In the cholesterol biosynthetic pathway, the rate-limiting step is the reduction of HMG-CoA, which takes place in the existence of the HMG-CoA reductase enzyme. In case, when mutation takes place within the mentioned enzyme, it loses the tendency to combine with the substrate HMG-CoA, and as a consequence, the pathway fails to move forward and synthesis of cholesterol stops.
When the levels of cholesterol are high within the organism than the mutation of the enzyme HMG-CoA reductase results in less synthesis of cholesterol within the body of the organism and will eventually assist in balancing the level of cholesterol present within the body.
Xylem is one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. Hope this helps you out! :)