enzyme ACC synthase and antisense technology in fruit ripening

Introduction

Introduction

Welcome to another class of agricultural biotechnology. In today's class, we will delve into the fascinating topic of fruit ripening and its manipulation through the enzyme ACC synthase. Ethylene, a gaseous hormone found in plants, plays a pivotal role in the ripening process. This comprehensive article will discuss the biochemical pathways involving ACC synthase that lead to ethylene production, and explore how antisense technology can be used to manipulate this process for extending fruit shelf life.

The Biochemistry of Fruit Ripening

Flavor and Fragrance

Fruit ripening is synonymous with flavor and fragrance. The enzyme amylase converts starchy substances into sweet, juicy products. Organic acids are broken down into low molecular weight volatile compounds such as ethers, alcohols, aldehydes, and ketones, contributing to the fragrance. Additionally, chlorophyll degradation brings about various colors in fruits, ranging from the reds of apples to the purples of blueberries.

Softening of the Cell Wall

An essential aspect for scientists is the softening of the fruit's cell wall for improved shelf life. Unripe fruits are hard due to more pectin, but as ripening progresses, pectin content decreases, making the fruit soft and susceptible to infection.

Ethylene Biosynthesis Pathway

Steps in Ethylene Production

1. Methionine to SAM: Adenosine methionine (SAM) is synthesized from methionine, a sulfur-containing amino acid, utilizing SAM synthetase and ATP.
2. SAM to ACC: SAM is converted into 1-aminocyclopropane-1-carboxylate (ACC) by ACC synthase.
3. ACC to Ethylene: ACC is converted to ethylene by ACC oxidase, producing carbon dioxide and hydrogen cyanide (a toxic byproduct detoxified to cyanoalanine by cyanoalanine synthase).

The full pathway starting from methionine leads to the production of ethylene and involves critical enzymes like SAM synthetase, ACC synthase, and ACC oxidase.

Key Enzyme and Inhibitors

ACC synthase is a rate-limiting step in the ethylene biosynthesis pathway. Its activity can be competitively inhibited by AVG (Aminoethoxyvinylglycine), effectively influencing the rate of ethylene production. For instance, applying AVG to fruit-bearing plants allows control over fruit ripening times.

Manipulation through Antisense Technology

Antisense RNA Technology

Antisense technology involves generating a complementary RNA strand to the mRNA, effectively forming a duplex that blocks translation. In the context of fruit ripening:

• The mRNA is single-stranded, containing coding information (sense).
• Scientists create an antisense RNA with a complementary sequence to the mRNA, preventing it from being read and translated by ribosomes.

Application Example: Tomato

In tomatoes, two essential ACC synthase genes are LEACC2 and LEACC4. By expressing antisense RNA for the LEACC2 gene, scientists achieved almost complete inhibition of its corresponding mRNA. This inhibition delayed ripening substantially, keeping the fruit firm for at least six weeks longer than non-genetically modified control fruits.

Experimental Observation

A comparative study between control and antisense tomatoes demonstrated a significant delay in ripening when using antisense RNA technology. While control fruits ripened completely by the 65th day, antisense-treated fruits only began the ripening process around the 63rd day.

Conclusion

The ethylene biosynthesis pathway is critical in fruit ripening, and manipulating key enzymes using antisense technology offers a promising method for extending fruit shelf life. Understanding how ACC synthase and related enzymes function helps develop innovative agricultural practices to benefit fruit production and storage.

Keywords

• Fruit ripening
• Ethylene biosynthesis
• ACC synthase
• Antisense RNA technology
• Methionine
• SAM synthetase
• AVG (Aminoethoxyvinylglycine)
• ACC oxidase
• Tomato

FAQs

What role does ethylene play in fruit ripening?

Ethylene is a gaseous plant hormone essential for triggering and regulating fruit ripening. It influences various ripening aspects such as color change, flavor, and texture.

How is ethylene synthesized in plants?

Ethylene is synthesized from methionine through a series of enzyme-mediated steps: SAM synthetase converts methionine to SAM, ACC synthase converts SAM to ACC, and ACC oxidase finally converts ACC to ethylene.

What is the function of ACC synthase in ethylene production?

ACC synthase is a key enzyme that converts SAM to ACC, a crucial intermediate in the ethylene biosynthesis pathway. It is considered a rate-limiting enzyme, meaning it controls the speed of the entire process.

How does antisense RNA technology delay fruit ripening?

Antisense RNA technology involves creating an RNA strand complementary to the mRNA for key ripening enzymes like ACC synthase. This duplex formation blocks translation, thereby reducing enzyme production and delaying ethylene synthesis and fruit ripening.

Can antisense RNA technology be used on other fruits?

Yes, antisense RNA technology can be adapted to various fruit species to control ripening and extend shelf life, though specific gene targets and effectiveness may vary among species.