Improving Nitrogen Efficiency of Meadow Bromegrass and Orchardgrass

Background

Giberellin (GA) is a plant hormone that is essential for regulating growth and reproduction. In rice, it promotes vertical growth but inhibits the gene NGR5, known to encourage tillering, meaning plant exposure to high levels of GA could result in a reduction in the plant’s response to nitrogen fertilizer-induced tillering. While this process is well understood in rice and has been used for strategic breeding decisions to improve grain yield and reduce plant size, there is potential that a similar mechanism could be working in other grass species, like meadow bromegrass and orchardgrass. Understanding this could help forage breeders make more strategic moves in the future to improve the yield and nitrogen responses of grasses common in Canadian pastures.

Objectives

• To evaluate if gibberellin regulates N-induced tillering in meadow bromegrass and orchardgrass

What they Did

Researchers investigated the impact of modulating GA on tillering, yield, and nitrogen needs in meadow bromegrass and orchardgrass. They grew these grasses under three nitrogen levels – no nitrogen (0 kg N/ha), moderate nitrogen (150 kg N/ha), and high nitrogen (300 kg N/ha), and five different plant growth regulators (PGRs) that either increase or inhibit GA. The study was conducted both in field and greenhouse conditions. Key metrics assessed included tiller numbers, plant height, yield, root growth, and forage quality, such as acid and neutral detergent fiber content and crude protein content.

What They Learned

This study aimed to understand how altering gibberellin (GA) levels affects the tillering potential of orchardgrass and meadow bromegrass, and whether their tillering mechanisms are similar to those in rice. Greenhouse results showed that reducing GA levels with a GA inhibitor suppressed shoot growth but increased tillering, particularly under moderate nitrogen levels. Similar trends were seen in the no and high nitrogen treatments. Meadow bromegrass showed increased plant area under GA biosynthesis inhibitor treatment. Although GA biosynthesis inhibitors improved tillering, GA application did not significantly impact results, likely due to already optimal GA levels in the plants, leaving the excess GA to be degraded and excreted by the plant. In the field experiments, no treatment effects were seen, possibly due to uneven plant growth, weed competition, and practical challenges in tiller counting.

No significant effects were observed on root growth or fiber content, but plants applied with a GA biosynthesis inhibitor had a higher crude protein content, possibly due to improved nitrogen availability from limited shoot growth. Additionally, the team was able to identify partial gene sequences similar to the rice NGR5 gene in orchardgrass and a related meadow bromegrass species, poverty brome.

What This Means

The increased tillering observed in response to reduced GA levels, along with the presence of genes like rice’s NGR5 in these forage grasses, suggests that their nitrogen-responsive tillering is regulated by a mechanism similar to that of rice. Therefore, increasing the NGR5 levels in these forages could potentially stimulate increased tiller production in response to nitrogen fertilizer. Ultimately, these high NGR5 grasses could have enhanced nitrogen use efficiency, yield potential, and yield tolerance. In rice, this biological pathway is manipulated to develop new cultivars that produce less GA. However, this approach is not practical for forage grasses, as low levels of GA equals more tillering, but shorter plants. Therefore, this team plans to build off this work and explore gene modifications to enhance the stability of the NGR5 protein in the presence of high GA levels.