By: Akshaya Ragukumar Ajitha
We all know that maize is juicy and its color is yellow but over time its taste, juiciness, size, color, and shape all are changing – why is this happening? Teosinte is a weedy grass that thrives in Mexico. It is the ancestor of modern maize. It is important because the plants that exhibited desirable traits were saved by the farmers, and their kernels were then planted for the harvest the following season and this technique is called artificial selection or selective breeding. Over time, maize cobs grew bigger and had more rows of kernels, finally taking on the shape of modern maize. The small seeds of teosinte do not resemble maize kernels in the least these seeds were protected from animal consumption by a tough shell and merely by looking at teosinte, we would not be able to determine that it is related to maize. Genetic research has allowed us to determine that teosinte, a type of grass, is the wild ancestor of corn. Teosinte does not resemble maize at all, especially when its kernels are contrasted with those of corn. However, the two are strikingly similar at the DNA level and they are identical in terms of chromosomal count and gene organization. Teosinte can actually interbreed with contemporary maize cultivars to create maize-teosinte hybrids that can procreate spontaneously.
The late Miocene steppe expansion was the period when the grass family originated and quickly expanded to a continental extent more than 80 million years ago. Wheat, corn and rice are the three major food crops in the world. In the maize plant, there are numerous branches with paired spikelets that each bear two florets (McSteen et al., 2022). The nucleotide differences between the two Zaya mays lines are more than the differences between humans and chimps. What’s more, the genes essential for maize’s structure are absent in wheat and rice, and not all grasses have the same complement of genes, which affects how they look.
Figure 1. Difference between Teosinte and Maize (Source)
There are two types of inflorescences in maize: tassel, which grows at the top of the plant and has male flowers; and ear, which grows in the leaf axils and has female flowers. Despite having distinct mature morphologies, tassel and ear grow from remarkably identical inflorescence primordia, and their patterning is largely regulated by the same set of developmental regulators. Some of these developmental regulators, such as various transcription factors, microRNAs, and plant hormones, were discovered through mutational analysis and clonal studies (Thompson et al., 2014).
Figure 2. Having both male and female flowers on the same plant, maize is a monoecious plant. Female flowers are produced lower on the branch, while male blooms are produced near the terminals (source).
Unlike other closely related monocot families where the embryo is a globular, hardly differentiated mass of cells at fruit maturation, the grass embryo develops well before the fruit is shed from the plant (McSteen et al., 2022). Not only in maize but also wild relatives of wheat and barley are the breakable inflorescence stalks prevalent. Both natural and human choices for grain size, quantity, and dispersion have been focused on the grain and the inflorescence that carries it. Wild grasses were a natural source of nourishment for early humans since they included grains’ starch and oil-filled embryos. Each pair of spikelets produces two small flowers, and the corn will develop several branches. In maize, mutations can produce single spikelets, which are comparable to those in rice and wheat, or they can change spikelet pairs into branches.
In maize, ramosa1 (ra1), which encodes a zinc-finger transcription factor and regulates the abrupt transition from creating branches to producing spikelet pairs, is upstream of ramosa2 (ra2), which encodes a transcription factor in the lateral organ boundary (LOB) domain family (McSteen et al., 2022).
Figure 3. Branching mutants of maize. Maize ears. (A) Wild type, (B) ramosa1 (ra1), (C) ra2 and (D) ra3 (source).
Increasing the size of the apical inflorescence meristem is one way to enhance the number of grains in maize. Cytokinin, a plant growth hormone, is regulated by a conserved signaling pathway that includes members of the CLAVATA (CLV) and WUSCHEL (WUS) families of proteins (McSteen et al., 2022). Cytokinin has an impact on the size and number of stem cells in the meristem. In maize, meristem size, row number, and yield can all be increased by mutations that change the signaling in the CLV-WUS pathway. Despite these variations, a scan for alleles in maize that had signs of selection revealed the same locus, which boosts cytokinin and cell division to improve yield in the species. Meristems that produce many spikelets are larger than meristems that only produce a single spikelet. The spikelet-pair meristem in maize is one example of these meristems. Defects in the CLV-WUS pathway or the plant growth hormone auxin might result in single spikelets rather than paired spikelet’s being produced in maize. Grasses are a success on both the economic and ecological fronts, it was hypothesized that the characteristically big endosperm and well-developed embryo of grasses.
Helpful links:
https://www.sciencedirect.com/science/article/pii/S1674205220304378
https://www.pnas.org/doi/pdf/10.1073/pnas.87.24.9888
https://www.science.org/doi/10.1126/science.abo5035
https://www.sciencedirect.com/science/article/abs/pii/B9780124171626000092
https://kids.frontiersin.org/articles/10.3389/frym.2017.00037
Works Cited
McSteen, Paula, and Elizabeth A. Kellogg. “Molecular, Cellular, and Developmental Foundations of Grass Diversity.” Science, vol. 377, no. 6606, 2022, pp. 599–602., doi:10.1126/science.abo5035.
Thompson, Beth. “Genetic and Hormonal Regulation of Maize Inflorescence Development.” The Molecular Genetics of Floral Transition and Flower Development, 2014, pp. 263–296., doi:10.1016/b978-0-12-417162-6.00009-2.