The oxidative burst in plants is mainly caused by reactive oxygen species (ROS) that are established to have many cellular implications in the cell cycle, such as, programmed cell death, hormone signaling, biotic and abiotic responses, including fruit ripening. Excessive quantities result in stress due to altered redox homeostasis, aberrant cell signaling and massive disturbances of orchestrated cellular functions (Bailey-Serres & Mittler, 2006). ROS in above threshold levels can damage cellular composition such as protein and lipids, which leads to the loss of membrane integrity and functionality with special significance in fruit ripening, as senescence triggering factor. Hence, the maintenance of ROS at cellular level in balanced and optimum level is necessary for maintaining the shelf life and quality of the fruits.
Oxidative damage in tissues during ripening is caused by the overproduction and accumulation of ROS or loss of the capacity of the antioxidant system to eliminate these radical. In spite of considerable efforts and huge resource commitments worldwide, the enhancement of fruit shelf life has met with limited success. In addition, post-harvest treatment that is not well regulated also affects the status of antioxidants in fruits or vegetables.
To fight the oxidative stress, many organisms including humans have developed an effective defense mechanism that reduces production of ROS and toxic to cells. The system is mainly composed of antioxidant enzymatic and non- enzymatic and enzymatic part of recovery. Antioxidants are compounds that are able to stabilize or deactivate free radicals before they attack cells. There are three main obstacles in the system when it comes to oxidative stress and how their impact could be minimized. Primary system serves to inhibit the oxidation of ROS through the termination of the chain reaction. Primary antioxidant containing OH or NH groups can inhibit reactive free radical reactions with proton transfer, such as, glutathione, ascorbic acid and phenol compounds.
Secondary system works for hydroperoxide species decomposition to form non-reactive and stable radical. This system includes catalase (CAT), superoxide dismutase (SOD), some peroxidases and the enzymes involved in the ascorbate-glutathione cycle: ascorbate peroxidase (APX), and glutathione reductase (GR) (Jimenez et al., 2002).The evolution of the antioxidant status and the oxidative stress of tomato at different stages of maturity were studied to improve the management and harvesting of this crop and to obtain fruit with higher nutritional content. Finally is the tertiary system that works to repair DNA damaged by ROS.
Carotenoids are common antioxidants. They play a major role in fruit colouring and act as antioxidants, reacting with free radicals, essentially peroxide radicals and singlet molecular oxygen (Namiki, 1990). In addition, Lycopene exhibits the highest rate physical quenching constant with singlet oxygen between dietary carotenoids (Stahl and Sies, 1996; Agarwal and Rao, 2000), which reduces the risk of several important pathologies of our times, for example cardiovascular diseases and some cancer typologies (Clinton, 1998; Rao, 2006). Another major carotenoid found in tomato is ??-carotene, there is much in vitro evidence of its interaction with free radicals, acting as a chain-breaking antioxidant and as a scavenger and quencher of singlet oxygen (Conn et al., 1992; Palozza & Krinsky, 1992).
Chilling injury (CI) is a physiological disorder of plants and plant organs caused by exposure to low, but non-freezing temperatures (Raison and Lyons, 1986). Conceptually, CI can be subdivided into two events; a primary event that is temperature-dependent and is initiated when the temperature falls below a threshold temperature for a specified period, and causes some metabolic dysfunction. The secondary event is time-dependent and includes several metabolic processes that can be adversely affected as a consequence of the primary event, and lead to the development of measurable symptoms characteristic of chilling injury (Orr and Raison, 1990). These detrimental changes reduce quality and consumer acceptability, leading to substantial economic loss.
The present research examines the transgenic RNAiACO1-21 (T2) which had blocking expression of specific gene encoding the ACC oxidase (Behboodian et al.,2012) during the course of ripening, on physical characteristics (colour, firmness and fruit weight) as well as nutritional composition (ascorbic acid, carotenoid contents, biodegradation and bioavailability) in tomato fruit during ripening. The transgenic tomatoes line-21 produced lower ethylene and exhibited longer shelf-life with more than 32 days as compared to the lowland Solanum lycopersicum cv.MT1 variety (wild-type) fruits, which was developed by the Malaysia Agriculture Research and Development Institute (MARDI). This variety bears smaller sized fruits with shorter shelf-life lasting only in 10 days.