**3. Virus preservation in seeds**

**2. Resistance-breaking tobamoviruses**

ferred by the *L1-3* genes [29–31] and rarely the *L4*

resistance were reported.

*Tm-22*

236 Advances in Seed Biology

Plant viruses belonging to the genus *Tobamovirus* (currently comprised of 37 species) are rod-shaped particles encapsulating a single-stranded RNA (+ssRNA) genome of ~6.4 kb. The genome encodes four ORFs. ORF1 and ORF2, separated by a leaky stop codon, encode the 126 and 183 kDa replicase protein complex (recently reviewed in [21]). ORF3 encodes the 30 kDa movement protein (MP), and ORF4 encodes the 17.5 kDa coat protein (CP). The tobamoviruses endanger cultivars in the world. For more than a century, tobacco and tomato plants that belong to the *Solanaceae* family are infected by TMV [1, 2] and ToMV [4]. Resistance to these viruses was introduced to tomato plants by introgression [22]. However, the durability of resistance genes is compromised by the pathogen selection pressure that gradually breaks

the plant defense system. In tomato plants, the durability of *Tm-1*, *Tm-2*, and *Tm-22*

tance genes has recently been jeopardized by the newly discovered tobamoviruses *Tomato mottle mosaic virus* (ToMMV), reported in Brazil [24], Mexico [25], and the USA [26], and the

the production of cucumber, melon, and watermelon has been endangered by the globalized spread of CGMMV. Since its discovery by Ainsworth in 1935 [6], no commercial cultivars fully resistant to CGMMV are available although temperature-sensitive strains with specific

**Figure 2.** *Tomato brown rugose fruit virus* (ToBRFV)-infected tomato (*Solanum lycopersicum*) plants. (**a, d**) Brown rugose symptoms developed on fruits. (**b, c**) yellow spots on fruits. (**a, e–g**) Mosaic pattern developed on leaves and narrowing

accompanied by mottling leaves. (**g**) Necrotic symptoms on pedicle, calyces, and petioles.

 resistance-breaking *Tomato brown rugose fruit virus* (ToBRFV) reported in Jordan [27] and Israel [28] (**Figure 2**). Within the *Solanaceae* family, pepper crops were also affected in the last decades by *Tobamovirus* species, mostly by PMMoV that overcame the resistance con-

[23] resis-

gene in Japan [32] and Israel [33]. In cucurbits,

The viral particles of tobamoviruses are extremely stable (**Figure 3(a)**), and infectivity is preserved in seeds for up to several years. Most of the *Tobamovirus* species display low percentage of seed transmission, but even very low occurrence of seed transmission is enough to start a spread of the disease [16]. Seed transmission of viruses occurs primarily via infected embryos through paternal or maternal pathways. However, tobamoviruses mostly infect the seed coat (testa) and the endosperm [4, 34, 35]. Transmission of the virus occurs primarily by mechanical means through transplanting the seedlings and causing cuts in the roots that are then susceptible to infection by the contaminated seed coat. The rout of *Tobamovirus* transmission to the seeds is not quite clear. While the testa is of maternal origin, the endosperm is the outcome of fertilization. It is possible that tobamoviruses partially follow the rout of the symplasmic pathway suggested to occur in seed infection of *Pea seed-borne mosaic virus* (PSbMV) without the accomplishment of embryo invasion.

**Figure 3.** *Cucumber green mottle mosaic virus* (CGMMV) morphology and localization in cucurbit seeds. (**a**) Electron micrograph illustration of viral particles, (**a1**) high resolution of a single viral particle. (**b–e**) In situ immunofluorescence of CGMMV in infected melon (*Cucumis melo*) seeds using CGMMV-specific antibodies and secondary antibodies conjugated with Alexa Fluor 488. The fluorescent signal indicates the presence of CGMMV coat protein. Bars, 200 μm.
