Epigenetic responses to Phytophthora citrophthora gummosis in citrus.

Studies show that DNA methylation is associated with plant immunity but little is known as to how this epigenetic mechanism assists plants in adjusting their responses to biotic stress, especially when interacting with an hemibiotrophic pathogen such as citrus Phytophthora. The aim of the present study was to assess the effects of scion-rootstock interaction on plant resistance to P. citrophthora infection and DNA methylation patterns in 'Pera' sweet orange and 'Tahiti' acid lime grafted onto 'Rangpur' lime and 'Tropical' sunki rootstocks reinoculated with P. citrophthora. Results showed that reinoculated plants of the 'Pera' sweet orange/'Rangpur' lime and 'Tahiti' acid lime/'Tropical' sunki combinations with more and less sensitive varieties to Phytophthora, presented smaller stem lesions and increased frequency of full methylation and hemimethylation rates, compared to inoculated plants. In contrast, 'Tahiti' acid lime/'Rangpur' lime, two highly sensitive varieties, and 'Pera'/'Tropical' sunki, two much less sensitive varieties, showed high increases in the frequency of hemimethylation and non-methylation levels. Results suggest that in citrus, both the scion-rootstock interaction and DNA methylation affect the response to P. citrophthora infection. Reinoculated plants, depending on the combination, showed changes in intracellular hyphae growth through the formation of sets of fibers and crystal accumulation in the periderm, cortex, and phloem. In addition, starch grain concentration was higher in reinoculated plants in comparison to inoculated plants. These findings support the assumption that DNA methylation is a plant defense mechanism and therefore may be exploited to improve the response of plants to the gummosis of P. citrophthora in citrus.

Vigor and mortality of citrus progenies with potential use as rootstocks

ABSTRACT: Rootstocks influence certain characteristics of the scion, such as vigor, fruit yield and quality, and tolerance to biotic and abiotic factors. In conventional citrus breeding programs, hybridization is the main method used for obtaining rootstock varieties. Between Sept and Oct 2006, controlled pollinations were carried out resulting in nine progenies, totaling 461 hybrid seedlings. The female parents were the Santa Cruz Rangpur lime (SCRL) and the Sunki of Florida mandarin (SFM), while the male parents, in addition to this mandarin, were the Indio and Riverside citrandarins, the Thomasville citrangequat (THOM), the Swingle citrumelo, the Argentina citrange (AGC) and the Benecke trifoliate orange (BKTO). Fruit setting rates of these crosses were measured and the hybrids generated were planted in the field in 2008 and evaluated from 2010 to 2014, for the variables of plant mortality, visual vigor, plant height and trunk diameter. The descriptive statistics mean, variation range, coefficient of variation and Pearson skewness coefficient were calculated. Data were submitted to analysis of variance and the means of the treatments were grouped by the Scott-Knott test, both at 5 % significance. The progenies, having the Sunki mandarin as the female parent, presented higher fruit setting rates and lower plant mortality rates than those obtained from the female parent Rangpur lime, while zero plant mortality was observed in the SFM × BKTO cross. The crosses SCRL × SFM, SFM × BKTO, SFM × AGC and SFM × THOM have the potential to generate vigorous hybrids, resulting in rootstock varieties adapted to the landscape unit of Coastal Tableland.

Citrus rootstocks influence the population densities of pest mites

Citrus are attacked by pest mites such as the citrus rust mite Phyllocoptruta oleivora (Ashmead) (Acari: Eriophyidae) and the spider mite Tetranychus mexicanus (McGregor) (Acari: Tetranychidae). However, little is known on citrus rootstocks influencing pest mites. We aimed to evaluate the influence of rootstocks on population densities of pest mites on the sweet oranges 'Pera CNPMF D-6' and 'Valencia Tuxpan' throughout time. Adults of both mite species were monthly counted during 19 months from June 2011 to February 2013. Rootstocks influenced the populations of pest mites, since lower densities of P. oleivora were found on 'Pera CNPMF D-6' sweet orange grafted on the hybrid TSKC × CTTR - 002 and on 'Swingle' citrumelo in comparison with the hybrid LVK × LCR - 010, 'Red' rough lime and 'Santa Cruz' rangpur lime as rootstocks. Similarly, lower densities of T. mexicanus were found on 'Valencia Tuxpan' sweet orange grafted on the hybrid HTR-051 in comparison to 'Indio' citrandarin, 'Sunki Tropical' mandarin and LVK × LCR - 010 as rootstocks. We concluded that densities of the mites P. oleivora and T. mexicanus on the sweet oranges 'Pera CNPMF D-6' and 'Valencia Tuxpan' were affected in some periods of the year by some rootstocks, suggesting influence of some genotypes on these pests.

