Micronutrients Affect Expression of Induced Resistance Genes in Hydroponically Grown Watermelon against Fusarium oxysporum f. sp. niveum and Meloidogyne incognita

The soil-borne pathogens, particularly Fusarium oxysporum f. sp. niveum (FON) and southern root-knot nematode (RKN, Meloidogyne incognita) are the major threats to watermelon production in the southeastern United States. The role of soil micronutrients on induced resistance (IR) to plant diseases is well-documented in soil-based media. However, soil-based media do not allow us to determine the contribution of individual micronutrients in the induction of IR. In this manuscript, we utilized hydroponics-medium to assess the effect of controlled application of micronutrients, including iron (Fe), manganese (Mn), and zinc (Zn) on the expression of important IR genes (PR1, PR5, and NPR1 from salicylic acid (SA) pathway, and VSP, PDF, and LOX genes from jasmonic acid (JA) pathway) in watermelon seedlings upon inoculation with either FON or RKN or both. A subset of micronutrient-treated plants was inoculated (on the eighth day of micronutrient application) with FON and RKN (single or mixed inoculation). The expression of the IR genes in treated and control samples was evaluated using qRT-PCR. Although, significant phenotypic differences were not observed with respect to the severity of wilt symptoms or RKN galling with any of the micronutrient treatments within the 30-day experimental period, differences in the induction of IR genes were considerably noticeable. However, the level of gene expression varied with sampling period, type and concentration of micronutrients applied, and pathogen inoculation. In the absence of pathogens, micronutrient applications on the seventh day, in general, downregulated the expression of the majority of the IR genes. However, pathogen inoculation preferentially either up- or down-regulated the expression levels of the IR genes at three days post-inoculation depending on the type and concentration of micronutrients. The results demonstrated here indicate that micronutrients in watermelon may potentially make watermelon plants susceptible to infection by FON and RKN. However, upon infection the IR genes are significantly up-regulated that they may potentially aid the prevention of further infection via SA- and JA-pathways. This is the first demonstration of the impact of micronutrients affecting IR in watermelon against FON and RKN infection.

Differential Transmission of Old and New World Begomoviruses by Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) Cryptic Species of Bemisia tabaci.

Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) are two of the most invasive members of the sweetpotato whitefly, Bemisia tabaci, cryptic species complexes and are efficient vectors of begomoviruses. Bemisia tabaci MEAM1 is the predominant vector of begomoviruses in open-field vegetable crops in the southeastern United States. However, recently B. tabaci MED also has been detected in the landscape outside of greenhouses in Florida and Georgia. This study compared the transmission efficiency of one Old-World (OW) and two New-World (NW) begomoviruses prevalent in the southeastern United States, viz., tomato yellow leaf curl virus (TYLCV), cucurbit leaf crumple virus (CuLCrV), and sida golden mosaic virus (SiGMV) between B. tabaci MEAM1 and B. tabaci MED. Bemisia tabaci MEAM1 efficiently transmitted TYLCV, CuLCrV, or SiGMV, whereas B. tabaci MED only transmitted TYLCV. Percent acquisition and retention of OW TYLCV following a 72 h acquisition access period was significantly higher for B. tabaci MED than B. tabaci MEAM1. In contrast, B. tabaci MEAM1 acquired and retained significantly more NW bipartite begomoviruses, CuLCrV or SiGMV, than B. tabaci MED. Quantitative analysis (qPCR) of virus DNA in whitefly internal tissues revealed reduced accumulation of CuLCrV or SiGMV in B. tabaci MED than in B. tabaci MEAM1. Fluorescent in situ hybridization (FISH) showed localization of CuLCrV or SiGMV in the midgut of B. tabaci MED and B. tabaci MEAM1. However, localization of CuLCrV or SiGMV was only observed in the primary salivary glands of B. tabaci MEAM1 and not B. tabaci MED. TYLCV localization was observed in all internal tissues of B. tabaci MEAM1 and B. tabaci MED. Overall, results demonstrate that both B. tabaci MEAM1 and B. tabaci MED are efficient vectors of OW TYLCV. However, for the NW begomoviruses, CuLCrV and SiGMV, B. tabaci MEAM1 seems to a better vector.

The Transcriptomic Profile of Watermelon Is Affected by Zinc in the Presence of Fusarium oxysporum f. sp. niveum and Meloidogyne incognita.

