Specific Physiological and Anatomical Traits Associated With Polyploidy and Better Detoxification Processes Contribute to Improved Huanglongbing Tolerance of the Persian Lime Compared With the Mexican Lime

Abstract

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.

Enhanced Photosynthetic Capacity, Osmotic Adjustment and Antioxidant Defenses Contribute to Improve Tolerance to Moderate Water Deficit and Recovery of Triploid Citrus Genotypes

“…Such trends were also observed in polyploid citrus under severe water deficit [31,48] or salinity stress [29]. Moreover, enhanced leaf gas exchange parameters were reported in 3x Persian Lime subjected to HLB stress [32]. These enhanced photosynthetic capacity was also related to morphological properties induced by polyploidy [27,49].…”

“…Increased SOD and APX activities were also observed in clementine grafted onto tetraploid Carrizo citrange under natural chilling stress [69]. In the same way, stronger APX and CAT activities were recorded in triploid Persian lime, which favored the oxidative stress-induced HLB limitation [32]. Moreover, these authors reported that CAT activity was the key marker that differentiate the triploid Persian lime from 2x Mexican lime under HLB stress.…”

“…Moreover, triploids are heterozygous, and each hybrid exhibits a specific allelic combination that can lead to high gene dosage and thus may contribute to better adaptation to stressful conditions [27]. While tetraploidy in citrus, especially in rootstocks, has been related to improved tolerance to abiotic stresses [28][29][30], the enhanced tolerance to biotic and abiotic stresses of triploid citrus scions remains to be confirmed [31,32].…”

PYL1- and PYL8-like ABA Receptors of Nicotiana benthamiana Play a Key Role in ABA Response in Seed and Vegetative Tissue

“…Indeed, polyploidy is considered a driving force in plant evolution and adaptation [ 43 ], and either auto- or allopolyploidization events have contributed to speciation and diversification [ 42 ]. Recent research has revealed the adaptive effect of polyploidy on plant response to global climate change as well as improving tolerance to plant pathogens [ 43 , 44 ]. Polyploidy also leads in woody plants to larger xylem and phloem conduits, lowering hydraulic resistance and facilitating long-distance transport between organs [ 45 ].…”

Transcriptome wide identification of CDR family in Citrus latifolia and its expression during HLB infection

“…Persian lime is still considered one of the least susceptible to HLB when compared to sweet citrus such as orange (C. sinensis), tangerine (C. clementina), and even to other limes such as Mexican lime (C. aurantifolia), as Persian lime trees are able of continue their production for many years after CLas has been detected (Evans et al 2014). Recently, HLB diseased Persian lime treed maintained high physiological performance in terms of photosynthesis, stomatal conductance and transpiration (Sivager et al 2021), suggesting that phloem activity can be maintained in this species during HLB infection.…”