Inhibition of malformin activity by hydroxyproline and restoration by proline

Abstract

MALFORMIN, (cyclo-D-cysteinyl-D-cysteinyl-L-valyl-D-leucyl-L-isoleucyl)¹, is produced by the fungus Aspergillus niger van Tiegh., induces severe malformations on stems and petioles of various plants, and causes root curvature². When ¹⁴C-malformin was supplied to cuttings of the bean Phaseolus vulgaris L. most of the ¹⁴C was found in the wall fraction and was liberated from a protein-like component in the walls as ¹⁴C-malformeic acid (SO₃H-malformin) by oxidation³. Previous results indicated that malformin inhibited wall synthesis and altered wall composition^4–6. We recently found that malformin is a potent stimulator of plant growth and inhibits several phytochrome- and ethylene-mediated responses. Because some effects of light and ethylene on plant growth have been attributed to an increase in extensin^8–11, a hydroxyproline-rich protein in plant walls, it was suggested that malformin may alter the synthesis or function of this wall component⁷. Because malformin activity is inhibited by reducing agents¹² and it reacts with sulphydryl compounds¹³, involvement of wall sulphydryl groups in the activity of malformin was also proposed⁷. We report here on the effect of hydroxyproline and proline, amino acids important in the synthesis of extensin, on the growth-stimulating and root-curving activities of malformin.