A new database reveals the secrets of the red fruit
Wild tomatoes grown in South America may have certain genetic advantages over the tomatoes we know - for example, traits related to taste, smell or resistance to pests. Farmers looking to restore some of their lost traits to the tomatoes they grow can now draw on a vast database devoted entirely to the red fruit and including genetic and metabolic mapping of both wild and cultivated tomatoes.
The database - established by a research team led by Prof. Assaf Aharoni from the Weizmann Institute of Science in collaboration with Prof. Danny Zamir's research group from the Hebrew University of Jerusalem and researchers from other institutions - built on the basis of 600 hybrids created by Prof. Zamir's group between wild tomatoes from Peru, Solanum pennellii, and among the common cultivated variety Solanum. The scientists conducted comprehensive genetic and metabolic mapping of the various hybrid strains and used computational tools to determine which segments of the genome, or even which individual genes, are responsible for the different traits in each strain. This database will help researchers and farmers create improved tomato varieties, and it is already helping the institute's scientists to unravel some mysteries related to the tomato.
sweet story
Green tomatoes are often bitter due to a high content of alpha-tomatin, a toxic substance that protects the tomato from bacteria and fungi and ensures that animals and birds do not eat them prematurely. Upon ripening, alpha-tomatin becomes a non-bitter metabolite, but until now it was not known how the two processes - the ripening and the sweetening - are timed. Also, it was not known how the toxic substance does not harm the tomato itself.
Dr. Yana Kazachkova, a post-doctoral researcher in Prof. Aharoni's group, examined these questions in a unique variety of wild tomatoes that remain bitter even after ripening. The researchers, who discovered this variety in the Andes in the 90s, assumed that the tomatoes are bitter due to mutations in the enzymes responsible for the production of alpha-tomatin. When Dr. Kazachkova and her colleagues compared the genome of the bitter tomatoes to that of non-bitter varieties, they discovered that the difference between them is indeed rooted in a mutation - but contrary to belief, the mutation is not in the gene that encodes the transition from bitter to sweet, but in the gene responsible for the protein that transports substances inside the cell; The researchers named this gene GORKY ("bitter" in Russian).
To make sure that the mutation in the gene is indeed the one responsible for the bitter taste of wild tomatoes, Dr. Kazachkova silenced this gene in some tomatoes and increased its activity in others. She then analyzed the composition of the ingredients in the engineered tomatoes using advanced mass spectrometry equipment in Prof. Aharoni's laboratory. During the experiments, she used to occasionally taste the tomatoes, and was excited to discover that the ones without GORKY were indeed bitter. "These were the most delicious bitter tomatoes I've ever tasted," she says.
During the research, it became clear that when green tomatoes produce alpha-tomatin in large quantities, mainly in the skin of the fruit, this toxic substance is kept inside closed bubbles that prevent it from harming the plant itself. When the tomato ripens, GORKY transports the alpha-tomatin out of the vesicles, allowing these molecules to become non-toxic metabolites in a series of five biochemical reactions that make the tomato delicious. Without the transport, the alpha-tomatin remains in bubbles, and the tomato remains bitter. These findings may promote the development of new tomato varieties as well as follow-up studies that will examine whether the transport mechanism also exists in other plants.
Tomato has a dilemma
During evolution, tomatoes faced a difficult challenge: on the one hand, their shiny and seductive appearance attracts animals, who eat the fruit and spread its seeds in their secretions. On the other hand, even fungi that take over the tomato in its prime and spoil its good appearance - they also help spread its seeds and continue the lineage. How, then, does the tomato bridge between two opposing goals: maintaining a bright and lustrous appearance over time at the same time as succumbing to fungi?
The post-doctoral researcher Dr. Jedrzej Szymanski in Prof. Aharoni's laboratory, along with his colleagues, first succeeded in identifying two new genes involved in the production of glycoalkaloids - a group of substances that play a role in protecting the tomato and ripening it. In addition, the scientists searched for genes and metabolites that increase the tomato's resistance to one of the most common fungi that attack its fruits. They conducted a genomic and metabolic analysis of the 600 strains in the new database and at the same time tested the spread of the fungus in these strains; To this end, mushrooms were injected into small cuts on top of the tomatoes in all 600 varieties - and a total of 25 thousand cuts. This is how they discovered a network of six metabolites responsible for the tomato's resistance to the fungus. The researchers showed that it is possible to increase this resistance by using only one of the metabolites - vitamin B5. Furthermore, the researchers identified three genes involved in resistance to the fungus: when they were silenced, susceptibility to the fungal infection increased.
This research May help in the development of new fungicides and promote the development of resistant strains. And what about the desired evolutionary balance between "nurture" and "neglect"? Well, the researchers discovered that when the tomato ripens, the activity of the genes responsible for resistance to fungi decreases. This fact points to the existence of mechanisms that precisely time these processes, which are apparently opposed to each other.
Dr. Shmuel Bukovza, Dr. Ilana Rogchev, Dr. Shantan Panda, Dr. Prashant D. Sunavan, Dr. Nir Shaf, Dr. Shagit Meir, Dr. Andrey Weiner, Dr. R. Yongwei Dong, Dr. Shafra Ben Dor and Dr. Simon Michaeli - all from the Department of Plant and Environmental Sciences at the Weizmann Institute of Science; Dr. Itai Tzemach and Prof. Dani Zamir from the Hebrew University of Jerusalem; Dr. Yangzhi Hu and Prof. Ilon Shani from Tel Aviv University; Dr. Pablo Cárdenas, Dr. Dorothea Vers, Dr. Krista Kanstroff, Dr. Sophie Constance Lamberts, Dr. Kristof Krukol and Dr. Hussam Hassan Noor-Aldin from the University of Copenhagen; Dr. Justin Lashbrook from Stellenbosch University in South Africa; Prof. Avinesh Kamble from Savitribai Phule University in Pune, India; Dr. Arno Bobi, Dr. Jules Beckwilder, Dr. Yuri Tikonov and Dr. Irena Romero de la Fuente from the Wageningen Institute for Plant Research in the Netherlands.
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