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Finding more green for corn crops

These results from a 119-year-old experiment could strengthen corn stalks and boost yields

The goal with the stay-green gene is to prolong photosynthesis without blocking movement of carbon and nitrogen to the seed.

What’s your definition of a “long-term” research project? Depending on its goals and its methodologies, five years might seem like a long time. Something like 10 or 20 years could be a real stretch. But here’s an experiment dating back 119 years that’s in a different category altogether, and it has just produced a new and exciting result.

The team of researchers from the University of Illinois, along with valuable input from Corteva Agriscience, has been trying to identify a specific “stay-green” trait that they’ve seen in the project’s corn plots, and they’re attempting to tie it to a specific gene that they could then export to new hybrids.

It’s a far cry from the project’s earlier goals, which included testing seeds taken from plants exhibiting low-protein and high-protein characteristics, with the goal of studying energy remobilization in the seed and the leaf.

Dr. Stephen Moose, co-author of the research paper “Identification and characterization of a novel stay-green QTL (qualitative trait locus) that increases yield in maize,” has been involved in work on the “stay-green” gene for much of the past 20 years. As Moose, a professor in the department of crop sciences at the University of Illinois, notes, most seed companies have hybrids with some sort of stay-green rating in their catalogues but this research is close to identifying a specific gene responsible for that trait.

The initial study began in 1896 but in the 1920s, the university stopped the project and discarded most of the seeds. Fortunately, says Moose, one researcher didn’t get the message, so he kept some of the seed and continued the work.

It was around 2005, while reading some of the notes from the 1930s, that Moose found a mention of the low-protein line staying greener longer than the high-protein line. That finding sparked a collaborative effort between Moose and researchers from Pioneer Hi-Bred, now part of Corteva Agriscience. The company’s work, he recalls, started slowly, focusing more on root structure. By 2012, Moose and Pioneer’s researchers were working on a program linked to nitrogen use and understanding the genetics involved. It was based on that work that a specific gene — NAC7 — was linked to the stay-green effect.

“We crossed the two plants, let them shuffle other genes like a deck of cards and then dealt out a couple of hundred hands,” says Moose. “The high-protein plant had this big bushy root and lots of branches and fine hairs, and the low-protein line was almost like a tap root with a few branches.”

The below-ground differentiation surprised researchers at Pioneer, and they tried to determine any differences in above-ground growth as well.

“There they noticed — and we had seen it too — that there were differences in how fast the first leaf was dying,” says Moose. “So the stay-green effect, which we knew happens later in the season, seemed like it was happening earlier. I had noticed on occasion in the field that very early the leaves would die. When Pioneer researchers were doing it, they were work- ing in a controlled environment where it became much more obvious.”

Digging deeper

Another group within Pioneer began examining ties to the corn leaf and continued their work beyond the end of Moose’s team’s project. That group started with the roots and shifted to nitrogen response and grain yield. But the stay-green trait wasn’t part of that project — and it ended before they had identified the specific gene. The researcher at Pioneer continued working on it and contacted Moose in 2015 to let him know he’d made progress on the genetic mapping and invited him to rejoin the research.

Work continued with the focus on determining the differences between the grain and the roots. In many crops, including corn, there are numerous genes that contribute to the stay-green effect.

“Stay-green can be a two-edged sword, where the plant has a problem sending to the seed the carbon and the nitrogen accumulated in the leaves,” explains Moose. The effect is almost cosmetic as the leaves will stay greener but grain-fill may not be as extensive. “The good stay-green is where the plant is continuing to live longer and photosynthesize, but not block the movement of the carbon and the nitrogen into the grain. That’s a very rare kind of stay-green but that’s what this NAC7 gene does — it prolongs photosynthesis and lets the other functions happen.”

Other researchers have found a stay-green component in wheat, rice and sorghum, where genes contribute to a similar effect. But they’re usually the more cosmetic type, so they block remobilization of the nutrients, with less than desired effects on yield.

According to Moose, the stay-green gene normally initiates a process to dismantle chlorophyll and discontinue photosynthesis. Its presence — when detected — tells the plant to start remobilization of energy to the grain.

“But you’re not capturing any more energy — no more photosynthesis means there’s no more carbon fixing,” he says. “You’re shutting down the leaf and when this gene is lower or reduced, that leaf stays alive longer. The program of taking down photosynthesis doesn’t happen but the program of starting to divert to the grain is continuing. There are actually multiple controls and NAC7 regulates the photosynthesis part.”

The NAC7 trait turns on genes that degrade the photosynthesis “machine” which is made up mostly of proteins and chlorophyll. It starts to dismantle and recycle the nitrogen tied up in the chlorophyll and photosynthesis system and sends it to the seed to promote its growth.

The researchers from Corteva were able to develop corn plants that expressed different levels of NAC7 and found those with the lowest levels had the higher stay-green effect. They tested the plants in greenhouses and under field conditions across the U.S. during a two-year period. In comparisons against conventional hybrids, the low-level NAC7 corn yielded 4.6 bu./ac. higher (on average) without added fertilizer (beyond what growers would typically apply).

Moose speculates that Corteva’s interest in this work points more to keeping the stalks and leaves greener longer while allowing the grain to continue filling. He believes it’s more a means of reducing lodging and ear-drop.

“The yield advantage might come from that as well,” he adds. “It might not be a physiological process where there are more nutrients to the grain or an increase in photosynthesis — that’s harder to measure. But less lodging could increase yields.”

The genetics involved

The focus on this work now turns to the potential for its commercial availability, and Moose concedes this is still a few years away. In studying the NAC7 trait, researchers also noted other genes that influence the remobilization process, changing the way the seed is making protein and storing it, and how the seed grows in response.

“These genes work in different ways and we’ve tracked down some of those,” says Moose. “Many of them are not yet defined but we have our favourite one and we’re trying to use CRISPR to do that.”

From a trade and international regulatory perspective, there is concern about the technologies used in the mapping and genetic work to establish this new NAC7-traited hybrid. The team of researchers at Corteva used RNAi (ribonucleic acid interference) technology to conduct the initial NAC7 study, to turn down or silence the gene. But results can be variable, leading Corteva researchers to opt for CRISPR (clustered regularly inter- spaced short palindromic repeats) technology to remove the gene completely. It’s simpler, less costly and more consistent than RNAi, and it’s more likely to meet import requirements in other countries.

“Those countries are not going to be able to track it because you could claim that it was out there already and that we just bred it in and there’s no way to tell the difference,” says Moose. All developers have to do is show potential trade partners that they’ve recreated something that already exists in nature — and that’s the advantage with CRISPR. “All we can say is that people have eaten the low-protein corn that already has a natural mutation in NAC7. In fact, a local brewery makes beer out of it and there’ve been no problems.”

This article was originally published in the January 2021 issue of the Corn Guide.

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CG Production Editor

Ralph Pearce

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