You haven't provided even sliiightly enough information.

What is your tissue/sample from?

How are you QCing your RNA, how are you preparing your cDNA, how much are you diluting your cDNA?

How have you validated your primers? Do you have a positive control?

When you say "Tubulin and 18s rRNA ) and was the same", it is not clear what they were the same as. Were they also poor, or also fine?

Dear John ,

Thank you so much for your comment.

indeed some missing informations.

The tissue is Basil plants.

I extract RNA using promega kits, before preparing cDNA I measure the RNA concentration so when use RNA for cDNA I have the same concentration in all samples. the dilution of cDNA is done 1/10 and 1/20;

I meant tubulin gave not clear results like ACTIN

and as I said I am still learning how to optimize the conditions; no positive control

the design of my experiment is : I have control not treated (0 g salt without fungus, and control treated 0 g salt with fungus , and then the salt treatment , 6 g salt without fungus and 6 g salt with fungus)

Thank you

More detail is appreciated (though you still haven't quite provided enough).

So, top tip for RNA QC:

Pay close attention to the 260/230 ratio, as this reflect guanidium salt contamination. Ideally you want this to be >2.0, but in my experience anything above ~1.7 works ok. If it's below this, you will get poor cDNA synthesis (as the guanidium inhibits the enzyme).

If this is the case, you will likely need to clean your RNA -simply reprecipitating it usually does the trick.

Now, it's a ratio, so if your RNA yield is poor (