Analysis of resistance to Xylella fastidiosa within a hybrid population of Pera sweet orange × Murcott tangor

Abstract

Resistance to Xylella fastidiosa was evaluated within a population of 20 interspecific hybrids of Pera sweet orange and Murcott tangor under greenhouse conditions. Efficiency of inoculation, multiplication of bacteria within the plants, xylem vessel morphology, and symptom expression were analysed. The rate of infection ranged from 40 to 100% (average 70%) for all genotypes analysed. Xylella fastidiosa populations ranged from log 0·59 to log 2·13 cells mg − 1 tissue for the resistant hybrids. These values were significantly different ( P = 0·05) from those obtained for the tolerant (no symptoms but bacteria recovered) or susceptible (symptoms and bacteria recovered) hybrids (log 3·02 to log 4·06 cells mg − 1 ). Xylella fastidiosa was recovered from all hybrids (log 2·31 to 5·03 CFU mg − 1 tissue) except the resistant ones. The first foliar symptoms appeared at least 90 days post-inoculation, the time varying according to genotype. No correlation between xylem vessel morphology and disease expression was observed, indicating that the resistance was the result of a genetic response of the host. According to this hypothesis, a high broad-based heritability index for resistance was obtained (0·96) at 210 days from X. fastidiosa inoculations, using bacterial quantification by real-time PCR, which indicated that the influence of the number of bacteria was the result of genetic rather than environmental variations.

Citrus genomics

“…However, this interaction is unusual compared with classical compatible interactions between bacteria/plant, since X. fastidiosa does not have avr genes from which avirulence (Avr) proteins from the pathogen are injected into the plant cells (Simpson et al 2000). On the other hand, mandarins (Citrus reticulata) and some hybrids with sweet orange seem to be tolerant or resistant to X. fastidiosa (Coletta-Filho et al 2007) characterized as an incompatiblelike interaction. Curiously, the persistence of X. fastidiosa seems to be brief inside mandarin plants, since the bacteria can be isolated at 30 days after inoculation (DAI) but not past 60 DAI (Coletta-Filho et al 2007;De Souza et al 2009).…”

“…On the other hand, mandarins (Citrus reticulata) and some hybrids with sweet orange seem to be tolerant or resistant to X. fastidiosa (Coletta-Filho et al 2007) characterized as an incompatiblelike interaction. Curiously, the persistence of X. fastidiosa seems to be brief inside mandarin plants, since the bacteria can be isolated at 30 days after inoculation (DAI) but not past 60 DAI (Coletta-Filho et al 2007;De Souza et al 2009). An explanation for the resistance of these plants could be the morphology of the vessels, but no correlation between diameters or numbers of vessels with resistance was found (Coletta-Filho et al 2007), suggesting that resistance may be related to active defense responses.…”

“…Curiously, the persistence of X. fastidiosa seems to be brief inside mandarin plants, since the bacteria can be isolated at 30 days after inoculation (DAI) but not past 60 DAI (Coletta-Filho et al 2007;De Souza et al 2009). An explanation for the resistance of these plants could be the morphology of the vessels, but no correlation between diameters or numbers of vessels with resistance was found (Coletta-Filho et al 2007), suggesting that resistance may be related to active defense responses. To understand molecular mechanisms that take place in the interaction between susceptible/resistant plants and X. fastidiosa, EST libraries were constructed from sweet orange plants with and without CVC symptoms, and 'Ponkan' mandarin in the presence of the pathogen (De Souza et al 2007a, b).…”

Susceptibility ofCitrusspp.,Quercus ilexandVitisspp. toXylella fastidiosastrain CoDiRO

“…pauca are able to systemically infect some Citrus species, specially Citrus sinensis (sweet orange), in which they cause a severe disease that has been named Citrus variegated chlorosis (CVC) (Laranjeira et al, 1998). However, acid lime (Citrus aurantifolia), lemon (Citrus limon), grapefruit (Citrus paradisi), pummelo (Citrus grandis), kumquats, Poncirus trifoliata and most mandarins (Citrus reticulata) and tangors (C. sinensis 9 C. reticulata) are highly tolerant or resistant to this pathogen (Coletta-Filho et al, 2007;Garcia et al, 2012). Caipira, for example, a CVC isolate was shown to multiply and move systemically over time, reaching concentrations that increased from log 4-5 CFU/g of tissue at 1 week to log 5-7 CFU/g at 2-4 months after mechanical inoculation.…”

“…Caipira, for example, a CVC isolate was shown to multiply and move systemically over time, reaching concentrations that increased from log 4-5 CFU/g of tissue at 1 week to log 5-7 CFU/g at 2-4 months after mechanical inoculation. pauca can be detected in resistant genotypes 2-3 months after inoculation, bacterial populations usually decline after several months (Coletta-Filho et al, 2007;Niza et al, 2015). However, acid lime (Citrus aurantifolia), lemon (Citrus limon), grapefruit (Citrus paradisi), pummelo (Citrus grandis), kumquats, Poncirus trifoliata and most mandarins (Citrus reticulata) and tangors (C. sinensis 9 C. reticulata) are highly tolerant or resistant to this pathogen (Coletta-Filho et al, 2007;Garcia et al, 2012).…”