Use the protocol as suggested by manufacturer. It works well and moreover you are using Arabidopsis leaves so you should get easily 500-1000 ng/ul when you start with 100 mg leaf tissue and finally resuspend the RNA pellet into 30 ul DEPC treated sterile water.

Did you try adding glycogen as a co-precipitant?  I believe that is recommended if your sample is more dilute.

i am using trizol mehod and the leaf sample is too low. almost 17 mg. i need a protocol that can even extract RNA even from less amount of tissue. 

Ensure that the ratio of 10 ml of trizol per 1gram of tissue is adhered to. In addition, your frozen tissues should be grounded into completely powderish form before mixing with the required amount of trizol. 

Lastly overnight isopropanol precipitation has proven very useful for me in obtaining high concentration of rna extracted from arabidopsis.

Give these areas some fine tuning and i believe you will be smiling in a couple of days with some great results. keep up the good work.

Thanks alot Robert Pokoo. i will try your suggestion

i recommend the Omega kit for plant Rna extraction. You can find the Cat. No on the internet. But I am not sure whether this kit be good for small amount of leave or not.