Phloroglucinol Improves Direct Rooting of In Vitro Cultured Apple Rootstocks M9 and M26

Jin-Ho Kim,So-Young Park,Bo-Min Kwon,Thanh-Tam Ho

doi:10.3390/agronomy10081079

Abstract

Advances in micropropagation techniques have helped produce true-to-type clones of many horticulturally important plants. However, several cultivars of apple are difficult to root in vitro. In these cases, adventitious roots are induced together with undesirable formation of callus, which decrease the acclimatization rate of in vitro produced plantlets. In this study, two apple rootstocks, M9 and M26, were subjected to different concentrations of indole-3-butyric acid (IBA) to induce root formation. Although addition of IBA to the medium induced root formation, rhizogenesis was accompanied by the undesirable formation of callus in both cultivars. On the other hand, in gene expression analysis, the indole-3-acetic acid (IAA) synthase genes AAO1 and YUC1 were expressed more highly in M9 than in M26. This suggests that endogenous auxin levels may be higher in M9, which may explain why M9 plantlets are difficult to root and experience high levels of callus formation during propagation. In addition, rooting medium containing 0.1 mg·L−1 IBA was supplemented with different concentrations of phloroglucinol (0, 0.5, 1.0, and 2.0 mM) to examine whether direct rooting efficiency in the M9 could be improved. Addition of 1.0 mM phloroglucinol increased rooting percentage and decreased callus formation in the M9 rootstock. The rootstock M9 is a desirable cultivar but presents a problem with true-to-type direct rooting. Addition of phloroglucinol may improve direct rooting and eliminate callus formation during propagation.

Highlights

Malus domestica is traditionally propagated by budding or grafting scions onto clonal rootstocks raised by stooling or layering [1]
To investigate the growth characteristics of M9 and M26 rootstocks, plantlets were grown in selected rooting medium, which consisted of Murashige and Skoog (MS) medium supplemented with 0.5 mg·L−1 indole-3-butyric acid (IBA), 3% (w/v) sucrose, and 7.4 g·L−1 agar
Apple M9 and M26 rootstocks were cultivated on growth medium containing 0, 0.1, 0.5, 1.0, or 2.0 mg·L−1 IBA, and callus formation and rooting percentages were recorded after 1, 2, 3, and 4 weeks cultivation

Summary

Introduction

Malus domestica is traditionally propagated by budding or grafting scions onto clonal rootstocks raised by stooling or layering [1]. Apple rootstocks and several well-known cultivars of fruiting apples can be propagated rapidly by in vitro culture, some reports indicate that initiation of tissue culture of apple cultivars presents major difficulties [1], with problems frequently seen in establishing shoot and root induction in vitro. In vitro tissue culture is a biotechnological technique that has been used to genetically improve cultivars (scions) and rootstocks [2]. An updated review presents a range of findings related to tissue culture of apple and other Malus spp. As in other plant species, micropropagation of apple generally includes four stages: establishment of in vitro cultures from ex vitro plants, shoot multiplication, rooting of Agronomy 2020, 10, 1079; doi:10.3390/agronomy10081079 www.mdpi.com/journal/agronomy

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