Dynamics of 'Candidatus Liberibacter asiaticus' Colonization of New Growth of Citrus

Early Population Dynamics of "Candidatus Liberibacter asiaticus" in Susceptible and Resistant Genotypes After Inoculation With Infected Diaphorina citri Feeding on Young Shoots

Huanglongbing is a highly destructive citrus disease associated with "Candidatus Liberibacter asiaticus" (Las), a phloem-limited and non-culturable bacterium, naturally transmitted by the psyllid Diaphorina citri. Although diverse approaches have been used to understand the molecular mechanisms involved in the pathogen-host interaction, such approaches have focused on already infected and/or symptomatic plants, missing early events in the initial days post-inoculation. This study aimed to identify the time course of Las multiplication and whole-plant colonization immediately following inoculation by infected psyllids feeding for 2 days. Thus, the experimental approach was to track Las titers after psyllid inoculation in new shoots (NS) of Citrus × sinensis (susceptible), Murraya paniculata (partially resistant), and Bergera koenigii (fully resistant). Soon after psyllid removal, Las titers dropped until the 10-12th days in all three species. Following this, Las titers increased exponentially only in C. × sinensis and M. paniculata, indicating active bacterial multiplication. In C. × sinensis, Las reached a stationary phase at ∼5 log Las cells/g of tissue from the 40th day onward, while in M. paniculata, Las increased at a lower rate of up to ∼3 log Las cells/g of tissue between the 40th and 60th days, decreasing gradually thereafter and becoming undetectable from the 160th day onward. In B. koenigii, Las titers decreased from the start and remained undetectable. In C. × sinensis, an average of 2.6 log of Las cells/g of tissue was necessary for Las to move out of 50% of the NS in 23.6 days and to colonize the rest of the plant, causing a successful infection. Conversely, the probability of Las moving out of the NS remained below 50% in M. paniculata and zero in B. koenigii. To our knowledge, this is the first study on Las dynamics and whole-plant colonization during the earliest stages of infection. Identification of critical time-points for either successful multiplication or Las resistance may help to elucidate initial events of Las-host interactions that may be missed due to longer sampling intervals and at later stages of infection.

Inoculation Period and Citrus Host Effect Establishment of New Infections of 'Candidatus Liberibacter asiaticus' Transmitted via Vegetative Grafting

Vegetative grafting is a common method of transmitting and propagating 'Candidatus Liberibacter asiaticus', the bacterial species accepted as the causal agent of the citrus disease huanglongbing (HLB). Generally, infected tissue that is grafted to a receptor tree remains in place indefinitely to ensure transmission. In this study, individual HLB-symptomatic leaves were grafted as 'Ca. L. asiaticus' inoculum sources to receptor trees of six citrus types and removed after an inoculation period (IP) of 21, 51, or 81 days. The goal was to assess the effect of varying IPs on transmission of bacteria to the receptor tree and on the successful establishment of a new infection. Survival analysis of data from three trials showed a significantly reduced proportion of infected trees with an IP of 21 days compared with IPs of 51 and 81 days but that there was no significant difference in the proportion of infected trees between IPs of 51 and 81 days. In addition, the time to first detection of pathogen DNA in an infected tree was delayed significantly for the 21-day IP when compared with the 51- and 81-day IPs. Survival analysis showed that the probability of infection of sweet orange trees was significantly higher than for trees of five other citrus types throughout the experiment. There was no significant difference between the infection probabilities of these latter five citrus types. The data from this study show that successful infection by grafting is dependent upon the time of exposure to the inoculum, that shorter IPs increase the time needed to establish a systemic infection, and that citrus types vary in their overall susceptibility to infection by 'Ca. L. asiaticus'.

Resistance to 'Candidatus Liberibacter asiaticus,' the Huanglongbing Associated Bacterium, in Sexually and/or Graft-Compatible Citrus Relatives

Huanglongbing (HLB) is the most destructive, yet incurable disease of citrus. Finding sources of genetic resistance to HLB-associated 'Candidatus Liberibacter asiaticus' (Las) becomes strategic to warrant crop sustainability, but no resistant Citrus genotypes exist. Some Citrus relatives of the family Rutaceae, subfamily Aurantioideae, were described as full-resistant to Las, but they are phylogenetically far, thus incompatible with Citrus. Partial resistance was indicated for certain cross-compatible types. Moreover, other genotypes from subtribe Citrinae, sexually incompatible but graft-compatible with Citrus, may provide new rootstocks able to restrict bacterial titer in the canopy. Use of seedlings from monoembryonic species and inconsistencies in previous reports likely due to Las recalcitrance encouraged us to evaluate more accurately these Citrus relatives. We tested for Las resistance a diverse collection of graft-compatible Citrinae species using an aggressive and consistent challenge-inoculation and evaluation procedure. Most Citrinae species examined were either susceptible or partially resistant to Las. However, Eremocitrus glauca and Papua/New Guinea Microcitrus species as well as their hybrids and those with Citrus arose here for the first time as full-resistant, opening the way for using these underutilized genotypes as Las resistance sources in breeding programs or attempting using them directly as possible new Las-resistant Citrus rootstocks or interstocks.