Tissue culture

In an earlier post, I discussed our efforts to select new breeding lines with genetic resistance to potato virus Y.  This is just one part of a larger strategy for virus management in the breeding program.  Another key part that has been gaining momentum in recent weeks is our tissue culture collection.  Tissue culture allows us to maintain and propagate many plants in a small space, and to do so under aseptic conditions (i.e., without introducing disease).  Each plant grows in a test tube filled with semi-solid growth media, and many racks of tubes can fit inside an insulated growth chamber with controlled lighting and temperature:
Once a new breeding line is deemed to have significant commercial potential, which typically occurs after 6 years of field testing, young sprouts from the tubers are introduced into tissue culture for the first time.  At this point the plants are most likely not disease-free because they have been propagated in the field for many years, so the plants are put through a virus eradication process that involves exposure to certain antiviral chemicals and environmental conditions.  Grace has just transferred our first batch of breeding lines onto the antiviral media, so we’ll provide more updates on their progress later this year.


Marker-assisted selection for PVY

One of the key developments in plant breeding in the last 20 years has been the use of molecular markers to select for genetic traits in segregating populations.  Disease resistance is perhaps the prime example of a trait amenable to marker-assisted selection.  Thanks to the diligent work of Grace Christensen (research specialist for the breeding program), we are now able to use genetic markers to track two resistance genes for potato virus Y (PVY).  One gene, designated Ry(adg), was introduced to cultivated potato (S. tuberosum ssp. tuberosum) from S. tuberosum ssp. andigena, and the other gene, Ry(sto), was introduced from the wild species S. stoloniferum.  The presence of either gene confers extreme resistance to PVY, which is very desirable as PVY is the leading reason why seed potatoes fail certification in Wisconsin.

The markers for both genes can be assayed using PCR and gel electrophoresis.  The gel image below shows our result for the Ry(sto) marker known as YES3-3B.

STO analysis 2

The YES3-3B marker linked to Ry(sto) produces a 284 bp fragment, which is visible as a third band on the left side.  To ensure the assay was working, we included W8946-1rus as a positive control, which is known to have inherited Ry(sto) from the USDA-ARS breeding line PA98V1-2.  For a negative control we used the PVY-susceptible line W6234-1rus.  Of the four new breeding lines tested in this gel, only AW08417-6rus showed a banding pattern indicating the presence of Ry(sto).  The other three did not inherit the resistance gene.

End of the Season, End of an Era

On Monday we harvested our last field trial at the Hancock Research Station, marking the end of the 2014 field season for the breeding program.  One of the key changes we made this year was to group all Year 4+ breeding lines from the same market category into a single trial, rather than evaluating the Year 4 and Year 5 cohorts separately (and lumping all market types together).  The biggest trial was our chip trial at Hancock with 229 entries and 401 plots, followed by the fry processing trial containing 100 entries and 221 plots (also at Hancock). Our fresh market trials were subdivided into russet, yellow, and red categories, and portions of these trials were replicated on commercial farms in Wisconsin.


One of the highlights from 2014 was the success of our crossing program. More than 500 cross-combinations were made, with a focus on elite x elite crosses to maximize the probability of finding progeny with commercial potential. We also enriched our breeding populations for PVY resistance by “backcrossing” PVY-resistant chip and russet breeding lines to elite types.


On a bittersweet note, 2014 marks the end of Bryan Bowen’s career with the breeding program.  More than twenty years ago, Bryan moved to Rhinelander to manage the UW research station and has led the transformation of that facility into an indispensable resource for the breeding program.  I am very grateful for his mentoring during my first year on the job.


Field Days 2014

On July 18 there was a field day at the Rhinelander Research Station to showcase the work of the potato breeding program and other research taking place there:



A few days later we participated in the potato field day at the Hancock Research Station to tell growers about our new varieties:


Crossing and “true” seed

Several weeks are spent each summer cross-pollinating our best breeding lines to generate tens of thousands of true seeds that are genetically distinct.  Here is a photo of Lance and Grace removing the anthers from collected flowers:


After drying overnight, the anthers are shaken and shed pollen that is collected in tubes.  The true seeds are inside the green fruit, or berries, that develop after pollination:




These seeds are called “true” because they are seeds in the botanical sense, whereas the “seed” that is typically used for planting potatoes is actually a piece of tuber.


Making a better French fry

Two weeks ago we broke new ground at UW-Madison by performing quality testing for French fries.  This grew out of our participation in a nationwide effort to develop new varieties for the French fry market with lower acrylamide, which is a suspected carcinogen that forms when potatoes (and other starchy foods) are fried.  It turns out that we have lots of breeding lines with lower acrylamide than current varieties--the problem is they don't make good fries.  This is not too surprising as we (and many other public breeding programs) have not been selecting for the textural quality traits required for success in the fry market.  To remedy this situation, we are working with commercial fry processing companies and the USDA Potato Research Worksite in East Grand Forks, MN, to develop a screening protocol that can be piloted this winter in our breeding program.

Thanks to the work of postdoctoral researcher Dr. Yi Wang (pictured at left, below), the initial tests of this protocol began two weeks ago.  Yi and I scored the incidence of defects due to color variation and internal texture problems in two dozen samples of fries.  My colleague Dr. Paul Bethke (pictured at right, below) joined the effort this past week.


Tuber-unit planting

The ground is finally ready for planting at the Rhinelander Research Station. Last week we planted the seed maintenance plots for our most advanced breeding lines. Each plot contains 80 plants (or hills), which is enough to repopulate an 80-hill plot next year and provide seed for production trials at several locations, such as the Hancock Research Station. The seed drill pictured below is a specialized machine that allows us to plant in "tuber-units," using seed tubers that have been partially cut (e.g., two cross-cuts to make four seed pieces). The tuber is broken into pieces at planting time and placed sequentially in the slots of a rotating wheel, with a skip between tubers. The horizontal wheel moves the seed pieces over a chute that drops them into the furrow.


The tuber-unit planting method is used to facilitate the removal of virus-infected plants during the growing season. If the foliage of one plant is observed to have mosaic virus symptoms, most likely all of the plants from the same seed tuber are infected, and the whole group of plants is removed from the plot.


We’re off to a slow start due to heavy rain this spring, but we did manage to get in one day of planting at the Hancock Research Station in April. Here is a photo of us planting a replicated yield trial for fresh market russets. From left to right: Umesh, Grace, Bryan, and Jeff.
Each potato “seed” piece is a cut tuber, around 2 oz. in weight, which we planted by hand at a spacing of 1 ft. within the row. The breeding program staff in Rhinelander spends many days each spring cutting seed.

Tuber internal quality

One of the key traits for a successful potato cultivar is to have a low incidence of internal defects.  To score this trait, the breeding program cuts open 30-50 tubers per plot during our initial post-harvest evaluation.



The photo below on the left shows the presence of hollow heart defect in a plot of Russet Burbank potatoes.  The internal quality of the UW breeding line W9133-1rus on the right looks better.