Non-host Plant Resistance against Phytophthora capsici Is Mediated in Part by Members of the I2 R Gene Family in Nicotiana spp.

The identification of host genes associated with resistance to Phytophthora capsici is crucial to developing strategies of control against this oomycete pathogen. Since there are few sources of resistance to P. capsici in crop plants, non-host plants represent a promising source of resistance genes as well as excellent models to study P. capsici - plant interactions. We have previously shown that non-host resistance to P. capsici in Nicotiana spp. is mediated by the recognition of a specific P. capsici effector protein, PcAvr3a1 in a manner that suggests the involvement of a cognate disease resistance (R) genes. Here, we have used virus-induced gene silencing (VIGS) and transgenic tobacco plants expressing dsRNA in Nicotiana spp. to identify candidate R genes that mediate non-host resistance to P. capsici. Silencing of members of the I2 multigene family in the partially resistant plant N. edwardsonii and in the resistant N. tabacum resulted in compromised resistance to P. capsici. VIGS of two other components required for R gene-mediated resistance, EDS1 and SGT1, also enhanced susceptibility to P. capsici in N. edwardsonii, as well as in the susceptible plants N. benthamiana and N. clevelandii. The silencing of I2 family members in N. tabacum also compromised the recognition of PcAvr3a1. These results indicate that in this case, non-host resistance is mediated by the same components normally associated with race-specific resistance.

The Arabidopsis synaptotagmin SYTA regulates the cell-to-cell movement of diverse plant viruses.

Synaptotagmins are a large gene family in animals that have been extensively characterized due to their role as calcium sensors to regulate synaptic vesicle exocytosis and endocytosis in neurons, and dense core vesicle exocytosis for hormone secretion from neuroendocrine cells. Thought to be exclusive to animals, synaptotagmins have recently been characterized in Arabidopsis thaliana, in which they comprise a five gene family. Using infectivity and leaf-based functional assays, we have shown that Arabidopsis SYTA regulates endocytosis and marks an endosomal vesicle recycling pathway to regulate movement protein-mediated trafficking of the Begomovirus Cabbage leaf curl virus (CaLCuV) and the Tobamovirus Tobacco mosaic virus (TMV) through plasmodesmata (Lewis and Lazarowitz, 2010). To determine whether SYTA has a central role in regulating the cell-to-cell trafficking of a wider range of diverse plant viruses, we extended our studies here to examine the role of SYTA in the cell-to-cell movement of additional plant viruses that employ different modes of movement, namely the Potyvirus Turnip mosaic virus (TuMV), the Caulimovirus Cauliflower mosaic virus (CaMV) and the Tobamovirus Turnip vein clearing virus (TVCV), which in contrast to TMV does efficiently infect Arabidopsis. We found that both TuMV and TVCV systemic infection, and the cell-to-cell trafficking of the their movement proteins, were delayed in the Arabidopsis Col-0 syta-1 knockdown mutant. In contrast, CaMV systemic infection was not inhibited in syta-1. Our studies show that SYTA is a key regulator of plant virus intercellular movement, being necessary for the ability of diverse cell-to-cell movement proteins encoded by Begomoviruses (CaLCuV MP), Tobamoviruses (TVCV and TMV 30K protein) and Potyviruses (TuMV P3N-PIPO) to alter PD and thereby mediate virus cell-to-cell spread.

Early and late gene expression in pepper huasteco yellow vein virus.

Viral infections usually take place in an orderly manner and can be divided into at least two phases: an early and a late stage. In geminiviruses, plant viruses with a circular, single-stranded DNA genome, expression of viral genes involves complex regulation strategies that suggest the existence of a pattern of temporal gene expression. In this work, the transcription of pepper huasteco yellow vein virus (PHYVV) genes was studied. Green fluorescent protein replacements and RT-PCR analyses were used to monitor PHYVV gene expression chronologically in suspension cells and plant tissue. A model is proposed to describe the order of geminivirus gene expression, where the genes that encode Rep, TrAP and REn are expressed during an early stage of infection. The genes that encode the coat protein and the nuclear shuttle protein are expressed during the late stage of infection.

Use of geminiviral vectors for functional genomics.

Virus-induced gene silencing (VIGS) can be used to study the function of a gene by downregulating its expression and analyzing the resulting phenotype. VIGS is a handy tool that is less time consuming and labor intensive than other methods for generating mutants. Geminiviruses are particularly convenient and valuable choices as VIGS vectors in functional genomics. The small size of their DNA genome, the simplicity of the methods for inoculation, their wide host range and their conserved genome organization are just a few of the advantageous characteristics that this group of viruses has to offer. Geminivirus-based vectors have proved to be very efficient in VIGS systems, and further development of these systems will most probably permit their application in studies of the functional genomics of important crops that are recalcitrant to other forms of analysis.