Indoor growing can be a breeding ground for many viral, bacterial and fungal plant pathogens. Without the natural benefits of Mother Nature, indoor growers must constantly monitor and protect their plant’s health from a variety of potential outbreaks, including plant pathogens. Just one major outbreak can cause an entire crop to be lost. Harpin proteins, a natural plant-health promoter and plant-growth stimulator, just might be your solution.

Harpin proteins activate a naturally occurring process in the plant, boosting the plant’s immune systems in order to fight off diseases. They do this by tricking your plant into thinking it is under attack by plant pathogens—except there are no harmful pathogens present. The benefits of harpin proteins include generating a hypersensitive response (HR), systemic acquired resistance (SAR) and increased photosynthesis. These results produce a plant with stronger and more fruitful yields.

Discovery of harpin proteins

Harpin proteins were discovered in 1992 at Cornell University from basic research attempting to understand how plant pathogenic bacteria interacted with plants. When a plant pathogen is introduced to a plant, the first line of defense is the HR within the plant. The scientists at Cornell University aimed to identify a specific bacterial protein responsible for triggering this response.

It was discovered that the protein was encoded by one of a group of bacterial genes called the hypersensitive response and pathogenicity (hrp) gene cluster. The hrp cluster in the bacterium Erwinia amylovora (Ea), which causes fire blight in pears and apples, was dissected and a single protein that elicited HR in certain plants was identified. In addition to the HR, this protein was also responsible for triggering systemic acquired resistance and, surprisingly, increased plant growth. The protein was given the name harpin from the corresponding gene designated hrpN and was the first example of its species.

What is a hypersensitive response?

The HR is a well-known mechanism in plants used to prevent the spread of infection by pathogens. When the HR is active within a plant, there is rapid cell death within the plant cells at the site of infection. This rapid death creates a physical barrier, meaning the pathogen is no longer able to spread its disease throughout the plant. In applying harpin proteins to the leaves of a plant, the plant is tricked into thinking a plant pathogen is present. This triggers a cascade reaction from the plants, which leads to another well-known plant resistance function: SAR.

What is systemic acquired resistance?

SAR is a resistance response throughout the entire plant resulting from a localized exposure to a plant pathogen. It is similar to the process found in the human immune system when one gets a flu shot or the chicken pox—once the body is introduced to a virus, it develops natural defense systems against future similar viruses. When a plant detects a pathogen present, the SAR generates a plant-wide resistance to this pathogen. So, the plant’s overall immune system is increased, which offers protection against plant diseases and results in increased plant health.

Other benefits of harpin proteins

In addition to the SAR, applying a HR elicitor like harpin proteins has also been found to produce a wide range of benefits, including:

  • Increased photosynthesis
  • Increased nutrient uptake
  • Cell-wall expansion
  • Increased root systems
  • Promotion of flower initiation and fruit set and size
  • Increased fruit sugars
  • Frost protection
  • Reduction in nematode populations
  • Reduction in cases of botrytis

Harpin protein field results

Harpin proteins have been subject to over 20 years of research, including over 500 university and field trials on more than 40 separate crops. Some trials results include:

  • Increased yields of 8 – 20% in row crops
  • Increased yields from 10 – 30% in vegetable crops
  • Reduction of nematodes up to 50%
  • Increased shelf life of fresh-cut vegetables up to 3 – 7 days.

EPA approval of harpin proteins

Since harpin proteins facilitate a naturally occurring process, it was approved by the Environmental Protection Agency in 2000 for use in disease management and yield enhancement. In 2002, EPA conferred the Green Chemistry Challenge Award to harpin technology.

Harpin proteins applications

The two most effective applications for harpin proteins are a foliar application and a seed treatment. In fact, it is recommended to use both applications together for optimal results.

For foliar applications, harpin proteins are diluted in water and applied to the plant every 2 – 3 weeks throughout the growing cycle. Harpin proteins are concentrated and completely absorbed within 24 hours of application; therefore, users can apply Harpin right up to harvest. In addition to the concentration level, harpin proteins generate an entire-plant SAR. As such, it is not essential to maintain full coverage on every part of the plant.

The seed treatment method involves mixing seeds in a highly concentrated solution of harpin proteins and water. It is recommended to dissolve the solution for 30 minutes prior to treating seeds and to plant the seeds within 10 minutes of treatment.

Growers benefit from harpin proteins

By using harpin proteins, indoor growers will be able to maintain their plants’ elevated immune system and strong internal resistance to viral, bacterial and fungal plant pathogens. This results in a healthier, stronger plant. In summary, harpin proteins should be used as a plant-health promoter and plant-growth stimulator to produce stronger and more fruitful yields.