Guest post by Siva Chudalayandi (@sianj)
I love plants and a walk through the woods never fails to refresh my mind. I have been fortunate to have spent the last several years researching aspects of plant genetics and development. The cells of the plant leaves house many compartments called chloroplasts, which are the factories that make organic matter using just sunlight, water and carbon dioxide in a process called photosynthesis. Through this blog I’d like to pay tribute to this unique organ of plants.
Leaves occur in myriad shapes and forms. Sometimes they are large (e.g. banana leaves) or serrated (tomato leaves), or are needle-like, as seen on pine trees. However, like many other structural forms in biology they are made up of distinct substructures that are found in many different groups of plants. I live in the USA in the state of Iowa, an area that is flush with corn fields. Corn (maize) is a fascinating model to study how the leaves of the grasses and other monocot plants develop. The flat and wide portion of the leaf is called the leaf blade, while the part of the leaf that hugs the stem is called the sheath. The blade and sheath are separated by an outgrowth called the ligule and a loosely defined region called the auricle (Figure 1). This sheath and blade pattern is repeated in every leaf.
Figure 1: Maize shoot. Image credit: Chudalayandi et al. (1).
Over the years many researchers have proposed varied explanations for the existence of a ligule in grasses, including that it protects the interior of the plant from water and insects. However, a more plausible explanation for ligules and auricles is that they might act as a pivot to help position the leaf blade at the correct angle to receive appropriate amounts of solar radiation. Using tools in genetics and molecular biology, scientists have uncovered many of the cellular aspects of maize leaf and ligule development.
Leaves emerge from a structure called the shoot apical meristem. Researchers at Cornell University (2) used a technique called laser microdissection on developing maize shoots to precisely isolate the emerging ligule/auricle, leaf blade and sheath. Then they examined which genes were active in both maize plants with normal ligules and mutants that lacked ligules. The scientists found that gene networks involved in specifying initiation of various lateral organs (leaf, branches etc.) in plants are also involved in the formation of ligules. To briefly elaborate, in the shoot apical meristem, KNOX proteins (3) accumulate at the base while a plant hormone called auxin accumulates in regions marking where a leaf will form. This pattern is repeated in each emerging leaf and lateral organ that develops from the meristem. The same network of genes also specifies the blade sheath boundaries and where the ligule will form.
Along with producing auxin, the gene networks control the production of another plant hormone called cytokinin. The interplay between auxin and cytokinin is partly responsible for maintaining the pattern of leaf growth. Many researchers have studied this pathway in different plants over the years and this has been the subject of many fine review articles (4,5). These and other studies show that during evolution, a similar network of genes is often slightly modified and redeployed to make a newer organ that offers an organism another way to adapt to changes in its environment.
References:
1: Chudalayandi, Sivanandan; Cahill, James; Scanlon, Michael; Muszynski, Michael. Cytokinin hypersignaling reprograms maize proximal-distal leaf patterning. Short talk abstract (T-20), 57th Annual Maize Genetics Conference, St. Charles, IL, USA (Mar 12-Mar 15, 2015).
2: Johnston R, Wang M, Sun Q, Sylvester AW, Hake S, Scanlon MJ. Transcriptomic analyses indicate that maize ligule development recapitulates gene expression patterns that occur during lateral organ initiation. Plant Cell. 2014 Dec; 26 (12):4718-32. (http://www.ncbi.nlm.nih.gov/pubmed/25516601)
3: Hay A, Tsiantis M. A KNOX family TALE. Curr Opin Plant Biol. 2009 Oct; 12(5):593-8. (http://www.ncbi.nlm.nih.gov/pubmed/19632142
4: Koenig D, Sinha N. Evolution of leaf shape: A pattern emerges. Curr Top Dev Biol. 2010;91:169-83. (http://www.ncbi.nlm.nih.gov/pubmed/20705182)
5: Lewis MW, Hake S. Keep on growing: building and patterning leaves in the grasses. Curr Opin Plant Biol. 2016 Jan 2;29:80-86. (http://www.ncbi.nlm.nih.gov/pubmed/26751036)
About the author: Siva Chudalyandi is a post-doctoral research associate at the Iowa State University. Siva studies maize genetics and genomics, and loves working at the intersection of biology and big data. Follow him on twitter (@sianj).
Plant Scientist 