ACL5 acquired strict thermospermine synthesis activity during the emergence of vascular plants

Norspermine (Nspm), one of the uncommon polyamines (PAs), was detected in bryophytes and lycophytes; therefore, the aminopropyltransferases involved in the synthesis of Nspm were investigated. The enzymatic activity was evaluated by the transient high expression of various aminopropyltransferase genes in Nicotiana benthamiana, followed by quantification of PA distribution in the leaves using gas chromatography-mass spectrometry. The bryophyte orthologues of ACL5, which is known to synthesise thermospermine (Tspm) in flowering plants, were found to have strong Nspm synthesis activity. In addition, two ACL5 orthologous with different substrate specificities were conserved in Selaginella moellendorffii, one of which was involved in Tspm synthesis and the other in Nspm synthesis. Therefore, further detailed analysis using these two factors revealed that the β-hairpin structural region consisting of β-strands 1 and 2 at the N-terminus of ACL5 is involved in substrate specificity. Through functional analysis of a total of 40 ACL5 genes in 33 organisms, including algae, it was shown that ACL5 has changed its substrate specificity several times during plant evolution and diversification. Furthermore, it was strongly suggested that ACL5 acquired strict Tspm synthesis activity during the emergence of vascular plants, especially through major changes around the β-hairpin structural region.

Functional identification of bacterial spermine, thermospermine, norspermine, norspermidine, spermidine, and N1-aminopropylagmatine synthases

Spermine synthase is an aminopropyltransferase that adds an aminopropyl group to the essential polyamine spermidine to form tetraamine spermine, needed for normal human neural development, plant salt and drought resistance, and yeast CoA biosynthesis. We functionally identify for the first time bacterial spermine synthases, derived from phyla Bacillota, Rhodothermota, Thermodesulfobacteriota, Nitrospirota, Deinococcota, and Pseudomonadota. We also identify bacterial aminopropyltransferases that synthesize the spermine same mass isomer thermospermine, from phyla Cyanobacteriota, Thermodesulfobacteriota, Nitrospirota, Dictyoglomota, Armatimonadota, and Pseudomonadota, including the human opportunistic pathogen Pseudomonas aeruginosa. Most of these bacterial synthases were capable of synthesizing spermine or thermospermine from the diamine putrescine and so possess also spermidine synthase activity. We found that most thermospermine synthases could synthesize tetraamine norspermine from triamine norspermidine, that is, they are potential norspermine synthases. This finding could explain the enigmatic source of norspermine in bacteria. Some of the thermospermine synthases could synthesize norspermidine from diamine 1,3-diaminopropane, demonstrating that they are potential norspermidine synthases. Of 18 bacterial spermidine synthases identified, 17 were able to aminopropylate agmatine to form N1-aminopropylagmatine, including the spermidine synthase of Bacillus subtilis, a species known to be devoid of putrescine. This suggests that the N1-aminopropylagmatine pathway for spermidine biosynthesis, which bypasses putrescine, may be far more widespread than realized and may be the default pathway for spermidine biosynthesis in species encoding L-arginine decarboxylase for agmatine production. Some thermospermine synthases were able to aminopropylate N1-aminopropylagmatine to form N12-guanidinothermospermine. Our study reveals an unsuspected diversification of bacterial polyamine biosynthesis and suggests a more prominent role for agmatine.

Effect of drought and combined drought and heat stress on polyamine metabolism in proline-over-producing tobacco plants

The roles of proline and polyamines (PAs) in the drought stress responses of tobacco plants were investigated by comparing the responses to drought alone and drought in combination with heat in the upper and lower leaves and roots of wild-type tobacco plants and transformants that constitutively over-express a modified gene for the proline biosynthetic enzyme Δ1-pyrroline-5-carboxylate synthetase (P5CSF129A; EC 2.7.2.11/1.2.1.41). In both genotypes, drought stress coincided with a decrease in relative water content (RWC) that was much less severe in the upper leaves than elsewhere in the plant. The drought also increased proline levels in both genotypes. A brief period of heat stress (2 h at 40 °C) at the end of the drought period did not significantly influence the proline levels in the upper leaves and roots but caused a further increase in the lower leaves of both genotypes. The rate at which these elevated proline levels returned to normal during the post-stress recovery period was slower in the transformants and plants that had been subjected to the combined stress. In both genotypes, drought stress significantly reduced the levels of spermidine (Spd) and putrescine (Put) in the leaves and roots relative to those for controls, and increased the levels of spermine (Spm) and diaminopropane (Dap, formed by the oxidative deamination of Spd and Spm). Spd levels may have declined due to its consumption in Spm biosynthesis and/or oxidation by polyamine oxidase (PAO; EC 1.5.3.11) to form Dap, which became more abundant during drought stress. During the rewatering period, the plants' Put and Spd levels recovered quickly and the activity of the PA biosynthesis enzymes in their leaves and roots increased substantially; this increase was more pronounced in transformants than WT plants. The high levels of Spm observed in drought stressed plants persisted even after the 24 h recovery and rewatering phase. The malondialdehyde (MDA) contents of the lower leaves of WTs increased substantially during the drought stress period; a less pronounced increase occurred in the transformants and after the application of the combined stress. After the post-stress recovery period, the MDA contents in the leaves of both genotypes were higher than those in the corresponding controls. The MDA contents of the upper leaves in plants of both genotypes remained relatively constant throughout, indicating that these leaves are preferentially protected against the adverse effects of oxidative stress and demonstrating the efficiency of the plants' induced antioxidative defense mechanisms.