I say tomato…

Researchers at the National Institute of Plant Genome Research in India have found a surprisingly simple way to extend the shelf life of fresh tomatoes. Most tomatoes will last about 10-15 days before going unappealingly squishy. The enhanced tomatoes last 45 days or more and are firmer than unmodified tomatoes, which I imagine makes for great tomato sandwiches!

Before getting into the how, let’s talk about why this research is important. According to Enhancement of fruit shelf life by suppressing N-glycan processing enzymes in this week’s PNAS, post-harvest fruit and vegetable softening is a big problem, with losses accounting for almost 50% of all produce in developing countries. India, the country that funded the research, and the world’s 2nd largest fruit and vegetable producer, loses 35-40% of produce to softening.

Squished tomato by limaoscarjuliet via Flickr.

We all know that post-consumer food waste is a big problem, and we can alleviate this somewhat in our homes and by choosing restaurants that try to reduce waste. But there isn’t much we can do about pre-consumer waste – from grain that rots in the silo due to fungus to tomatoes that rot in transit due to ripening. By reducing pre-consumer food waste, we can reduce the number of acres needed to produce the same amount of food. In India, preventing all fruit and vegetable softening would be like reducing the amount of land needed to grow fruits and vegetables by 35-40%!

So, how could that softening be prevented?

Researchers have been working for a long time on different parts of the ripening and spoiling process, trying to find ways to slow it down. Nothing has been really effective in getting produce to last longer, and we’ve ended up with produce that is more bland than it used to be, especially when it comes to tomatoes. In short, neither breeding nor genetic engineering has been successful… until now.

In Enhancement of fruit shelf life by suppressing N-glycan processing enzymes, Meli* and fellow researchers found two enzymes that contribute to fruit softening. The enzymes are α-mannosidase and β-D-N-acetylhexosaminidase, α-Man and β-Hex for short. Both of these enzymes break the glycosidic bonds between carbohydrates, as well as between carbohydrate and noncarbohydrate. The role of these enzymes in ripening and softening is to help break down the cell walls that keep the fruit firm. If the enzymes are stopped from breaking down the cell walls, the tomato stays fresh!

Meli and fellow researchers turned off the genes that code for these two enzymes α-Man and β-Hex with biotechnology, but they didn’t use any whole genes from tomatoes or any other species. Instead, they used some pieces of the tomato α-Man and β-Hex genes. These gene fragments are transcribed into RNA under control of the constitutive (always on) CaMV 35S promoter. They then twist and bind with themselves, resulting in double stranded RNA, which activate the RNA interference mechanism that plants and other organisms naturally use to combat double stranded RNA viruses.

The results are pretty striking, as you can see from these pictures. The control tomatoes were unappealingly wrinkly by 20 days, and rotten by 45 days. The tomatoes with α-Man or β-Hex turned off were still firm even at 45 days.

Control and experimental tomatoes over time, from the PNAS article "Enhancement of fruit shelf life by suppressing N-glycan processing enzymes" by Meli, et. al.

RNAi can be used just about any time you want to turn off a gene – it’s even being tested for human use to help combat genetic diseases. For an overview of RNAi that’s a little more detailed than the picture below, check out the RNA Interference interactive video by Nature Reviews (via ERV. Note: the video wouldn’t play on my Mac in Firefox but worked great in Safari).

Overview of RNAi from Huntington's Outreach Project for Education, at Stanford.

The researchers used Agrobacterium to carry the DNA sequences into very young tomato plants, along with a marker gene for kanamycin resistance. Biotech plants can be made without markers but it’s much easier to use them, and there is no risk (for more on antibiotic resistance markers, see GMO Compass).

This work, as far as I can tell, is funded purely by the Indian government – not by private corporations. Specifically, it is funded by the Department of Biotechnology which is part of the Ministry of Science and Technology. They have some pretty impressive goals, as listed in the Plant Biotechnology section, including:

  • Genetic engineering and molecular biology tools for forest tree improvement including reduction of generation time, production of horticultural and plantation crops with desired characteristics.
  • Transgenics for improved yield, stress tolerance, balanced nutrition, keeping quality of flowers, fruits and vegetables, better nutrient and water utilization capacity should be produced.
  • Cataloguing of accessions of wild and land races to study genetic diversity for resolving taxonomic problems.

Tomatoes at Union Square by Lindsay Beyerstein via Flickr.

One of the biggest arguments against biotechnology is that it has been under corporate control. Unfortunately, that’s been true in the United States, where publicly funded research in agriculture has been all but ended. Happily, that’s not the case in India and China. These governments are researching biotech traits for the benefit of their farmers, not for the benefit of shareholders.

If this biotech trait is available royalty-free, then it will presumably be available for breeding by small seed companies and by farmers. I’m imagining beautiful genetically-diverse heirloom tomatoes that have this amazing ability to stay firm on your counter well past the tomato growing season. This means that fewer tomatoes will need to be shipped around the world, and that more can be grown locally. I hope to see some long-lasting tomatoes in my CSA share soon!

* You may have noticed that I usually use the name of the first author rather than the name of the last author when I’m referring to a peer-reviewed paper. In biology-related papers, the first author is the graduate student, or sometimes post-doctoral researcher, who did most if not all of the labwork and writes most if not all of the paper.The last author is the PI (Primary Investigator), who generally provides guidance, helps with experimental design, and edits the paper. The authors in the middle are usually other grad students and their PIs who helped with the project. While all of the authors usually have put in a lot of time and effort, it’s that first author who worked the hardest, and I like to recognize that.

ResearchBlogging.orgMeli V, Ghosh S, Prabha T, Chakraborty N, Chakraborty S, & Datta A. (2010). Enhancement of fruit shelf life by suppressing N-glycan processing enzymes Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0909329107