![]() S-ADENOSYLMETHIONINE DECARBOXYLASEs (SAMDCs) catalyze the formation of an important intermediate of spermidine and spermine biosynthesis, the decarboxylated S-adenosylmethionine (dSAM). Interestingly, in Arabidospsis thaliana there is no ODC and only the ADC-catalyzed pathway contributes to Put biosynthesis. In plants, Put is synthesized by two pathways: either directly from ornithine via ORNITHINE DECARBOXYLASE (ODC EC 4.1.1.17) or indirectly from arginine via ARGININE DECARBOXYLASE (ADC EC 4.1.1.19). Finally, the promeristem matures and develops into a fully functional shoot meristem. This is followed by the conversion phase in which the root meristem is converted into a shoot promeristem with a characteristic cell division pattern. First, exogenous cytokinin transiently pauses cell division in the LRPs (mitotic pause) and the regulators of shoot development start to be expressed. Three successive phases of this process could be distinguished. In recent years, 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. In the model plant, Arabidopsis thaliana, shoot regeneration is usually achieved via indirect organogenesis from root explants. Based on the recognition that high auxin to cytokinin ratios promote root while high cytokinin to auxin ratios shoot formation, the establishment of in vitro plant regeneration systems were elaborated for hundreds of plant species. Among the plant hormones, auxin and cytokinin are the most important to regulate plant morphogenesis including organ formation. During these processes, explants or calli first form ectopic apical meristems, which subsequently develop into shoots or roots, respectively. The potential ways how PAO5 may influence direct shoot organogenesis from Arabidopsis LRPs are discussed.ĭe novo organogenesis from somatic plant tissues occurs both in nature or in vitro, either directly or indirectly through callus formation. This was correlated with Spd accumulation in the roots and ROS accumulation in the converting LRPs. 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. However, the effect of PAs on shoot meristem formation might also be dependent on their catabolism. 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. 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. In this system, no callus formation takes place. Interestingly, the lateral root primordia (LRPs) of Arabidopsis can be directly converted to shoot meristems by exogenous cytokinin application. Besides plant hormones, the role of polyamines (PAs) has been implicated in these processes. Most of these regeneration pathways are indirect and involve callus formation. Plants can be regenerated from various explants/tissues via de novo shoot meristem formation. ![]()
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