Mucilaginous exudate from the apical root sections of Allium ascalonicum provokes autoinhibition of proliferation and subsequent bud regeneration

Abstract

Mucilage secreted from in vitro-cultured organs has been frequently reported in numerous Allium species. Despite its adverse effect on cell proliferation and subsequent de novo regeneration, this phenomenon has never been thoroughly studied. In Allium ascalonicum (shallot), only the uppermost part of the root tip responds to a callus induction treatment by forming callus with bud-forming capacity. The very same site of the root secretes mucilage during the course of its cultivation on callus induction medium (CIM). To address this issue, mucilaginous exudate (10 μL), collected from previously cultured root-tips of shallot, was applied directly to the root-tips isolated from in vitro-cultured shallot plants. Untreated root-tips were used as a control. All explants were cultivated on CIM containing 5 μM 2,4 D + 5 μM BA for 10 weeks, and then on regeneration induction medium supplemented with 5 μM BA for 4 weeks. A remarkable difference in cell proliferation and subsequent bud regeneration between treated and control root-tips were observed. Calli formed from treated root-tips were 3-fold lighter (53.7 mg vs. 169.9 mg) and regenerated buds with lower frequency (14.64% vs. 89.2%) and smaller mean bud number per explant (0.1 vs. 15.4), and these buds were shorter (0.65 mm vs. 16.4 mm) compared to the control. Root-tips isolated from older roots secreted the mucilage at the higher frequency, formed smaller calli and regenerated fewer buds than those isolated from younger roots. Inhibition of cell proliferation indicates the potential of this exudate in control of cell proliferation in the treatment of some diseases.