Résumé
The bacterial agents associated with “huanglongbing” (HLB) disease belong to 'Candidatus Liberibacter' and are transmitted by psyllids. The 'Ca. L. asiaticus' (CLas) lead to a reaction of the tree associated with the synthesis of callose at the phloem sieve plates. Thus, the obstruction of pores providing connections between adjacent sieve elements limits the symplastic transport of the sugars and starches synthesized through photosynthesis. The tolerance to HLB of the Persian triploid lime (Citrus latifolia) compared to diploid Mexican lime (C. aurantifolia) was studied. Analysis of the petiole sieve plate in control petiole samples showed that pores were larger in the Persian than in the Mexican lime. Scanning electron microscopy (SEM) analyses of petiole samples of symptomatic leaves showed important deposition of callose into pores of Mexican and Persian limes, whereas biochemical analyses revealed better detoxification in Persian limes than in Mexican limes. HLB-infected diploid and tetraploid citrumelo rootstocks grafted with Mexican and Persian limes were also investigated. Secondary roots were anatomically studied by SEM to observe callose deposition at the phloem sieve plates. Voltammetry of immobilized microparticles (VIMP) in roots for defining electrochemolomic profiles was applied to determine the oxidative stress status of root samples. SEM analyses of root samples did not allow observation of any callose deposition into sieve pores of diploid and tetraploid genotypes. VIMP revealed more limited oxidative stress in tetraploid samples compared to diploids. These results were even stronger when rootstocks were grafted with triploid Persian lime compared to Mexican lime and were corroborated by stronger polyphenol contents. Taking together these results underline the interest of the use of polyploid scion and rootstock to tackle HLB and reduce its impact in the fields.