Which Plant?

Hello! this may be due to the increasing age of the plant these develop thicker cuticles and have reservations structures and therefore sometimes become more tolerant and not necessarily more resistant to the stress

Don't know which plant and research you are studying. But, a transgenic plant can be chimeric, such as transgenic tobacco plants. We can propagate them vegetatively in tissue culture by cutting a tip and re-plant it into another box. If the tip/shoot derived from a 'good' part (with 'resistance') of the chimeric plant, you can obtain a progeny with resistance. You can also cut a tip, tissue-culture it, get roots, and move it to greenhouse set seeds for seedlings. If this cut is from the 'bad' part (no resistance) of the chimeric plant, you can end up obtaining non-resistant seedlings.

Well, trait phenotypic behavior can change shifting from vegetative to grain propagation in virtue of the Mendelian segregation. even though you did not expand on your situation, I will illustrate my point: if the trait (disease resistance) is dominantly inherited, and the plant is heterozygous, the progeny from vegetative propagules will express resistance to the disease, while the progeny from seeds will be a mixture of resistant and susceptible individuals. Hope this helps.     

Thank you for your answer Mr Habyarima. As far as my plant is concerned the resistance is heterozygous thats why the seedlings are showing susceptible reaction

Have a good time there in Hyderabad. I have been there, in Patancheru, at ICRISAT headquarters and Hyderabad University.

As pointed by others your question is quite vague. Seedling were generated by vegetative propagation or sexual reproduction is most important of all. Although seedling should come from seed which should be produced by sexual reproduction unless plant you are dealing with is apomictic. If the seeds are by vegetative reproduction, there is no chance of segregation. Susceptibility in seedling can also be due to varied response to pathogen at seedling stage. A grown up plant is more hardy and will be more resistant.  

Mr Goel Sir, the seedlings are from seeds only ie fertilized seeds.I mean they are from sexually reproduced. The screening we did in small seedlings and they are 4 months old. So is the screening valid in 4 months old plant?. The plant studied here is Piper colubrinum a wild species of Piper nigrum(Black pepper). The cuttings taken from the mother plant are showing resistance to Phytophthora when leaf inoculation is done, But when the seeds from the plants are germinated and when seedlings where screened at 5 leaf stage they are showing susceptibility in the first generation but in subsequent generation they are showing an intermediate response(resistance) to Phytophthora. The resistance reaction is measured by the lesion diameter produced on the leaf upon inoculation. The vegetative cuttings from the mother plant is showing reduced lesion size and HR reaction upon inoculation.

Hi Vijesh,

This is getting interesting... Isn't the black pepper self-pollinated plants? Is the disease-resistant gene genetically transformed recently (This can have segregation in the offspring and progeny.)? or Is it just a disease-resistant line (not through genetic transformation) (the 'resistant' allele might have been 'fixed')? Do you know the genetic background of this line? Since you mentioned that it is a 'wild' species (a common germplasm for resistant-gene source), I bet it is not a transgenic one.

Hi Dr Yau

The particular plant in question Piper colubrinum is a wild species of Black pepper or Piper nigrum. The Piper colubrinum has been designated as resistant to Phytophthora by studies done on it previously....there is no genetic transformation done as yet. Breeding between Piper colubrinum and Piper nigrum has not been sucessful as yet. it is not a transgenic one just a wild species resistant to Phytophthora as per the latest information available till now from other research conducted on p.colubrinum. Both Black pepper and OUR plant in question are self pollinated. Hope this clarifies...

Hi Vijesh,

Thanks for clarifying it. Since they are self-pollinated plants. The plants may have already reached the status that most its genomic loci are homozygous. And no segregating resistant (vs. non-resistant) trait can be observed in the progeny if this locus is also homozygous (assuming this trait is controlled by a single gene). Therefore, theoretically the resistant phenotype of offsprings should resemble the parental lines.

I still thought it might be because the cuttings have some morphological characteristics that make them resistant. As you say that medis the resistance reaction produced by the lesion diameter on the leaf upon inoculation, perhaps making anatomical cuts inoculated leaves and not, find something that will help explain ...

Although your plant is highly self pollinated, it still can have hetrozygosity. Self pollinating plants can be mostly self pollinating but still can cross (unless flower is cleistogamous) albeit at low percentage. At the same time due to various reasons, heterozygosity can prevail even after generations of selfing. So it is possible that you are seeing segregation. You have to be careful in your biological assay, as pointed by Elisa.

Dr.Goel:

Can the segregation be due to the resistance being heterozygous in nature

During the first generation plants(seedlings) are showing susceptibility but in subsequent generation there is an intermediate phenotype visible. How is that possible?

It is not necessary that resistance is being controlled by only one locus, there might be an involvement of multiple loci. This can explain intermediate phenotype also. You need to study the inheritance of the trait. Is it being inherited in a Mendelian  fashion or some kind of gene interaction. As I said earlier, if I am in your place, I will first pay attention to Elisa's advice, do some anatomical study of the infected plant in time dependent manner and see what is causing the resistance. It is quicker than a gene interaction study and might answer a lot of questions. Make sure to include parents in your study, it will give you a comparative account of anatomical differences between the parents which might be reason for resistance. Resistance in many instances is just tolerance due to anatomical features like thicker cuticle etc.

As I understand you stated that during the first generation (I guess you meant at the seedling stage), seed-derived plants showed susceptibility; but, in subsequent generation (I guess you meant at later growth stages) there was an intermediate phenotype visible. Well, if I correctly interpreted your statement, your situation here would imply you are in front of "APR" phenomena, that is Adult Plant Resistance. APR is explained by a combination of multiple minor genes conferring a kind of durable resistance. This is known even in other crops species like in wheat (APR) resistance to stripe rust. I think this hypothesis can be confirmed in further investigations. You can easily dissect your trait by means of either conventional QTL analysis or association studies. If you have interest venturing in this research area, I am happy to offer assistance. There are several mathematic algorithms to help process your data. Best regards.