% start out with plain.tex,
% having TFM-based CM fonts preloaded in \fam 0...3

% load OT math font in \fam 4
\input luaotfload.sty
\font\4={name:XITS Math:mode=base;script=math} at 10pt
\font\5={name:XITS Math:mode=base;script=math;ssty=0} at 7pt
\font\6={name:XITS Math:mode=node;script=math;ssty=1} at 5pt
\textfont4=\4 \scriptfont4=\5 \scriptscriptfont4=\6

%\tracingall
$$\Uradical "4 "221A {x}$$
$$\Uroot    "4 "221A {3}{x^3+y^3}$$

$$
  \Udelimiterover  "4 "23DE {a+b}
+ \Udelimiterunder "4 "23DF {a+b} = C
$$

$$
  \Umathaccent    "0 "4 "23DE {a+b}
+ \Umathbotaccent "0 "4 "23DF {a+b} = C
$$

$$
A \mathrel{\Uoverdelimiter  "4 "2192 {a+b}}
B \mathrel{\Uunderdelimiter "4 "2192 {a+b}}
$$

\bye