Effect of Grafting on Viral Resistance of Non-transgenic Plum Scion Combined With Transgenic PPV-Resistant Rootstock

Abstract

In stone fruit trees, resistance to Plum pox virus (PPV) can be achieved through the specific degradation of viral RNA by the mechanism of RNA interference (RNAi). Transgenic virus-resistant plants, however, raise serious biosafety concerns due to the insertion and expression of hairpin constructs that usually contain various selective foreign genes. Since a mature stone tree represents a combination of scion and rootstock, grafting commercial varieties onto transgenic virus-tolerant rootstocks is a possible approach to mitigate biosafety problems. The present study was aimed at answering the following question: To what extent are molecular RNAi silencing signals transmitted across graft junctions in transgrafted plum trees and how much does it affect PPV resistance in genetically modified (GM)/non-transgenic (NT) counterparts? Two combinations, NT:GM and GM:NT (scion:rootstock), were studied, with an emphasis on the first transgrafting scenario. Viral inoculation was carried out on either the scion or the rootstock. The interspecific rootstock “Elita” [(Prunus pumila L. × P. salicina Lindl.) × (P. cerasifera Ehrh.)] was combined with cv. “Startovaya” (Prunus domestica L.) as a scion. Transgenic plum lines of both cultivars were transformed with a PPV-coat protein (CP)-derived intron-separate hairpin-RNA construct and displayed substantial viral resistance. High-throughput sequence data of small RNA (sRNA) pools indicated that the accumulation of construct-specific small interfering RNA (siRNA) in transgenic plum rootstock reached over 2%. The elevated siRNA level enabled the resistance to PPV and blocked the movement of the virus through the GM tissues into the NT partner when the transgenic tissues were inoculated. At the same time, the mobile siRNA signal was not moved from the GM rootstock to the target NT tissue to a level sufficient to trigger silencing of PPV transcripts and provide reliable viral resistance. The lack of mobility of transgene-derived siRNA molecules was accompanied by the transfer of various endogenous rootstock-specific sRNAs into the NT scion, indicating the exceptional transitivity failure of the studied RNAi signal. The results presented here indicate that transgrafting in woody fruit trees remains an unpredictable practice and needs further in-depth examination to deliver molecular silencing signals.

Effective transfer of plum pox virus resistance from transgenic plum rootstocks to apricot scions

“…They found that transgenic rootstocks remained virus free but could not protect the scion due to the lack of an efficient transfer of transgene-derived siRNAs from the rootstocks to the scions. However, scions accumulated specific endogen sRNAs characteristic of the rootstocks (Sidorova et al , 2021). Conversely, Zhao and Song, (2014) showed that PNRSV-hpRNA-derived siRNAs were transmitted up to 1.2 m from the transgenic sweet cherry rootstocks to the non-transgenic scions conferring enhanced PNRSV resistance.…”

“…However, despite this encouraging evidence, contradictory results were also obtained in herbaceous and woody plants (Leibman et al , 2015;Sidorova et al , 2021). In particular, Sidorova et al , (2021) showed that although the transgenic rootstocks of the interspecific Elita cv. [(Prunus pumila L. × P.…”

Biotechnological Advances to Improve Abiotic Stress Tolerance in Crops

“…The trans-grafting method comprises the grafting of a non-genetically modified scion onto a genetically altered rootstock. The scion obtains benefits and traits conferred by transgenes in the rootstock, but the end products, such as fruits, do not contain the transgene and hence are not genetically modified [ 112 , 113 ]. Another innovation is micrografting, which involves the in vitro grafting of small shoot apices or lateral buds onto decapitated rootstock seedlings [ 110 , 114 , 115 ].…”

“…The molecular basis of grafting signaling is not fully understood, but recent research suggests that different plant hormones, proteins, epigenetic events, and several types of RNA could be responsible for changes in scion [ 103 , 113 , 115 , 116 , 117 , 118 , 119 , 120 ].…”

Genome Editing for Plasmodesmal Biology

“…A recent report showed that even the genomes could transfer horizontally via organelle travel during the remodeling of PD and vascular connection at the root-scion junction ( Hertle et al, 2021 ). Previous reports have successfully used transgenic rootstocks to transfer transgene-mediated traits to the wild-type scion parts- for example, the development of CMV resistance in tomato ( Bai et al, 2016 ), Pierce’s disease resistance in grape ( Dandekar et al, 2019 ), PPV resistance in plum ( Sidorova et al, 2021 ), and increased nitrogen levels in walnut overexpressing an ammonium transporter gene ( Liu et al, 2021 ). In such studies, engineering of PD trafficking and the use of transgene-free CRISPR techniques to attain desired traits would be highly desirable because the non-transgenic genome editing approach may easily avoid the GM issues and related-regulatory hurdles.…”