In this article we will discuss about the role of biotechnology in enhancing the shelf-life of fruits.
Fruits contain high percentage of water and hence show high metabolic activity compared with seeds. During post harvesting stage this high activity continues and they are vulnerable to easy decay and perishability.
Thus short shelf-life makes it difficult to transport and market the fresh fruits at distant places. In many of the crops harvesting of fruits is done during immature or green mature stage or by using refrigeration and controlled atmosphere storage, the shelf life is prolonged.
Such methods are impractible in many fruits e.g., mangoes, etc. Alternatively biotechnological methods are being evolved to extend the shelf life of fruits. Such manipulations have been successfully done in tomato fruits where fruit softening has been influenced by the down-regulation of the polygalacturonase gene.
Further, ethylene hormone which has a significant effect on fruit ripening, can also be manipulated. Gradually this technology will be extended to other fruits.
Most available studies pertain to tomato since it has many advantages e.g., easy to transform genetically; is an important crop; annual, widely consumed, etc. Between 20-30 genes have been identified so far. We shall confine ourselves to those genes which are associated with ripening changes or have featured in transgenic experiments.
Tomato like many other fruits like mango, water melon, banana, apple is climacteric and its ripening is associated with a burst in respiratory activity at the onset of ripening. It synthesizes ethylene then. This gas functions as a hormone to initiate and coordinate expression of ripening genes and development of quality characters.
Two enzyme steps are involved in ethylene production (see below). In order to prolong shelf life of fruits it is essential to prevent ethylene production or to remove it immediately around the fruits.
Conversely, it is possible to stimulate ripening of some unripe fruits by treating them with ethylene gas or supplying compounds such as ACC or ethephon that lead to ethylene production.
It is assumed that there is ethylene signal transduction pathway during ripening and atleast two genes are involved during ripening. The first ripening cDNA to be cloned and sequenced, originally called TOM 6 encoded polygal-acturonase (PG). This is cell modifying enzyme that catalyzes the hydrolysis of polygalacturonic acid chains in un-methylated regions of pection.
With the onset of ripening, the PG gene is transcriptionally activated “and the mRNA and protein are extremely abundant in ripening fruit. Pectinesterase (PE) is a second cell wall and modifying enzyme that removes the methyl groups from methylated polygalacturonic in pectin. Carotenoid biosynthesis genes are also reported and they help in the conversion of chloroplasts into chromoplasts.
It has been demonstrated that the level of PSY is the chief factor in determining lycopene production in ripening fruits. Several other cDNAs encoding mRNAs enhance during tomato ripening but their exact role is still not understood. Ripening genes have also been cloned in other climacteric fruits like avocado, melon.
Promoter (s) have to be used in raising transgenics with a specific feature. In tomato four fruit gene promoters have been used and these include CaMV 35S, 2A11, E8 and ACOl promoter. In addition PG gene promoter has also been used.
The processes of texture change, hormone synthesis and colour production have all been manipulated genetically and several commercial products based on these modifications have reached the market.
These modifications have been generated mainly by gene silencing, inactivation of a specific gene in a targeted manner. The plant gene could be inactivated either by antisense technology or by sense suppression or co-suppression technology. There are at least two types of sense suppression and both involve the transfer of additional copies of sense genes of transgenic plants.
Using antisense gene driven by a constitutive promoter, the inhibition of PG gene expression was achieved. PG antisense plants had reduced levels of PG mRNA and enzyme activity. But it did not affect other ripening traits.
It is now well known that PG does play an essential role in cell wall metabolism and texture change during fruit ripening; other enzymes like cellulase, galacturanases are involved in softening; low PG fruits offer some benefits for both fresh market and processed tomatoes.
Three genes encoding the major form of pectin esterase are reported in tomato and on inhibition of their expression through antisense gene silencing in transgenics, the production of PE is reduced by 90% in ripe fruit.
In tomato genes concerned with ethylene biosynthesis have been inactivated by down-regulation with antisense or sense genes and fruits with variable levels of ethylene production have been obtained. This technology has also been used in cut flowers to enhance shelf life. Complete inhibition of ACS has been accomplished. Besides fruit ripening even colour change is also inhibited in tomato.
The fruits maturity is normal but fruit ripening is prevented and hence cracking, spoilage is checked. The first genetically modified fruit to enter the market was the ‘Flavr Savr’ tomato having low PG first marketed by Calgene in 1994 in USA.
Delayed or reduced ethylene production would be useful in fruits of apple, pear, melon, banana, peach, papaya. If fruits remain attached to the parent plant without ripening, they would accumulate more sugars or organic acids and improved flavour.