I want to do RNA seq comparison between two situation in case of plant. is it necessary to use technical replicates in this case. RNA seq is new emerging technology to detect allelic isoform expression as well as DGE in different situation.?
Since the technical reproducibility of RNA-seq is rather good technical replicates are not really necessary. This means that if one skilled person uses the same library prep kit on 5 aliquots of the same sample and creates 5 libraries, and sequences on the same sequencer, the result should be 5 times the same data, approximately. There will be some "Poisson variability", but technical variability is limited.
(Note that if a different kit is used results will be different, obviously.)
But the difference between two plants (of the same species) will be bigger, because of biological variability. So in order to accurately quantify a difference between two groups, you need to assess the intra-group variability, the biological variability. To properly characterize the biological variability you need biological replicates.
The very minimum of replicates would be 3 samples per group. But if you can afford it, definitely do more samples.
I don't think "RNA seq is new emerging technology". You don't need technical replicates but you need biological replicates. For allele isoform expression, you need many samples ( as you need to work with genotypes ) with more sequencing depth than that is required for DGE.