Analysis of the systemic colonization of cucumber plants by Cucumber green mottle mosaic virus

Abstract

Systemic movement of Cucumber green mottle mosaic virus (CGMMV) in cucumber plants was shown to be from photoassimilate source to sink, thus indicating phloem transport. Nevertheless, CGMMV was not detected by immunocytochemical procedures in the intermediary cell-sieve element complex in inoculated cotyledons, where photoassimilate loading occurs. In stem internodes, CGMMV was first localized in the companion cells of the external phloem and subsequently in all tissues except the medulla, therefore suggesting leakage of the virus from, and reloading into, the transport phloem during systemic movement. In systemically infected sink leaves, CGMMV was simultaneously detected in the xylem and phloem. Interestingly, CGMMV accumulated to high levels in the differentiating tracheids of young leaves implying that the xylem could be involved in the systemic movement of CGMMV. This possibility was tested using plants in which cell death was induced in a portion of the stem by steam treatment. At 24 6C, steam treatment effectively prevented the systemic movement of CGMMV, even though viral RNA was detected in washes of the xylem above the steamed internode suggesting that xylem circulation occurred. At 29 6C, CGMMV systemically infected steam-treated cucumber plants, indicating that CGMMV can move systemically via the xylem. Xylem transport of CGMMV was, however, less efficient than phloem transport in terms of the time required for systemic infection and the percentage of plants infected. INTRODUCTIONSystemic transport through the vascular system is a crucial step in plant virus infection. Most plant viruses are thought to move systemically through the phloem in parallel with photoassimilate transport (reviewed by Haywood et al., 2002;Lucas & Wolf, 1999;Nelson & Van Bel, 1998;Oparka & Turgeon, 1999;Thompson & Schulz, 1999). Phloem transport includes the loading (entry) of the virus into the phloem at source tissues, its circulation in the transport phloem and its unloading (exit) from the phloem at the sink tissues. Photoassimilate loading occurs presumably in the minor veins of source organs (Nelson & Van Bel, 1998;Santa Cruz, 1999) and in the case of cucurbits is thought to follow a symplastic route involving the specialized intermediary cell-sieve element complex (Turgeon, 1996). Knowledge of phloem circulation of plant viruses is far from complete, yet it is known that it parallels photoassimilate flow (Leisner et al., 1992;Oparka & Turgeon, 1999). The requirement of a functional capsid protein (CP) for systemic movement is common but not universal, depending on specific virus-host interactions (e.g. Dalmay et al., 1992;McGeachy & Barker, 2000;Petty & Jackson, 1990;Ryabov et al., 2001), and for most viruses the precise form in which the virus is transported has not been determined. Phloem unloading in the sink organs also parallels photoassimilate unloading and proceeds basipetally in the developing leaf during the sink-to-source transition (Cheng et al., 2000;Leisner et al., 1992; Roberts et al., 1997; Santa Cruz e...

Internalization and Dissemination of Human Norovirus and Animal Caliciviruses in Hydroponically Grown Romaine Lettuce

“…Hence, internalization of pathogens in fresh produce during field production remains poorly understood and is highly debated. The feasibility of internalization of human enteric viruses by plants is supported by the ability of plants to internalize their own viral pathogens, which can be taken up from soil and water (22,36,39). Many plant viruses are transported with water, minerals, and photosynthetic products throughout the plant tissues (36).…”

“…Many plant viruses are transported with water, minerals, and photosynthetic products throughout the plant tissues (36). In addition, many plant viruses encode the viral movement (M) protein that assists in their movement within the plant system (36). As the size of a virus is approximately 1,000 times smaller than that of a bacterium, in theory, the efficiency of a smaller pathogen to enter and disseminate in plants would be elevated.…”

“…The feasibility of internalization of human enteric viruses by plants is supported by the ability of plants to internalize their own viral pathogens, which can be taken up from soil and water (22,36,39). Many plant viruses are transported with water, minerals, and photosynthetic products throughout the plant tissues (36). In addition, many plant viruses encode the viral movement (M) protein that assists in their movement within the plant system (36).…”

A New Source of Resistance to Cucumber Green Mottle Mosaic Virus in Melon

“…' Temperature influences the systemic movement of plant viruses and the expression of symptoms in the host plants (Mallor et al, 2003). Moreno et al (2004) showed that the systemic movement of CGMMV in cucumber plants is more rapid at 29°C than at 24°C conditions. Therefore, we screened plants at 30°C for early symptom expression and examined the response of 'Chang Bougi' plants to CGMMV-SH under different temperature conditions.…”

Análisis funcional y localización subcelular de las proteínas implicadas en el movimiento intra e intercelular del virus de las manchas necróticas del melón (MNSV)

“…De acuerdo con esta observación, se ha demostrado que los plasmodesmos existentes entre las células de la vaina y las células del mesófilo no suponen una barrera significativa para el movimiento de los virus (Nelson y van Bel, 1998). Sin embargo, en la mayoría de los estudios realizados en venas menores siempre hay un mayor porcentaje de células del parénquima floemático infectadas con respecto a las células acompañantes, sugiriendo que incluso en plantas susceptibles la invasión de las células acompañantes es un paso limitante de la infección sistémica (Nelson y van Bel, 1998;Moreno et al, 2004). Además, se sabe que los factores del huésped que interaccionan con las proteínas virales y ácidos nucleicos son diferentes a aquellos que están implicados en el movimiento célula a célula en el mesófilo.…”