The tobacco plasma membrane NADPH oxidase and the extracellular polyamine oxidase interact functionally to regulate the homeostasis of reactive oxygen species.

In vitro regeneration of Arabidopsis from roots is generally achieved via indirect organogenesis. First, transdifferentiation of lateral root primordia to calli is achieved by a balanced auxin-to-cytokinin ratio that is followed by the induction of shoot meristem formation using a high cytokinin level. Here we demonstrate that if the root explants were transferred onto a hormone-free medium after a transient (4-days) cytokinin treatment, embryogenic marker genes (LEC1, LEC2, FUS3) started to be expressed. App. 50% of the regeneration foci developed into plantlets with trichome-less cotyledon-like leaves. Moreover, the somatic embryogenesis defective lec1 mutant could regenerate only shoots with trichome-bearing leaves under this condition. Based on these observations, the mixed accomplishment of shoot organogenesis and somatic embryogenesis is hypothesized in the Arabidopsis root explants cultured under hormone-free conditions following cytokinin induction. Using whole seedlings instead of root explants in the same experimental set up, no regenerates were formed on the roots. Applying the auxin transport inhibitor TIBA to the root-to-shoot junction of the seedlings, the regeneration ability of the root could be restored. The observations indicate that shoot-derived endogenous auxin blocks the cytokinin-induced regeneration process in the roots of whole seedlings. The expression of the wound-induced transcription factor WIND1 could be detected in the roots of unwounded seedlings if the shoot-to-root auxin transport was inhibited. Manipulating the exogenous cytokinin level together with the endogenous shoot-to-root auxin transport therefore could mimic the effect of wounding (removal of shoot) on plant regeneration from roots.

Polyamines play an important role in developmental and environmental stress responses in plants. Polyamine oxidases (PAOs) are flavin-adenine-dinucleotide-dependent enzymes associated with polyamine catabolism. In this study, 14 genes were identified in the tobacco genome that code for PAO proteins being named based on their sequence homology with Arabidopsis PAOs (AtPAO1-5): NtPAO1A-B; NtPAO2A-C, NtPAO4A-D, and NtPAO5A-E. Sequence analysis confirmed that the PAO gene family of the allopolyploid hybrid Nicotiana tabacum is not an exact combination of the PAO genes of the maternal Nicotiana sylvestris and paternal Nicotiana tomentosiformis ones. The loss of the N. sylvestris homeolog of NtPAO5E and the gain of an extra NtPAO2 copy, likely of Nicotiana othophora origin, was revealed. The latter adds to the few pieces of evidence suggesting that the paternal parent of N. tabacum was an introgressed hybrid of N. tomentosiformis and N. othophora. Gene expression analysis indicated that all 14 PAO genes kept their expression following the formation of the hybrid species. The homeologous gene pairs showed similar or opposite regulation depending on the investigated organ, applied stress, or hormone treatment. The data indicate that the expression pattern of the homeologous genes is diversifying in a process of subfunctionalization.

Zinc oxide nanoparticles (ZnO NPs) are promising tools for improving Zn delivery to plants, but their efficiency depends strongly on their physicochemical properties. Here, we compared four ZnO NP formulations differing in origin, particle size and surface charge for their short-term interactions with Arabidopsis thaliana roots. Seedlings were exposed to 2.5–25 mg/L ZnO NPs for 2 h, and root Zn-associated fluorescence was visualized using ZinPyr-1. All formulations increased fluorescence in a concentration-dependent manner, but positively charged, chemically synthetized ZnO NPs (CHEM1, CHEM2) were more effective than negatively charged formulations. CHEM2, which is larger and less positive than CHEM1, induced the strongest response, particularly in the root tip and middle root zone, and produced a distinct Zn-associated signal in vascular tissues. No significant increase was detected near the root–shoot junction, indicating limited redistribution during short-term exposure. Plant exposure also altered suspension chemistry. In case of CHEM2, it caused the strongest acidification and Zn depletion, although changes in suspension Zn did not consistently correspond to root fluorescence or pH. These findings demonstrate that Zn delivery from ZnO NPs is highly formulation-dependent and likely reflects the combined influence of surface charge, particle size, dissolution and root–particle interactions. Thus, physicochemical properties should be carefully considered when designing ZnO nanomaterials for plant Zn delivery.