Os citros são atacados por ácaros-praga como o ácaro-da-falsa-ferrugem Phyllocoptruta oleivora (Ashmead) (Acari: Eriophyidae) e Tetranychus mexicanus (McGregor) (Acari: Tetranychidae). No entanto, pouco é conhecido sobre o efeito de porta-enxertos sobre populações de pragas. O objetivo deste trabalho foi avaliar a influência de porta-enxertos nas densidades populacionais de ácaros-praga nas laranjeiras 'Pera CNPMF D-6' e 'Valência Tuxpan' ao longo do tempo. Adultos das espécies de ácaro citadas foram quantificados mensalmente, durante 19 meses, de junho de 2011 a fevereiro de 2013. Os porta-enxertos influenciaram as populações dos ácaros-praga, uma vez que menores densidades de P. oleivora foram encontradas em laranjeira 'Pera CNPMF D-6', enxertada no híbrido TSKC × CTTR - 002 e no citrumelo 'Swingle', em comparação com o que se observou em relação aos porta-enxertos LVK × LCR - 010, limoeiro 'Rugoso vermelho' e limoeiro 'Cravo Santa Cruz'. Similarmente, menores densidades de T. mexicanus ocorreram em laranjeira 'Valência Tuxpan' enxertada no híbrido HTR-051, em comparação com o que se deu em relação aos porta-enxertos citrandarin 'Indio', tangerineira 'Sunki Tropical' e LVK × LCR - 010. Concluiu-se que as densidades dos ácaros P. oleivora e T. mexicanus nas laranjeiras 'Pera CNPMF D-6' e 'Valencia Tuxpan' foram afetadas pelo porta-enxerto em determinados períodos do ano, sugerindo influência de alguns genótipos sobre essas pragas.

Citrus rootstocks influence the population densities of pest mites

Citrus are attacked by pest mites such as the citrus rust mite Phyllocoptruta oleivora (Ashmead) (Acari: Eriophyidae) and the spider mite Tetranychus mexicanus (McGregor) (Acari: Tetranychidae). However, little is known on citrus rootstocks influencing pest mites. We aimed to evaluate the influence of rootstocks on population densities of pest mites on the sweet oranges 'Pera CNPMF D-6' and 'Valencia Tuxpan' throughout time. Adults of both mite species were monthly counted during 19 months from June 2011 to February 2013. Rootstocks influenced the populations of pest mites, since lower densities of P. oleivora were found on 'Pera CNPMF D-6' sweet orange grafted on the hybrid TSKC × CTTR - 002 and on 'Swingle' citrumelo in comparison with the hybrid LVK × LCR - 010, 'Red' rough lime and 'Santa Cruz' rangpur lime as rootstocks. Similarly, lower densities of T. mexicanus were found on 'Valencia Tuxpan' sweet orange grafted on the hybrid HTR-051 in comparison to 'Indio' citrandarin, 'Sunki Tropical' mandarin and LVK × LCR - 010 as rootstocks. We concluded that densities of the mites P. oleivora and T. mexicanus on the sweet oranges 'Pera CNPMF D-6' and 'Valencia Tuxpan' were affected in some periods of the year by some rootstocks, suggesting influence of some genotypes on these pests.

Os citros são atacados por ácaros-praga como o ácaro-da-falsa-ferrugem Phyllocoptruta oleivora (Ashmead) (Acari: Eriophyidae) e Tetranychus mexicanus (McGregor) (Acari: Tetranychidae). No entanto, pouco é conhecido sobre o efeito de porta-enxertos sobre populações de pragas. O objetivo deste trabalho foi avaliar a influência de porta-enxertos nas densidades populacionais de ácaros-praga nas laranjeiras 'Pera CNPMF D-6' e 'Valência Tuxpan' ao longo do tempo. Adultos das espécies de ácaro citadas foram quantificados mensalmente, durante 19 meses, de junho de 2011 a fevereiro de 2013. Os porta-enxertos influenciaram as populações dos ácaros-praga, uma vez que menores densidades de P. oleivora foram encontradas em laranjeira 'Pera CNPMF D-6', enxertada no híbrido TSKC × CTTR - 002 e no citrumelo 'Swingle', em comparação com o que se observou em relação aos porta-enxertos LVK × LCR - 010, limoeiro 'Rugoso vermelho' e limoeiro 'Cravo Santa Cruz'. Similarmente, menores densidades de T. mexicanus ocorreram em laranjeira 'Valência Tuxpan' enxertada no híbrido HTR-051, em comparação com o que se deu em relação aos porta-enxertos citrandarin 'Indio', tangerineira 'Sunki Tropical' e LVK × LCR - 010. Concluiu-se que as densidades dos ácaros P. oleivora e T. mexicanus nas laranjeiras 'Pera CNPMF D-6' e 'Valencia Tuxpan' foram afetadas pelo porta-enxerto em determinados períodos do ano, sugerindo influência de alguns genótipos sobre essas pragas.

Karyotype diversity and the origin of grapefruit.

Grapefruit is a group of citrus of recent origin, probably resulting from a cross between pummelo and sweet-orange. Aiming to investigate this putative origin and the genetic variability among grapefruit cultivars, the karyotype of six grapefruits, two pummelos, and one tangelo cultivar (grapefruit x tangerine) were analyzed using sequential CMA/DAPI double staining and FISH with rDNA probes. The karyotypes of grapefruit 'Duncan' and 'Foster' differ from those of 'Flame', 'Henderson', 'Marsh' and 'Rio Red'. The former have two chromosomes with a single CMA(+) band in both terminal regions (C type chromosome) and six chromosomes with only one CMA(+) terminal band (D type), whereas the latter have three C and five D type chromosomes. All accessions investigated exhibited two chromosomes with 5S rDNA but a variable number of 45S rDNA. The two former grapefruits displayed four 45S rDNA sites, whereas the remaining grapefruit cultivars had five. The two pummelos showed identical karyotypes, homozygous for CMA(+) bands and their four rDNA sites. From each pummelo chromosome pair one chromosome seems to be present in grapefruit karyotypes. The different grapefruit karyotypes might result from independent crosses between pummelos of different karyotypic constitution and sweet-oranges. The chromosome markers found in the tangelo 'Orlando' and the position of their two 45S rDNA confirm the grapefruit 'Duncan' and the tangerine 'Dancy' as their parents.