**8. Seed-transmitted tobamoviruses**

and induced caspase-like proteinases cause local cell death [22, 111]. In the case of *N* NB-LRR gene, translational arrest is involved [112]. The helicase domain of the replicase protein, MP,

A new approach that introduces resistance may circumvent the resistance braking tobamoviruses. The clustered regularly interspaced short palindromic repeats (CRISPR-Cas) system is mutagenesis targeted by guide RNAs [113, 114]. The specific guide RNA has sequence homology to a desired DNA site, thus enabling the use of the CRISPR-Cas system to edit specific genome sites. Editing occurs by the introduction of double-stranded DNA breaks by the endonuclease Cas9 at a locus of interest while exploiting cellular repair mechanisms to cause high fidelity heritable genome sequence changes [113, 115]. Successful application of the Cas9 methodology was reported for the virus genera: *Geminivirus* [116], *Potyvirus*, and

The viral MP is a target for various regulatory events. During viral passage through PD, the MP is transiently synthesized, and it is degraded by 26S proteasome [118]. MP binding to viral RNA is associated with repression of viral RNA translation [119]. Phosphorylation of MP apparently has diverse regulatory consequences for MP. Phosphorylated MP no longer represses RNA translation [120], which may indicate temporal regulation of viral movement and synthesis. Kinases that phosphorylate the MP carboxy terminus stabilize the protein and promote its activity on PD dilation [73]. Interestingly the carboxy terminus of MP is susceptible to phosphorylation by the host cell wall-associated protein kinase that may be sequestering the MP to the cell wall [121]. Phosphorylation of ToMV MP at serine 37 apparently confers

Suppression of posttranscriptional gene silencing (PTGS) is apparently an elementary mechanism for viral spread. Viruses are initiators of the endogenous PTGS, which degrades RNA. The viral RNA is processed into small interfering RNA (siRNA) by a drosophila Dicerlike enzyme, which is then incorporated into RNA-induced silencing complex (RISK) that degrades RNAs with sequence similarity to the siRNAs. A silencing signal, which may be the siRNAs, spreads between cells via PD and phloem [123]. This signal spread can be prevented by TMV [124]. Study of ToMV suppression of PTGS showed accumulation of siRNAs, sug-

As mentioned above, indirect pathway to seed invasion is limited by meristematic exclusion. In addition, seeds apparently have an intrinsic inhibitory effect on virus infection [126].

RNA viruses have high mutation rate [127] that contributes to rapid evolution dynamics which may ensure adaptation to new host plants or to stressful and fluctuating environments [128, 129]. Seed transmission may comprise a genetic bottleneck, which may reduce population size and induce the immergence of new virus strains [130]. Analysis of the effect of the vertical transmission bottleneck on *Zucchini yellow mosaic virus* (ZYMV), for example, showed that although high variability was observed in the 5′ untranslated region, the regions

stabilization to the protein and is essential for its intracellular localization [122].

gested to indicate a block in PTGS downstream of siRNA production [125].

**7. Virus preservation in seeds**

and CP is the avr proteins that elicit the HR of *N*, *Tm*, and *L* genes, respectively [22].

*Ipomovirus* [117].

244 Advances in Seed Biology

Selected tobamoviruses reported to be transmitted via seeds are listed in **Table 1**. Very low transmission ratios or no transmission at all occurs in most *Tobamovirus* species although seeds are infected. Nevertheless, even a low transmission percentage may be critical for new growing areas. Seed transmission primarily occurs via transplantation procedure, commonly practiced by growers. Most of the tobamoviruses contaminate the seed coat. Study of PMMoVcontaminated pepper seeds using fluorescence probe revealed that the seed coat epidermis and parenchyma cells and the endothelium that surrounds the endosperm all are invaded by PMMoV, leaving the endosperm and the embryo clear of the virus [134, 135]. These cells are of maternal origin. This observation is unlike the reports on ToMV and CGMMV that invade the seed coat as well as the endosperm or the perisperm-endosperm envelope (PEE), respectively. Indeed the seed coat originates from maternal tissues, but the endosperm and its envelope are the result of fertilization process [136]. In most cases, washes of externally attached viruses are not sufficient to prevent seed transmission [4, 34].

CGMMV-contaminated seeds are morphologically modified, as observed by optical coherence tomography. Infected melon seeds show irregularities in the aleurone layer outside the endosperm, and cucumber seeds showed a narrower gap between the seed coat and the endosperm [137]. Interestingly, hair-like structures were observed on the infected seed surface of cucumber and muskmelon [138].

High seed transmission ratio of up to 16% was observed in transmission of *Tropical soda apple mosaic virus* (TSAMV) in tropical soda apple weeds (*Solanum viarum*) [139]. Importantly, the virus can cause leaf deformation and even plant death to the *Solanaceae* plants *Capsicum annuum* cv. Capistrano and *C. annuum* cv. Enterprise, respectively [139]. Weed preservation of tobamoviruses [140, 141] and weed seed transmission may comprise a hurdle difficult to handle by growers [10]. Interestingly, seeds treated with trisodium phosphate (TSP) did not transmit the virus, indicating that TSAMV infection occurred at the outer layers of the seed coat.

Most conspicuous are the results regarding seed transmission of *Sunn-hemp mosaic virus* (SHMV) in sunn-hemp and cowpea plants. While little viral transmission occurred through sunn-hemp seeds, up to 20% transmission ratio occurred when cowpea chlorotic spot isolate infected cowpea seeds. The virus was present in all seed parts including the embryo [142].
