You could also doFinally, to get the whole thing with GNUvalue of both grepNote and some regex magicID, you could do:
$ grepawk -oPF"\t" '\tmRNA\t'$3=="mRNA"{n=$9; sub(/.*Note="\K[^"]+'*Note="/,"",n); sub(/"$/,"",n); sub(/.*ID=/,"",$9); sub(/;.*/,"",$9); print $9","n}' file.gff
Nitab4.5_0006317g0010.1,Peptidase S59%2C nucleoporin
Nitab4.5_0006317g0020.1,Putative S-adenosyl-L-methionine-dependent methyltransferase
Nitab4.5_0006317g0030.1,Unknown
Nitab4.5_0006317g0040.1,Heavy metal-associated domain%2C HMA
Nitab4.5_0006317g0050.1,Unknown
Nitab4.5_0002367g0010.1,Auxin response factor%2C B3 DNA binding domain%2C DNA-binding pseudobarrel domain%2C AUX/IAA protein%2C Aux/IAA-ARF-dimerisation
Nitab4.5_0002367g0020.1,Cellulose synthase
Nitab4.5_0002367g0030.1,Cellulose synthase
Nitab4.5_0002367g0040.1,Zinc finger%2C RING-type%2C Zinc finger%2C RING/FYVE/PHD-type
Nitab4.5_0002367g0050.1,SAC3/GANP/Nin1/mts3/eIF-3 p25%2C 26S proteasome non-ATPase regulatory subunit Rpn12
The -o makes grep print only the matching portion of the line, and the -P enables Perl Compatible Regular Expressions (PCRE). The \K, a PCRE featurePersonally, means "discard anything matched before this point". Sohowever, the regex looks for mRNA surrounded by tabs (\tmRNA\t; note thatI would do this means it will match if mRNA is the entire value of any field, but that should never happen in a GFF file), then 0 or more characters (.*) until it finds a Note=". Everything up to here is now discarded by the \K, and we only look for the longest stretch of non-" characters ([^"]+).perl instead:
$ perl -F'\t' -lane 'if($F[2] eq "mRNA"){/ID=([^\;]+).*Note="([^"]+)/; print "$1,$2"}' file.gff
Nitab4.5_0006317g0010.1,Peptidase S59%2C nucleoporin
Nitab4.5_0006317g0020.1,Putative S-adenosyl-L-methionine-dependent methyltransferase
Nitab4.5_0006317g0030.1,Unknown
Nitab4.5_0006317g0040.1,Heavy metal-associated domain%2C HMA
Nitab4.5_0006317g0050.1,Unknown
Nitab4.5_0002367g0010.1,Auxin response factor%2C B3 DNA binding domain%2C DNA-binding pseudobarrel domain%2C AUX/IAA protein%2C Aux/IAA-ARF-dimerisation
Nitab4.5_0002367g0020.1,Cellulose synthase
Nitab4.5_0002367g0030.1,Cellulose synthase
Nitab4.5_0002367g0040.1,Zinc finger%2C RING-type%2C Zinc finger%2C RING/FYVE/PHD-type
Nitab4.5_0002367g0050.1,SAC3/GANP/Nin1/mts3/eIF-3 p25%2C 26S proteasome