RESUMO

Huanglongbing (HLB) is presently a major threat to the citrus industry. Because of this disease, millions of trees are currently dying worldwide. The putative causal agent is a motile bacteria belonging to Candidatus Liberibacter spp., which is transmitted by psyllids. The bacteria is responsible for the synthesis of callose at the phloem sieve plate, leading to the obstruction of the pores that provide connections between adjacent sieve elements, thus limiting the symplastic transport of the sugars and starches synthesized in leaves to the other plant organs. The Persian triploid lime (Citrus latifolia) is one of the most HLB-tolerant citrus varieties, but the determinants associated with the tolerance are still unknown. HLB-infected diploid Mexican lime (Citrus aurantiifolia) and Persian lime were investigated. The leaf petiole was analyzed using scanning electron microscopy (SEM) to observe callose deposition at the phloem sieve plate. Leaf starch contents and detoxification enzyme activities were investigated. In the field, Persian lime leaves present more limited symptoms due to HLB than the Mexican lime leaves do. Photosynthesis, stomatal conductance, and transpiration decreased compared with control plants, but values remained greater in the Persian than in the Mexican lime. Analysis of the petiole sieve plate in control petiole samples showed that pores were approximately 1.8-fold larger in the Persian than in the Mexican lime. SEM analyses of petiole samples of symptomatic leaves showed the important deposition of callose into pores of Mexican and Persian limes, whereas biochemical analyses revealed better detoxification in Persian limes than in Mexican limes. Moreover, SEM analyses of infected petiole samples of asymptomatic leaves showed much larger callose depositions into the Mexican lime pores than in the Persian lime pores, whereas biochemical traits revealed much better behavior in Persian limes than in Mexican limes. Our results reveal that polyploids present specific behaviors associated with important physiological and biochemical determinants that may explain the better tolerance of the Persian lime against HLB compared with the Mexican lime.

RESUMO

Combination of biotic and abiotic stress is a major challenge for crop and fruit production. Thus, identification of genes involved in cross-response to abiotic and biotic stress is of great importance for breeding superior genotypes. Lectins are glycan-binding proteins with a functions in the developmental processes as well as in the response to biotic and abiotic stress. In this work, a lectin like gene, namely ClLectin1, was characterized in Volkamer lemon and its expression was studied in plants exposed to either water stress, hormonal elicitors (JA, SA, ABA) or wounding to understand whether this gene may have a function in the response to multiple stress combination. Results showed that ClLectin1 has 100% homology with a L-type lectin gene from C. sinensis and the in silico study of the 5'UTR region showed the presence of cis-responsive elements to SA, DRE2 and ABA. ClLectin1 was rapidly induced by hormonal treatments and wounding, at local and systemic levels, suggesting an involvement in defence signalling pathways and a possible role as fast detection biomarker of biotic stress. On the other hand, the induction of ClLectin1 by water stress pointed out a role of the gene in the response to drought. The simultaneous response of ClLectin1 expression to water stress and SA treatment could be further investigated to assess whether a moderate drought stress may be useful to improve citrus performance by stimulating the SA-dependent response to biotic stress.

Assuntos

RESUMO

Scion/rootstock interaction is important for plant development and for breeding programs. In this context, polyploid rootstocks presented several advantages, mainly in relation to biotic and abiotic stresses. Here we analyzed the response to drought of two different scion/rootstock combinations presenting different polyploidy: the diploid (2x) and autotetraploid (4x) Rangpur lime (Citrus limonia, Osbeck) rootstocks grafted with 2x Valencia Delta sweet orange (Citrus sinensis) scions, named V/2xRL and V/4xRL, respectively. Based on previous gene expression data, we developed an interactomic approach to identify proteins involved in V/2xRL and V/4xRL response to drought. A main interactomic network containing 3,830 nodes and 97,652 edges was built from V/2xRL and V/4xRL data. Exclusive proteins of the V/2xRL and V/4xRL networks (2,056 and 1,001, respectively), as well as common to both networks (773) were identified. Functional clusters were obtained and two models of drought stress response for the V/2xRL and V/4xRL genotypes were designed. Even if the V/2xRL plant implement some tolerance mechanisms, the global plant response to drought was rapid and quickly exhaustive resulting in a general tendency to dehydration avoidance, which presented some advantage in short and strong drought stress conditions, but which, in long terms, does not allow the plant survival. At the contrary, the V/4xRL plants presented a response which strong impacts on development but that present some advantages in case of prolonged drought. Finally, some specific proteins, which presented high centrality on interactomic analysis were identified as good candidates for subsequent functional analysis of citrus genes related to drought response, as well as be good markers of one or another physiological mechanism implemented by the plants.

Assuntos