Breeding for horizontal resistance

[DarJones]

Nathan, if you read the potato article, you will see that when it was written, they already were realizing that the horizontal/vertical paradigm was a nodigm. Better terminology would be to refer to single gene barrier type resistance as a key/lock resistance mechanism based on hypersensitive response, to chemical protectant mechanisms such as tomatene or solanine as antifungal mechanisms, and to tissue modifications such as trichome or stomata changes as structural resistance mechanisms.

[nathanp]

I haven't read it in it's entirety yet, but am I correct that what is commonly meant by Horizontal Resistance is the accumulation of multiple resistance mechanisms. Like you said, multiple single gene mechanisms (lock/key or otherwise), plus chemical mechanisms and tissue modification. Even if the original article is slightly simplistic, the principle seems to be that these are things that you can might be able to mass select for over time, rather than just focusing on inbred characteristics.

[Tom Wagner]

Thanks to everyone talking about "BREEDING FOR HORIZONTAL RESISTANCE" and please forgive my absence in this discussion of late. I have been focused (or has it been unfocused?) on training I disagreeociates, cooperators and interns on my agenda for this years breeding projects. Rather than trying to make my response expository, I am using near bullet points so that I can explain them point by point to my interns on a one on one basis.

Organization of genes controlling disease resistance

Nineteen single dominant genes (R genes) for resistance to viruses, nematodes, and fungi

Fourteen of those genes are located in five "hotspots" for resistance

Quantitative trait loci (QTL) for resistance have been found on all 12 chromosomes of potato
genetic clustering with R genes

polygenic resistance vs single gene resistance

19th century epidemic was caused by a unique genotype, HERB-1, that persisted for over 50 years. HERB-1 is distinct from all examined modern strains, but it is a close relative of US-1,
The mtDNA haplotypes of the plant pathogen Phytophthora infestans-86% percent of the herbarium specimens from historic epidemics were infected with the Ia mtDNA haplotype

R1, the first of many, is located within a hot spot for pathogen resistance on potato chromosome V
effector-R gene interactions trigger a range of plant defenses, such as the production of compounds toxic to the pathogen

Some so-called resistant varieties can resist some strains of blight and not others, so their performance may vary depending on which are around. These crops have had polygenic resistance bred into them, and are known as "field resistant".

The importance of allowing organically grown infested fields of potatoes to have refuse or volunteer plants to help spread the late blight into the next season.

Ridging (or hilling) is minimized to encourage tuber contamination by blight. Shallow planting forces the tubers to be close to the surface allowing better tuber infestation with rain washing the spores into the horizon of the tubers.

Massive resistance (R) gene stacking is considered to be one of the most promising approaches to provide durable resistance to potato late blight

The R3 complex locus on chromosome XI in potato is an example of natural R gene stacking, because it contains two closely linked R genes (R3a and R3b) with distinct resistance specificities to Phytophthora infestans. Remembering that R1 is on chromosome 5. The example of an R-gene cluster with multiple genes recognizing different races of P. infestans.

Sequences with unknown function have been defined as resistance gene analogues (RGAs)

a series of 11 R genes derived from S. demissum
Of these, R1, R2, R3a/b, R6, and R7
have been localized on the genetic maps of potato
However, these R genes confer race-specific resistance and those that were
introgressed into potato varieties, mainly R1, R2, R3, R4, and
R10, were quickly overcome by late blight after a time. However,with additive gene introgressing and recurrent breeding selection some could be useful again in a horizontal way.




Here in the PNW I can increase the chances of encouraging late blight to screen my accessions with the best humidities and temperatures to help make the late blight more relevant…..check out the following environments..

• A Beaumont Period is a period of 48 consecutive hours, in at least 46 of which the hourly readings of temperature and relative humidity at a given place have not been less than 10 °C (50 °F) and 75%, respectively.[20][21]
• A Smith Period is at least two consecutive days where min temperature is 10 °C (50 °F) or above and on each day at least 11 hours when the relative humidity is greater than 90%.

Vertical vs Horizontal is a confusing subject....not that using qualitative and quantitative is much better...but......in my TPS breeding work I am always trying to add qualitative traits into my vast pool of quantitative trait laden clones and true seed. It gets to the point where I don't even talk about a specific genes...what do I know when all I know is what I see?