A starch‐accumulating mutant of Arabidopsis thaliana deficient in a chloroplastic starch‐hydrolysing enzyme

Summary The aim of this work was to identify enzymes that participate in the degradation of transitory starch in Arabidopsis . A mutant line was isolated by screening leaves at the end of the night for the presence of starch. The mutant had a higher starch content than the wild‐type throughout the diurnal cycle. This accumulation was due to a reduction in starch breakdown, leading to an imbalance between the rates of synthesis and degradation. No reduction in the activity of endo‐amylase (α‐amylase), β‐amylase, starch phosphorylase, maltase, pullulanase or D‐enzyme could be detected in crude extracts of leaves of the mutant. However, native PAGE in gels containing amylopectin revealed that a starch‐hydrolysing activity, putatively identified as an endo‐amylase and present in wild‐type chloroplasts, was absent or appreciably reduced in the mutant. This is the first time that a specific enzyme required for starch degradation has been identified in leaves.