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Why Heterosis Gene Generate Higher Yield in Tomato?

Heterosis, also known as hybrid vigor, was first identified about a century ago and since then it has been widely used in plant breeding. However, the molecular mechanism behind this technology, especially the mechanism involving single or multiple genes still remains unclear. Recently, Zachary Lippman, a researcher at the Cold Spring Harbor Laboratory, discovered some possible mechanisms from flowering plants like tomato. Their study has been published on recent PNAS.

In plants’ growth cycles, the obtained nutrition and energy are used for two applications: vegetative growth, which allows plants to grow bigger, and reproductive growth, which allows plants to foster offspring. People can interfere plants’ growth with approaches such as manual pruning, but plants themselves have their own gene control to distribute energy reasonably to each of the two under different circumstances. Such gene control includes self-pruning (sp) gene, which facilitate vegetative growth, and single flower truss (sft) gene, which can produce florigen. The growth of flowers is the most significant incident to distinguish vegetative growth and reproductive growth. Although the molecular mechanism of the interaction between sp and sft genes is not yet clearly understood, their interaction can change the budding rules of plants and further affect nutrition and energy distributions.

The researchers found that tomatoes with sp mutation have fewer leaves in each coaxial unit but have more flowers. Based on this observation, the researchers further found that if one of the sft gene also mutated, the growth of flowers on the main bud and lateral buds delays, resulting in uniform ripening time and more time for leaves to accumulate nutrition. Does this suggest that the lack of gene lead to florigen level appropriate for higher yield? The researchers then used micro RNA to simulate different levels of sft gene expression in sp-mutants and found that at similar expression level of heterozygosis, the tomatoes grew more leaves before their first flowering and in the same time, the leaves didn’t affect the number of flowers.

Tomato heterosis
Schematic diagrams showing architectures of a wild type (WT) indeterminate tomato plant (left) and an sp determinate mutant (right). The colors of the fruits indicate maturity and the fruit gets more ripened from yellow to green to orange to red. The sp mutants have more densely packed fruits and close ripening time. *Image source: Ke Jiang et al. (2013) PLoS Genet.

Growing tomatoes is not as simple as we thought. Food crops like rice, wheat and corn have been screened for long time and they have agreed ripening time, but tomatoes are different. The tomatoes in market have been carefully looked after: peasants have to prune terminal buds regularly and separate flowers and fruits appropriately to ensure high quality of fruits. In the meanwhile, they also need to care for the maturity of fruits during the growth and pick them when proper. The indeterminacy of their flowering and growing is as same as their wild ancestors.

However, tomatoes for food production need large amount and relatively low requirements on taste and smell, making the tomato growing almost impossible to perform in the same with that in the study: tomato growers want to harvest each quarter and they want to harvest even more fruits with similar maturity – under such circumstance, it is an good approach to make the tomatoes flower and bear fruits as many as possible during flower season. This study provides new insight in improving tomato yield – since the reason heterosis gives high yield is because they set proper time for tomatoes to flower and bear fruits, if we can genetically modify related genes and optimize gene expression, we may produce more tomatoes even faster. The paper pointed out that the manipulation technique on florigen might exert profound influence on crops industry.

Reference:

  1. Jiang K, Liberatore KL, Park SJ, Alvarez JP, Lippman ZB (2013) Tomato Yield Heterosis Is Triggered by a Dosage Sensitivity of the Florigen Pathway That Fine-Tunes Shoot Architecture. PLoS Genet 9(12): e1004043. doi:10.1371/journal.pgen.1004043
  2. Peralta IE, Spooner DM (2005)Morphological characterization and relationships of wild tomatoes (Solanum L. Section Lycopersicon). Monogr Syst Bot Missouri Bot Gard:227-257.

Source: EurekAlert

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