Abstract
Plants can be regenerated from various explants/tissues via de novo shoot meristem formation. Most of these regeneration pathways are indirect and involve callus formation. Besides plant hormones, the role of polyamines (PAs) has been implicated in these processes. Interestingly, the lateral root primordia (LRPs) of Arabidopsis can be directly converted to shoot meristems by exogenous cytokinin application. In this system, no callus formation takes place. We report that the level of PAs, especially that of spermidine (Spd), increased during meristem conversion and the application of exogenous Spd improved its efficiency. The high endogenous Spd level could be due to enhanced synthesis as indicated by the augmented relative expression of PA synthesis genes (AtADC1,2, AtSAMDC2,4, AtSPDS1,2) during the process. However, the effect of PAs on shoot meristem formation might also be dependent on their catabolism. The expression of Arabidopsis POLYAMINE OXIDASE 5 (AtPAO5) was shown to be specifically high during the process and its ectopic overexpression increased the LRP-to-shoot conversion efficiency. This was correlated with Spd accumulation in the roots and ROS accumulation in the converting LRPs. The potential ways how PAO5 may influence direct shoot organogenesis from Arabidopsis LRPs are discussed.
Highlights
De novo organogenesis from somatic plant tissues occurs both in nature or in vitro, either directly or indirectly through callus formation [1]
Arabidopsis thaliana, shoot regeneration is usually achieved via indirect organogenesis from root explants [2,3,4]
It has been recognized that callus formation is not required for de novo shoot formation from root tissues and the direct conversion of lateral root primordia (LRP) to shoot meristem can take place in response to cytokinin application [5,6,7]
Summary
De novo organogenesis from somatic plant tissues occurs both in nature or in vitro, either directly or indirectly through callus formation [1]. During these processes, explants or calli first form ectopic apical meristems, which subsequently develop into shoots or roots, respectively. It has been recognized that callus formation is not required for de novo shoot formation from root tissues and the direct conversion of lateral root primordia (LRP) to shoot meristem can take place in response to cytokinin application [5,6,7]. Exogenous cytokinin transiently pauses cell division in the LRPs (mitotic pause) and the regulators of shoot development start to be expressed
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