Auxins and ABA Promote Vascular Function and Reduce Bitter Pit of Honeycrisp Apples

May 06, 2022

By Anna Wallis

Todd Einhorn, associate professor in Michigan State University’s Department of Horticulture, recently shared his M.S. student, Chayce Griffith’s, work demonstrating that auxin and abscisic acid (ABA) applications can be used to mitigate bitter pit. This article summarizes the main findings of his presentation. You can also watch the recording of Einhorn’s presentation.

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Honeycrisp with increasing levels of bitter pit. Photo by Beaudry and Yildiz-Ocal 2012.

Bitter pit is a calcium-related disorder that commonly occurs in Honeycrisp. Symptoms include sunken pits at the calyx end of the fruit caused by calcium deficiency and nutrient imbalance. Many factors can contribute to this nutrient imbalance and effective delivery of calcium within the fruit. First, calcium is only transported via xylem (not phloem). Xylem within the fruit is under significant strain, beginning as early as 30 days after full bloom. By the end of the season, there may be few remaining intact xylem connections. In addition, calcium competes with other nutrients, notably potassium (K), magnesium (Mg) and nitrogen (N).

Auxins, a group of plant hormones, play an important role in calcium transport in several ways. Indole acetic acid (IAA), the native auxin in plants, is well known to be involved in xylem differentiation as well as the flexibility of xylem tissue. Therefore, IAA promotes better vascular structures needed for effective Ca delivery. In addition, auxin mediates the transport of calcium.

Abscisic acid (ABA) is another plant hormone that plays a role in calcium delivery. ABA increases Ca delivery directly by increasing xylem function and by regulating Ca genes. It may also act indirectly. ABA reduces transpiration, leading to the direction of more water and calcium to the fruit (as opposed to leaves). Further, it indirectly inhibits shoot growth by closing stomata which reduces gas exchange. Both of which result in better Ca supply to fruit.

Based on this information, the Einhorn lab has been testing the hypothesis that applications of auxin and ABA will increase xylem differentiation and the functional life of vascular tissues in fruit, which will in turn improve fruit calcium concentration and ultimately reduce bitter pit.

In experiments using Honeycrisp on G.11 rootstock (bitter pit-susceptible trees), IAA, NAA or ABA was applied three times: 30, 45 and 60 days after full bloom (DAFB). Applications timings were no earlier than 30 DAFB so that there was no thinning effect, and no later than 60 DAFB because xylem differentiation is complete by this point. Fruit were sampled and dyed to visualize xylem bundles at 86, 107 and 136 DAFB (approximately three weeks after the last treatment, two weeks later and at harvest). Each material was tested at different rates.

As expected, overall vascular bundle functionality of non-treated Honeycrisp apples diminished both over time. In addition, vascular bundles were fewer closer to the calyx end of the fruit (see photo below).

All three materials had a positive effect on xylem functionality and bitter pit. IAA significantly increased vascular bundles at the earlier sampling times (86 and 107 DAFB) in a rate-dependent manner (5, 10 or 20 ppm). NAA applied at a rate of 5 or 10 ppm was effective at all sampling times, but 20 ppm was toxic to the fruit. ABA applied at a rate of 75 and 150 ppm was also effective at increasing the number of functional bundles at 107 and 136 DAFB.

Both IAA and ABA significantly reduced bitter pit by 65% in both harvest and post-harvest evaluations. NAA reduced bitter pit by about 50% at harvest and post-harvest, albeit not at the level of significance tested. These changes were accompanied by an increase in xylem-mobile nutrients (Ca and Fe) in peel tissue, as well as an improvement in K+Mg/Ca ratios.

Einhorn’s group is continuing work in 2022 evaluating IAA, ABA and NAA. In addition, they are investigating other forms of auxin that may be commercially available, such as IBA and the gibberellin inhibitor, PcA. We expect growers will be excited about this work and interested in trying this on their farms. However, research results are early, and we recommend waiting for an additional year of research to better inform commercial applications.

Source : msu.edu

Video: Can Gene Editing Give Seeds a Stronger Start?

Reagan Reed has a way of describing his research that might make a plant breeder pause.

“I’m the sort of person who likes to break things to understand them,” he says.

What he breaks are genes involved in seed germination. Reed, a Ph.D. candidate at the University of California, Davis, is investigating whether changing those genes can help seeds establish healthy plants under a wider range of conditions.

Reed recently attended the National Association for Plant Breeding meeting in College Station, Texas, as an NAPB Borlaug Scholar. His work speaks to a question that matters well beyond a research lab: what does it take for a seed to get a crop off to a strong start?

A seed that germinates quickly, alongside the others planted with it, spends less time vulnerable to stresses before seedlings become established.

“If you’re able to bridge that gap between germination to stand establishment, then that’s going to give you the highest probability of having a solid yield that year,” Reed says. “There’s a bunch that can go wrong after that, but it maximises your yield from the start, at least.”

Reed works in Dr. Imtiyaz Khanday’s lab at UC Davis, where his research focuses on seed quality in tomatoes and potatoes. He brings an unusual academic background to the work: at Skidmore College, he studied both biology and Latin.

Reed believes that combination prepared him for research. Translating an ancient text means studying each word, weighing its possible meanings and using context to reach the best interpretation. There is rarely a way to know with complete certainty what an author intended. Science, he says, requires a similar willingness to work through uncertainty.

“Attention to detail and comfortability working with uncertainty,” is how he sums up what Latin taught him.

In the lab, uncertainty can produce a surprise. Reed describes changing one gene and seeing little effect. Changing a second gene also appears to do little. Put both changes in the same genetic background, though, and the result can be substantial.

“I break this gene, nothing really happens. I break this other gene, nothing really happens. I combine them into the same genetic background, and now there’s a massive effect,” he says.

Those interactions matter as researchers gain the ability to edit multiple genes at once. They also complicate any simple account of how a trait works: improving a seed may depend on understanding what several genes do together.

For Reed, the question is whether that knowledge could help seeds germinate reliably when conditions are difficult. Some of the genes he studies prevent germination under certain conditions. That response may serve a plant well in one setting while limiting its usefulness in agriculture. He wants to learn what happens when those genes are changed, and whether the result improves establishment.

Yet a discovery that works in a lab still has a long way to travel before it reaches a field. Reed argues that agricultural biotechnology faces an economic challenge as well as a scientific one. New tools can create value for farmers, he says, but developing them requires a way to support the work in an industry where margins can be thin.

His fellowship with Flagship Pioneering gave him a view across AI, agriculture-related work and human health research. It also reinforced the importance of bringing people with different expertise into the same conversation—a benefit of gatherings such as the NAPB meeting.

“You need to have a team of people with different backgrounds who are willing to work together and approach the same problem in different perspectives and different ways,” he says.

That thinking connects the different parts of Reed’s path. Latin taught him to examine details without expecting a perfect answer. Gene editing lets him test what individual genes do—and what happens when their effects combine. Working with people beyond his own field helps him ask what those findings could become.

He came to agriculture because of its reach: the possibility that work by a small group of researchers could eventually make a difference on a much larger scale. His immediate focus is smaller and more precise. He wants to understand what helps a seed get through its first days and become a healthy plant.