Retargeting a maize ß-glucosidase to the vacuole--evidence from intact plants that zeatin-O-glucoside is stored in the vacuole.

Cytokinin (CK) activity is regulated by the complex interplay of their metabolism, transport, stability and cellular/tissue localization. O-glucosides of zeatin-type CKs are postulated to be storage and/or transport forms. Active CK levels are determined in part by their differential distribution of CK metabolites across different subcellular compartments. We have previously shown that overexpressing chloroplast-localized Zm-p60.1, a maize ß-glucosidase capable of releasing active cytokinins from their O- and N3-glucosides, perturbs CK homeostasis in transgenic tobacco. We obtained tobacco (Nicotiana tabacum L., cv Petit Havana SR1) plants overexpressing a recombinant Zm-p60.1 that is targeted to the vacuole. The protein is correctly processed and localized to the vacuole. When grown on medium containing exogenous zeatin, transgenic seedlings rapidly accumulate fresh weight due to ectopic growths at the base of the hypocotyl. The presence of the enzyme in these ectopic structures is shown by histochemical staining. CK quantification reveals that these transgenic seedlings are unable to accumulate zeatin-O-glucoside to levels similar to those observed in the wild type. When crossed with tobacco overexpressing the zeatin-O-glucosyltransferase gene from Phaseolus, the vacuolar variant shows an almost complete reversion in the root elongation assay. This is the first evidence from intact plants that the vacuole is the storage organelle for CK O-glucosides and that they are available to attack by Zm-p60.1. We propose the use of Zm-p60.1 as a robust molecular tool that exploits the reversibility of O-glucosylation and enables delicate manipulations of active CK content at the cellular level.

Cytokinins modulate auxin-induced organogenesis in plants via regulation of the auxin efflux.

Postembryonic de novo organogenesis represents an important competence evolved in plants that allows their physiological and developmental adaptation to changing environmental conditions. The phytohormones auxin and cytokinin (CK) are important regulators of the developmental fate of pluripotent plant cells. However, the molecular nature of their interaction(s) in control of plant organogenesis is largely unknown. Here, we show that CK modulates auxin-induced organogenesis (AIO) via regulation of the efflux-dependent intercellular auxin distribution. We used the hypocotyl explants-based in vitro system to study the mechanism underlying de novo organogenesis. We show that auxin, but not CK, is capable of triggering organogenesis in hypocotyl explants. The AIO is accompanied by endogenous CK production and tissue-specific activation of CK signaling. CK affects differential auxin distribution, and the CK-mediated modulation of organogenesis is simulated by inhibition of polar auxin transport. CK reduces auxin efflux from cultured tobacco cells and regulates expression of auxin efflux carriers from the PIN family in hypocotyl explants. Moreover, endogenous CK levels influence PIN transcription and are necessary to maintain intercellular auxin distribution in planta. Based on these findings, we propose a model in which auxin acts as a trigger of the organogenic processes, whose output is modulated by the endogenously produced CKs. We propose that an important mechanism of this CK action is its effect on auxin distribution via regulation of expression of auxin efflux carriers.

Inhibitory effect of fluoranthene on photosynthetic processes in lichens detected by chlorophyll fluorescence.

The effect of increasing concentration (0.01, 1 and 5 mg l(-1)) of fluoranthene (FLT) and the duration of exposure (12, 24, 36, 48 and 60 h) on the chlorophyll fluorescence parameters (F0, F(V)/F(M), Phi II and (F(M) - F(S))/F (S)) of symbiotic algae in thalli of two foliose lichens Lasallia pustulata and Umbilicaria hirsuta was investigated. Also the FLT concentration in thalli of both lichen species and the proportion of symbiotic algae (photobionts) in thalli was studied. The results obtained demonstrated that the applied concentration of FLT (1 and 5 mg l(-1)) affected primary photochemical processes of photosynthesis of algae in both lichen species. The F0 values increased and the F(V)/F(M) and Phi II values decreased. The fluoranthene content in thallus of both lichen species was increased with increasing FLT concentration in the environment. Chlorophyll fluorescence parameters showed different sensitivity to the content of FLT in Lasallia pustulata and Umbilicaria hirsuta, respectively. Higher sensitivity, detected in U. hirsuta, can be related to its diverse anatomical structure (higher portion of symbiotic algae in thalli) and to the different (nearly 2.5 times higher) content of FLT after exposure.