Zinc (Zn) accumulation and deficiency affect plant response to pests and diseases differently in varying pathosystems. The concentrations of Zn in plants aid in priming defense signaling pathways and help in enhanced structural defenses against plant pathogens. Studies are lacking on how concentrations of Zn in watermelon plants influence defense against two important soil-borne pathogens: Fusarium oxysporum f. sp. niveum (FON) and southern root-knot nematode (RKN, Meloidogyne incognita). In this study a comparative transcriptomics evaluation of watermelon plants in response to high (1.2 ppm) and low (0.2 ppm) levels of Zn were determined. Differential transcript-level responses differed in watermelon plants when infected with FON or RKN or both under high- and low-Zn treatment regimes in a controlled hydroponics system. Higher numbers of differentially expressed genes (DEGs) were observed in high-Zn-treated than in low-Zn-treated non-inoculated plants, in plants inoculated with FON alone and in plants inoculated with RKN alone. However, in the co-inoculated system, low-Zn treatment had higher DEGs as compared to high-Zn treatment. In addition, most DEGs were significantly enriched in hormone signal transduction and MAPK signaling pathway, suggesting an induction of systemic resistance with high-Zn concentrations. Taken together, this study substantially expands transcriptome data resources and suggests a molecular potential framework for watermelon-Zn interaction in FON and RKN.

Virus-virus interactions in a plant host and in a hemipteran vector: Implications for vector fitness and virus epidemics.

Mixed virus infection in host plants can differentially alter the plant phenotype, influence vector fitness, and affect virus acquisition and inoculation by vectors than single-virus infection. Vector acquisition of multiple viruses from multiple host plants could also differentially affect vector fitness and virus inoculation than acquisition of one virus. Whitefly-virus pathosystems in the southern United States include both the above-stated facets. For the first facet, this study examined the effects of single and mixed infection of cucurbit leaf crumple virus (CuLCrV, a begomovirus) and cucurbit yellow stunting disorder virus (CYSDV, a crinivirus) infecting squash on whitefly (Bemisia tabaci Gennadius MEAM1) host preference and fitness. Mixed infection of CuLCrV and CYSDV in squash plants severely altered their phenotype than single infection. The CYSDV load was reduced in mixed-infected squash plants than in singly-infected plants. Consequently, whiteflies acquired reduced amounts of CYSDV from mixed-infected plants than singly-infected plants. No differences in CuLCrV load were found between singly- and mixed-infected squash plants, and acquisition of CuLCrV by whiteflies did not vary between singly- and mixed-infected squash plants. Both singly- and mixed-infected plants similarly affected whitefly preference, wherein non-viruliferous and viruliferous (CuLCrV and/or CYSDV) whiteflies preferred non-infected plants over infected plants. The fitness study involving viruliferous and non-viruliferous whiteflies revealed no differences in developmental time and fecundity. For the second facet, this study evaluated the effects of individual or combined acquisition of tomato-infecting tomato yellow leaf curl virus (TYLCV, a begomovirus) and squash-infecting CuLCrV on whitefly host preference and fitness. Whiteflies that acquired both CuLCrV and TYLCV had significantly lower CuLCrV load than whiteflies that acquired CuLCrV alone, whereas TYLCV load remained unaltered when acquired individually or in conjunction with CuLCrV. Whitefly preference was not affected following individual or combined virus acquisition. Viruliferous (CuLCrV and/or TYLCV) whiteflies preferred to settle on non-infected tomato and squash plants. The mere presence of CuLCrV and/or TYLCV in whiteflies did not affect their fitness. Taken together, these results indicate that mixed infection of viruses in host plants and acquisition of multiple viruses by the vector could have implications for virus accumulation, virus acquisition, vector preference, and epidemics that sometimes are different from single-virus infection or acquisition.

Low Frequency of Horizontal and Vertical Transmission of Cucurbit Leaf Crumple Virus in Whitefly Bemisia tabaci Gennadius.

Cucurbit leaf crumple virus (CuLCrV), a bipartite begomovirus, is transmitted by whiteflies in a persistent and circulative manner. Like other begomoviruses, CuLCrV transmission via feeding is well understood; however, whether and how CuLCrV is transmitted by horizontal and vertical modes in its vector, Bemisia tabaci, remains unexplored. We studied transovarial and mating transmission of CuLCrV, and comparatively analyzed virus accumulation in whiteflies through feeding and nonfeeding modes. Furthermore, we quantified CuLCrV DNA A accumulation at different time points to determine whether this virus propagates in whiteflies. CuLCrV DNA A was transmitted vertically and horizontally by B. tabaci, with low frequency in each case. Transovarial transmission of CuLCrV DNA A was only 3.93% in nymphs and 3.09% in adults. Similarly, only a single viruliferous male was able to transmit CuLCrV DNA A to its nonviruliferous female counterparts via mating. In contrast, viruliferous females were unable to transmit CuLCrV DNA A to nonviruliferous males. Additionally, the recipient adults that presumably acquired CuLCrV transovarially and via mating were not able to transmit the virus to squash plants. We further report that the CuLCrV DNA A viral copy numbers were significantly lower in nonfeeding modes of transmission than in feeding ones. The viral copy numbers significantly decreased at succeeding time points throughout adulthood, suggesting no CuLCrV propagation in B. tabaci. Altogether, the low frequency of nonfeeding transmission, reduced virus accumulation in whiteflies, and absence of plant infectivity through nonfeeding transmission suggest that transovarial and mating CuLCrV transmission might not substantially contribute to CuLCrV epidemics.