root system, plants must form offshoots of their roots at the right moment. The VIB researchers describe how this process is controlled in the prominent professional journal Science. A key player in this process is a protein called ACR4. Depending on the signals that it receives from its environment, this protein triggers the formation of a root offshoot. Now that we know the control mechanism, we can begin to stimulate plant roots to form more, or fewer, offshoots. This can lead to a more ecological agriculture and to the production of better crops at the same time. An efficient network

It is difficult to overstate the importance of plants in our lives − they are responsible for our oxygen and for food, clothing, energy, and countless other things. And in turn, the importance of a plant’s roots is unquestionable: they provide the plant with necessary nutrients and moisture. The more the roots are subdivided, in breadth and depth, the better they can do their work. So, a well-coordinated, controlled formation of root offshoots is crucial to a plant. But, until now, how a plant determines when and where an offshoot should be formed was unknown.

Asymmetric cell division

The presence of stem cells is very important in the development of plants and animals. Stem cells are cells that can transform themselves into various types of cells. In animals, tissues and organs are formed before birth; but in fully-grown plants, stem cells continue to play a major role in the formation of new organs or tissues, such as root offshoots.

These stem cells are found inside the root, and several of them will induce the formation of an offshoot. These ‘root-founder’ cells undergo an asymmetric cell division. In contrast to the usual cell division, which gives rise to two identical cells, asymmetric cell division produces two different cells: a stem cell that is identical to the original cell, and a cell that is ready to become a specialized cell – in this case, a secondary root cell.

The decisive signal

With the aid of the mouse-ear cress (Arabidopsis thaliana), a frequently used model plant, Ive De Smet and Valya Vassileva in Tom Beeckman’s group have been studying how a plant determines which cells will trigger offshoots. To do this, the VIB researchers in Ghent have employed a special technology that makes it possible to make synchronous offshoots develop at different moments. This allowed them to isolate the cells that induce the formation of offshoots. They found out which genes are active in these cells and compared them with the genes that are crucial to normal cell division. In this way, the researchers identified a specific set of genes that control asymmetric cell division and send the signal for the formation of offshoots.

ACR4: control over asymmetric division

The researchers then examined one of these genes more closely. The ACR4 gene contains the DNA code for a receptor, a protein that is often located on the exterior of a cell to pick up signals from the outside and transmit them to the controlling mechanisms within the cell. When the researchers disrupted the function of ACR4 in plant cells, the precisely orchestrated asymmetric cell division was also disturbed. From this finding, De Smet and Vassileva inferred that ACR4 plays a key role in the creation of offshoots. Because the protein has a receptor function, triggering the formation of offshoots depends on its reaction to signals from the environment.

Desired or undesired

With this research, the scientists have discovered a fundamental mechanism − fundamental for the plant, and very important for plant-breeders as well. This new knowledge enables us to promote, or retard, the formation of offshoots − both activities are useful in a large number of applications.

Promoting an extensive root system helps plants absorb nutrients more readily, and thus they need less fertilizer. Such plants can also grow more easily in dry or infertile soils. Furthermore, plants with a well-developed root system are more firmly anchored in the soil and can be used to counteract erosion.

On the other hand, slowing down secondary root formation can be advantageous in tuberous plants, like potatoes or sugar beets. This enables these food crops to invest all their energy in the production of nutrients. Fewer root offshoots also makes it easier for farmers to harvest these crops.

Plant research with medical possibilities?

This plant research sheds light on the control of asymmetric cell division − and this kind of cell division is similar to the cell division of stem cells in animals, too. So, these results can also provide greater insight into how animal stem cells specialize.

For example, irregular cell division plays a role in the development of various types of cancer, and similar control mechanisms might underlie this process as well. This is clearly an important area for future research.

Scientists already knew that flowering plants, unlike animals require not one, but two sperm cells for successful fertilisation.

The mystery of this ‘double fertilization’ process was how each single pollen grain could produce ‘twin’ sperm cells. One to join with the egg cell to produce the embryo, and the other to join with a second cell in the ovary to produce the endosperm, a nutrient-rich tissue, inside the seed.

Double fertilisation is essential for fertility and seed production in flowering plants so increased understanding of the process is important.

Now Professor David Twell, of the Department of Biology at the University of Leicester and Professor Hong Gil Nam of POSTECH, South Korea report the discovery of a gene that has a critical role in allowing precursor reproductive cells to divide to form twin sperm cells.

Professor Twell said: “This collaborative project has produced results that unlock a key element in a botanical puzzle.

The key discovery is that this gene, known as FBL17, is required to trigger the destruction of another protein that inhibits cell division. The FBL17 gene therefore acts as a switch within the young pollen grain to trigger precursor cells to divide into twin sperm cells.

“Plants with a mutated version of this gene produce pollen grains with a single sperm cell instead of the pair of sperm that are required for successful double fertilization.

“Interestingly, the process employed by plants to control sperm cell reproduction was found to make use of an ancient mechanism found in yeast and in animals involving the selective destruction of inhibitor proteins that otherwise block the path to cell division.

“Removal of these blocks promotes the production of a twin sperm cell cargo in each pollen grain and thus ensures successful reproduction in flowering plants.

“This discovery is a significant step forward in uncovering the mysteries of flowering plant reproduction. This new knowledge will be useful in understanding the evolutionary origins of flowering plant reproduction and may be used by plant breeders to control crossing behaviour in crop plants.

“In the future such information may become increasingly important as we strive to breed superior crops that maintain yield in a changing climate. Given that flowering plants dominate the vegetation of our planet and that we are bound to them for our survival, it is heartening that we are one step closer to understanding their reproductive secrets.”

Researchers at the University of Leicester are continuing their investigation into plant reproduction. Further research underway in Professor Twell’s laboratory is already beginning to reveal the answers to secrets about how the pair of sperm cells produced within each pollen grain aquires the ability to fertilize.