The Anti-Senescence Activity of Cytokinin Arabinosides in Wheat and Arabidopsis Is Negatively Correlated with Ethylene Production

Leaf senescence, accompanied by chlorophyll breakdown, chloroplast degradation and inhibition of photosynthesis, can be suppressed by an exogenous application of cytokinins. Two aromatic cytokinin arabinosides (6-benzylamino-9-β-d-arabinofuranosylpurines; BAPAs), 3-hydroxy- (3OHBAPA) and 3-methoxy- (3MeOBAPA) derivatives, have recently been found to possess high anti-senescence activity. Interestingly, their effect on the maintenance of chlorophyll content and maximal quantum yield of photosystem II (PSII) in detached dark-adapted leaves differed quantitatively in wheat (Triticum aestivum L. cv. Aranka) and Arabidopsis (Arabidopsisthaliana L. (Col-0)). In this work, we have found that the anti-senescence effects of 3OHBAPA and 3MeOBAPA in wheat and Arabidopsis also differ in other parameters, including the maintenance of carotenoid content and chloroplasts, rate of reduction of primary electron acceptor of PSII (QA) as well as electron transport behind QA, and partitioning of absorbed light energy in light-adapted leaves. In wheat, 3OHBAPA had a higher protective effect than 3MeOBAPA, whereas in Arabidopsis, 3MeOBAPA was the more efficient derivative. We have found that the different anti-senescent activity of 3OHBAPA and 3MeOBAPA was coupled to different ethylene production in the treated leaves: the lower the ethylene production, the higher the anti-senescence activity. 3OHBAPA and 3MeOBAPA also efficiently protected the senescing leaves of wheat and Arabidopsis against oxidative damage induced by both H2O2 and high-light treatment, which could also be connected with the low level of ethylene production.

Efficient Micropropagation Protocol for the Conservation of the Endangered Aloe peglerae, an Ornamental and Medicinal Species

A number of Aloe species are facing an extremely high risk of extinction due to habitat loss and over-exploitation for medicinal and ornamental trade. The last global assessment of Aloe peglerae Schönland (in 2003) ranked its global conservation status as ‘endangered’ with a decreasing population trend. In the National Red List of South African Plants, the extremely rapid decline of this species has resulted in its conservation status being elevated from ‘endangered’ to ‘critically endangered’ based on recent or new field information. This dramatic decline necessitates the development of a simple, rapid and efficient micropropagation protocol as a conservation measure. An in vitro propagation protocol was therefore established with the regeneration of 12 shoots per shoot-tip explant within 8 weeks using Murashige and Skoog (MS) medium supplemented with 2.5 µM meta-topolin riboside (an aromatic cytokinin). The rooting of the shoots with a 100% frequency on half-strength MS medium without any plant growth resulted in additional six shoots produced per cultured shoot. The resultant plantlets were successfully acclimatized with a 100% survival frequency after 6 weeks. Overall, the developed protocol can result in the production of 3906 transplantable shoots that are ready for rooting per annum from a single shoot-tip explant. It is simple and efficient for seedling production in the ex situ cultivation and conservation of the endangered A. peglerae.

Micropropagation of Camptotheca Acuminata Decne (Nyssaceae) – Endangered Ornamental and Medicinal Tree

Camptotheca acuminata Decne (Nyssaceae) (happy tree, tree of life, cancer tree) is a rapidly growing deciduous and endangered tree endemic to east Tibet and southern China (Liu et al., 2002). C. acuminata is an ornamental tree with monopodial growth, beautiful leaves and shade- and cold- tolerant in their natural environment. Recently, its demand has grown rapidly due to secondary metabolites production like Camptothecin and its derivatives with anti cancer and antivirus activity. The aim of the present study was to develop an efficient protocol for in vitro micropropagation of this valuable plant. Different cultural media based on both MS (Murashige and Skoog, 1962) or DKW (Driver and Kuniyuki, 1984) formulations with different cytokinins (6-benzylaminopurine, BAP or 2-isopentenyladenine, 2iP or meta-topolin, mT) have been involved. The aromatic cytokinin meta-topolin stimulated plant growth. The optimum multiplication rate of Camptotheca shoots occurred on the DKW basal medium, supplemented with 2.5 µM meta-topolin, grown under mixed LED light. The highest number of roots per plantlets was recorded on the rooting medium with 0.3 mgl-1 NAA.

Comparing the Effects of Benzyladenine and meta-Topolin on Sweet Basil (Ocimum basilicum) Micropropagation

Micropropagation of aromatic plants reveals an effective way of obtaining high volume, virus-free plant material of uniform quality. The application of meta-Topolin (mT) (N6-(2-hydroxybenzyl) adenine-9-riboside) and aromatic cytokinin as Benzyladenine (BAP) in the micro propagation of sweet basil (Ocimum basilicum L.) was tested for the first time and plant growth parameters assessed to determine the optimum level of these cytokinins. Additionally, the rate of root-growth inhibition due to these two cytokinins was also assessed. Our results show that 1 mg/l (4.43 mM) BAP and 0.5 mg/l (2.07 mM)mT produced the most favourable effects on new shoot developments. Meta-Topolin was shown to increase the quality of the plants and in comparison with BAP fewer distortions were observed. No significant differences in root-growth inhibition between the mT and BAP were detected.

Effects of cytokinins on antioxidant enzymes in in vitro grown shoots of Pelargonium hortorum L. H. Bayley

<p>The aim of this study was to determine the influence of <em>meta</em>-topolin (<em>m</em>T) and 6-benzyl-aminopurine (BAP) on the hydrogen peroxide (H₂O₂) level and antioxidant enzymes activities in relation to the shoot formation and senescence process in <em>Pelargonium hortorum</em> cultivars, which differ in their susceptibility to leaf yellowing under <em>in vitro</em> conditions.</p><p>In an early senescing cultivar ‘Grand Prix’, the addition of an aromatic cytokinin <em>m</em>T to abscisic acid (ABA)-enriched Murashige and Skoog (MS) basal medium more efficiently inhibited leaf yellowing than BAP. In both genotypes, <em>meta</em>-topolin was also the most effective in shoot formation. It was found that <em>Pelargonium</em> species varying in their susceptibility to senescence differ in H₂O₂ production and antioxidant enzymes activities. Generally, <em>meta</em>-topolin more effectively enhanced H₂O₂production and POD activity than BAP and control medium, but its effect depended on genotype. The highest H₂O₂ production stimulated by <em>m</em>T was observed on day 5 of subculture in late senescing cv. ‘Bergpalais’. In both geranium genotypes, superoxide dismutase (SOD) and catalase (CAT) levels were highest at the beginning of the subculture period, during the initiation of shoot formation. SOD showed the highest activity on day 5 of subculture on the medium without cytokinin and generally being higher in cv. ‘Bergpalais’ than in cv. ‘Grand Prix’. CAT activity was positively regulated by both cytokinins. POD activity was most effectively enhanced by <em>m</em>T, but on different days of subculture - on the 2^nd day of subculture in cv. ‘Bergpalais’ and on the 22^nd day of subculture in cv. ‘Grand Prix’. The enhanced activity of POD in the presence of <em>m</em>T, 4-fold higher than on control medium, at the end of subculture in <em>P. hortorum </em>‘Grand Prix’ coincided with the inhibition of leaf senescence.</p>