Dror, Orit

‘ Candidatus Liberibacter solanacearum’ (Lso), haplotype D, is an insect-transmitted, phloem-limited bacterium that induces developmental abnormalities in carrots, including witches’ broom and hairy root symptoms. We hypothesize that these symptoms result from Lso-induced hormonal imbalances. To investigate this, we analyzed the spatial and temporal distribution of Lso in carrot plants and assessed its effects on hormone-related gene expression and phytohormone levels. Our findings revealed that Lso first accumulates in the shoot apical meristem before spreading to root tissues, aligning with phloem flow dynamics. Transcriptomic and biochemical analyses indicated that cytokinin (CK) biosynthesis and response genes were upregulated, whereas gibberellin biosynthesis genes were downregulated. In contrast, no significant changes were observed in auxin biosynthesis or signaling. Hormone quantification further demonstrated increased CK levels in lateral roots and decreased CK levels in the root meristem of infected plants, with no detectable changes in auxin levels. Additionally, salicylic acid and jasmonic acid were significantly elevated following Lso infection, suggesting a persistent plant defense response. To validate the role of CK and auxin in symptom development, we applied synthetic growth regulators to infected and uninfected plants. CK treatment exacerbated witches’ broom symptoms, whereas auxin application mitigated this phenotype but enhanced lateral root formation. These results suggest that Lso manipulates phytohormone homeostasis to induce disease symptoms, offering a potential avenue for symptom mitigation through targeted hormone applications. Our findings provide novel insights into the role of plant hormones in Lso pathogenesis and highlight new strategies for managing carrot yellows disease.

‘ Candidatus Liberibacter solanacearum’ is an insect-transmitted bacterium associated with several plant diseases. In the Mediterranean Basin, ‘ Ca. L. solanacearum’ haplotype D is vectored by Bactericera trigonica and can severely infect carrot plants leading to abnormal growth phenotypes and significant yield losses. Insecticide applications are insufficient to suppress disease spread and damage, and additional means for disease control are needed. In the current study, we evaluated the resistance of 97 carrot accessions to the bacterial pathogen ‘ Ca. L. solanacearum’ and its associated symptoms. Accessions (Western and Asian types) were first screened in two commercial carrot fields. We found that Western type accessions were less prone to develop disease symptoms in both fields and were less frequently visited by the insect vector in one field. Overall, 22 Asian and five Western accessions with significantly lower disease incidence compared with the commercial cultivar were found. These accessions were then inoculated with ‘ Ca. L. solanacearum’ under controlled conditions and were assessed for disease incidence, insect oviposition, and bacterial relative titer. Five accessions (three Asian and two Western) had significantly lower disease incidence compared with the reference cultivar. Interestingly, disease incidence was not necessarily in line with insect oviposition or in planta bacterial titer, which may indicate that other, perhaps physiological, differences among the accessions may govern the susceptibility of plants to the disease. The resistant accessions found in this study could be used in future resistance breeding programs and to better understand the underlying mechanisms of resistance to ‘ Ca. L. solanacearum’.

A decade ago, shoot proliferation symptoms (i.e., witches’ broom) in carrots were believed to be the cause of ‘Candidatus Phytoplasma’ and Spiroplasma infection, yet in recent years this association appeared to have weakened, and a closer association was found with the yet-unculturable, psyllid-transmitted Gram-negative bacterium ‘Candidatus Liberibacter solanacearum’. In Israel, carrots are grown throughout the year, yet shoot proliferation symptoms tend to appear only in mature plants and mostly in late spring to early summer. We hypothesized that factors such as plant age, temperature, and vector load, which vary during the year, have a critical effect on symptom development and examined these factors under controlled conditions. Here we show that young carrot seedlings are as prone as older plants to develop shoot proliferation symptoms after ‘Ca. L. solanacearum’ inoculation. Surprisingly, we found that the local ‘Ca. L. solanacearum’ haplotype was extremely sensitive to constant temperature of 30°C, which led to a significant reduction in bacterial growth and symptom development compared with 18°C, which was very conducive to symptom development. We have also found that inoculations with 10 or 20 psyllids per plant results in faster symptom development compared with inoculations with two psyllids per plant; however, the difference between vector loads in disease progress rate was not significant. These data provide important insights to the effects of plant age, growth temperature, and vector load on ‘Ca. L. solanacearum’ and its associated symptoms and further strengthen the notion that ‘Ca. L. solanacearum’ is the main responsible agent for carrot witches’ broom in Israel. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .