Introduction & Status

Introduction

Tomato is a model crop for study of fruit ripening. Tomato has short generation period (~5-100 days) and year-round growth potential, due to its photo-neutral nature. Further, sequences of many cDNAs and ESTs related to fruit ripening is known. Fruit ripening is associated with softening of fruits, increased susceptibility to various pathogens leading to around 25% loss of the crop yield. The study of ripening is very interesting because it can allow development of tools to delay fruit ripening. Attempts have been made in the past to delay fruit ripening by antisense suppression of genes for HMG-CoA reductase, polygalactouronase (PG), pectin methylase transferase, ACC synthase, ACC oxidase, phytoene synthase and ethylene receptor. Suppression of ethylene biosynthesis results in prevention of fruit ripening, and requires external application of the hormone. However, once fruits are gassed with ethylene to ripen them, the shelf life is as short as for naturally matured fruits. In order to manipulate fruit ripening in such a way that fruits do not become soft and infected by pathogens but develop desirable traits of ripened fruits, such as accumulation of pigments, sugars, volatiles and organic acids to proceed in a natural manner, better understanding of the process of fruit ripening. Though ethylene is a major player in regulation of fruit-ripening, all the responses induced by ethylene can not be considered to have any impact on fruit ripening. It is essential to identify ethylene responses which are directly related to fruit ripening, so that they can be targeted for manipulation of fruit ripening.


Tomato plants harbouring rin mutation fail to ripen and show inhibition of all measured ripening phenomena, including the respiratory climacteric and associated ethylene evolution, carotenoid accumulation, softening and production of flavour compounds. Though rin mutants do not ripen in response to ethylene, these mutants retain other ethylene responses like seedling triple response, floral abscission and petal and leaf senescence. This suggests that RIN product controls fruit development upstream of ethylene. Recently, it has been found that rin mutant has a lesion in LeMADS-RIN gene. The present proposal aims at creation of transgenic lines of tomato having LeMADS-RIN expressed in antisense orientation under the control of constitutive or fruit-specific promoters. Some of the transgenic lines would be analyzed for delay of fruit ripening. Strong phenotypes would be used for study of ethylene responsive genes using tomato microarrays, which are commercially available at Cornell University, USA. Expression of ethylene responsive genes of tomato would be studied in these transgenic plants to identify genes which are related specifically to fruit ripening, and not to general ethylene responses. Since rin mutation affects only ripening-related subset of ethylene responses, it would be very important to study ethylene mediated
responses in plants under-expressing RIN product. This study would help in creating some tomato lines with delayed ripening and dissection of ethylene signal transduction into ripening related and ripening independent signalling events.

Current Status

Attempts are being made to understand the mechanism of fruit ripening by using microarray approach. One of the area of interest is to find out a sub-set of ethylene responses which are specific to fruit ripening and are not general ethylene responses. More than 80 genes related to ripening of tomato fruits have been cloned. Some of the genes of interest related to fruit ripening and biotic stress would be analyzed by transgenic approach. An efficient system for tomato transformation system is available, and has already been utilized for expression of antigens of Vibrio cholerae, targeted towards development of edible vaccine against cholera. A micro-array facility is already functional at the site