Plant genetics is a very powerful tool to study the process of life.
The pathway for a certain biological process can be pieced together through mutants, which are organisms that have a change in gene sequence that often lead to phenotypic alterations.
One example is the discovery of the stomata development pathway.
Stomata (i.e. pore structures on the leaf surface) are formed by two jellybean-shaped cells called guard cells. They function to facilitate gas and water exchange that is important for photosynthesis and transpiration. When observing leaf surface area under a microscope, one can see that there are two cell types: guard cells that form stomata and pavement cells that are shaped like puzzle pieces (Figure ). Occasionally cell types called meristemoids and guard mother cells, which are precursors of mature guard cells, can also be spotted in developing tissues.
Several mutants that do not have stomata were identified from chemical mutant populations (Figure). One had pavement cells only, and was named speechless (spch). Another had structures that resembled rose petals. These rose-petal structures are meristemoids that have gone through multiple rounds of cell division. Scientists named this mutant mute. Finally, a mutant line that had caterpillar-like structures that are access division of guard cell-like structures was named fama, after the ancient Roman Goddess of Rumor.The mutated genes that caused the phenotype in spch, mute and fama were cloned, and they were identified based on their sequences as three transcription factors that are known to regulate gene expression.
Characterization of these three mutants, along with the observation of meristemoids, guard mother cells, and mature guard cells in wild type leaves, led to the current stomatal development pathway:
SPCH controls the transition from undifferentiated epidermal cells to meristemoids. MUTE then facilitates the differentiation of meristemoids to guard mother cells. Finally, FAMA regulates the maturation of guard mother cells to two mature guard cells that shape a stoma (Figure).
Research of the stomatal development pathway and many other pathways has been mainly conducted in the model plant Arabidopsis thaliana, which is a small weed in the dicot Brassicaceae family. Because of its small genome size and rapid life cycle, Arabidopsis thaliana has been a favorite model for plant biologists to understand molecular mechanisms in plants. However, nature has provided a rich diversity of plants that differ morphologically, ecologically, and physiologically. To better understand panicoid grasses that compromise the world’s important food, feed, and bioenergy crops such as maize, sorghum, sugarcane, and switchgrass, Setaria viridis has emerged as a model to represent panicoid grasses. Like Arabidopsis thaliana, Setaria viridis has a short life cycle (six week seed-to-seed), small genome size, and a small stature that is easy to grow and maintain in controlled environments. Genetic tools such as transgenic lines, crossing tools, and a chemical mutant (NMU) population that contains many plants of interesting phenotypes were generated in Setaria viridis for characterization of pathways that control important traits (e.g. biomass).
Do you have a favorite pathway that you would like to understand? Look carefully for interesting phenotypes in your NMU population and perhaps you will find some puzzles to piece together!
Figure Source: Modified from Peterson et al. 2010. Plant Cell 22, 296-306. This material is copyrighted by the American Society of Plant Biologists and is reprinted with permission.
MacAlister CA, Ohashi-Ito K, Bergmann DC. 2007. Transcription factor control of asymmetric cell divisions that establish the stomatal lineage. Nature 445, 537-540.
Ohashi-Ito K, Bergmann DC. 2006. Arabidopsis FAMA controls the final proliferation/differentiation switch during stomatal development. Plant Cell 18, 2493-2505.
Peterson KM, Rychel AL, Torii KU. 2010. Out of the Mouths of Plants: The Molecular Basis of the Evolution and Diversity of Stomatal Development. The Plant Cell 22, 296-306.
Pillitteri LJ, Sloan DB, Bogenschutz NL, Torii KU. 2007. Termination of asymmetric cell division and differentiation of stomata. Nature 445, 501-505.
Contributed by Christine Shyu Ph.D