<i>Solanum nigrum</i> L. is a potato spindle tuber viroid reservoir
Background. Among the weeds of the Solanaceae family, a potential viroid infection reservoir, black nightshade (<i>Solanum nigrum</i> L.), is ubiquitous in Russia. Materials and methods. Molecular diagnostics of the viroid was performed using RT-PCR with the P3/P4 primers specific for PSTVd and 6Pospi F/R specific for the Pospiviroidae family of viroids. The amplification products were cloned and sequenced, and the nucleotide sequences were deposited in the GenBank database. Results. RT-PCR showed that 80-100% of plants were infected with PSTVd. The cDNA amplicons of two plants from Nizhny Novgorod (Sn1 and Sn2) and one plant from Tula Province (Sn3) were cloned (3 clones each) and sequenced. An analysis of the nucleotide variability in the clones revealed significant genetic differences between the PSTVd strains, independent of the geographical origin. Mutations U120C and U192C, compared with the reference strain VP35 (LC523658), were found in all 9 clones. The Sn1 isolate was close to the “potato” VP35 isolate and differed from it only by three mutations. The Sn2 isolate was represented by three identical clones and differed from VP35 by nine mutations, while Sn3 had 22 mutations in the region of 49-310 nucleotide positions in total for three clones. After artificial infection of healthy <i>S. nigrum</i> plants in a greenhouse, the presence of PSTVd in infected plants was confirmed and it was proved that the viroid was transmitted by seeds to plants of the next generation. Conclusion. For the first time in externally healthy plants of <i>S. nigrum</i>, widely distributed in the potato fields of Russia (Nizhny Novgorod and Tula Province), the potato spindle tuber viroid, PSTVd, was detected. It was proved that the viroid was preserved in the seeds of <i>S. nigrum</i> and transmitted to their progeny. Genetic heterogeneity of PSTVd strains from <i>S. nigrum</i> plants was found. Two mutations, U120C and U192C, were common for all tested clones of PSTVd strains.