February 1, 2010
Source: Proceedings of the National Academy of Sciences of the United States of America (PNAS)
Researchers have extended the shelf life of tomatoes by 30 days by suppressing enzymes that promote ripening. Asis Datta and colleagues identified two enzymes, α-mannosidase (α-Man) and β-D-N-acetylhexosaminidase (β-Hex), which accumulate in tomatoes at critical stages during the fruit's ripening. The researchers linked α-Man and β-Hex specifically with ripening-associated fruit softening, a phenomenon of interest because excessive softening accounts for as much as 40 percent of post-harvest fruit loss. The authors used genetic engineering to "silence" the enzymes in tomatoes and report that experimental tomatoes lacking α-Man were approximately 2.5 times firmer than conventional tomatoes, and those lacking β-Hex were 2 times firmer. Both types of transgenic tomatoes retained their texture and firmness for up to 45 days, compared with conventional tomatoes, which started shrinking and losing texture after 15 days. The researchers found that the transgenic tomato plants grew normally and produced typical amounts of vegetation and fruit, which matured at the typical rate. Manipulating the enzymes that alter the speed and duration of ripening may help enhance the shelf life of tomatoes and, potentially, other fruit, according to the authors.
Enhancement of fruit shelf life by suppressing N-glycan processing enzymes
Vijaykumar S. Meli, Sumit Ghosh, T. N. Prabha, Niranjan Chakraborty, Subhra Chakraborty, and Asis Datta
10.1073/pnas.0909329107
http://www.pnas.org/content/early/2010/01/27/0909329107
Abstract
In a globalized economy, the control of fruit ripening is of strategic importance because excessive softening limits shelf life. Efforts have been made to reduce fruit softening in transgenic tomato through the suppression of genes encoding cell wall–degrading proteins. However, these have met with very limited success. N-glycans are reported to play an important role during fruit ripening, although the role of any particular enzyme is yet unknown. We have identified and targeted two ripening-specific N-glycoprotein modifying enzymes, α-mannosidase (α-Man) and β-D-N-acetylhexosaminidase (β-Hex). We show that their suppression enhances fruit shelf life, owing to the reduced rate of softening. Analysis of transgenic tomatoes revealed ≈2.5- and ≈2-fold firmer fruits in the α-Man and β-Hex RNAi lines, respectively, and ≈30 days of enhanced shelf life. Overexpression of α-Man or β-Hex resulted in excessive fruit softening. Expression of α-Man and β-Hex is induced by the ripening hormone ethylene and is modulated by a regulator of ripening, rin (ripening inhibitor). Furthermore, transcriptomic comparative studies demonstrate the down-regulation of cell wall degradation- and ripening-related genes in RNAi fruits. It is evident from these results that N-glycan processing is involved in ripening-associated fruit softening. Genetic manipulation of N-glycan processing can be of strategic importance to enhance fruit shelf life, without any negative effect on phenotype, including yield.
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