The role of roots in cysteine biosynthesis by Arabidopsis thaliana

O-Acetylserine(thiol)lyase (OASTL) enzyme catalyzes the cysteine biosynthes is in photosynthetic organisms. In mature Arabidopsis thaliana the highest activity level is observed in non-photosynthetic tissues like roots, that a lso show significant amount of protein detected by Western blot analysis. B y means of specific probes for cytosolic and plastidic OASTL isoenzyme tran scripts, cytosolic isoenzyme has been determined to be predominantly expres sed in roots, while the expression of the plastidic isoform is high in both green and non-green tissues. Sulfur starvation produces an increase on the OASTL activity level in all t he Arabidopsis organs examined, this effect being particularly significant in the aerial parts of the plant.