Nutrient Uptake in Huanglongbing-affected Sweet Orange: Transcriptomic and Physiological Analysis

Abstract

Since the advent of Huanglongbing [HLB (Candidatus Liberibacter asiaticus)] in Florida, several preliminary reports have emerged about the positive effects of mineral nutrition on the performance of HLB-affected citrus (Citrus sp.) trees. HLB-affected trees are known to undergo significant feeder root loss. Therefore, studies have focused on foliar nutrient application instead of soil-applied nutrients speculating that the HLB-affected trees root systems may not be competent in nutrient uptake. Some studies also suggest that HLB-affected trees benefit from micronutrients at higher than the recommended rates; however, the results are often inconclusive and inconsistent. To address this, the goal of the present study was to evaluate the nutrient uptake efficiency and the quantitative and qualitative differences in nutrient uptake of HLB-affected trees. HLB-affected and healthy sweet orange (Citrus sinensis) trees were grown in a 100% hydroponic system with Hoagland solution for 8 weeks. The trees were deprived of any fertilization for 6 months before the transfer of trees to the hydroponic solution. Altogether, the four treatments studied in the hydroponic system were healthy trees fertilized (HLY-F) and not fertilized (HLY-NF), and HLB-affected trees fertilized (HLB-F) and not fertilized (HLB-NF). HLY-F and HLY-NF trees were found to have similar levels of leaf nutrients except for N, which was found to be low in nonfertilized trees (HLY and HLB). Both HLB-F and HLB-NF trees had lower levels of Ca, Mg, and S compared with HLY trees. In addition, HLB-NF trees had significantly lower levels of micronutrients Mn, Zn, and Fe, compared with HLY-NF trees. The hydroponic solution analysis showed that HLB-F and HLY-F trees had similar uptake of all the nutrients. Considering that HLB-affected trees have a lower root-to-shoot ratio than healthy trees, nutrient uptake efficiency per kilogram of root tissue was significantly higher in HLB trees compared with HLY trees. Under nutrient-deficient conditions (day 0) only nine genes were differentially expressed in HLB roots compared with HLY roots. On the other hand, when fertilizer was supplied for ≈1 week, ≈2300 genes were differentially expressed in HLB-F roots compared with HLY-F roots. A large number of differentially expressed genes in HLB-F were related to ion transport, root growth and development, anatomic changes, cell death, and apoptosis compared with HLY-F trees. Overall, anatomic and transcriptomic analyses revealed that HLB-affected roots undergo remarkable changes on transitioning from no nutrients to a nutrient solution, possibly facilitating a high uptake of nutrients. Our results suggest the roots of HLB-affected trees are highly efficient in nutrient uptake; however, a small root mass is a major limitation in nutrient uptake. Certain micronutrients and secondary macronutrients are also metabolized (possibly involved in tree defense or oxidative stress response) at a higher rate in HLB-affected trees than healthy trees. Therefore, a constant supply of fertilizer at a slightly higher rate than what is recommended for micronutrients and secondary macronutrients would be beneficial for managing HLB-affected trees.

Rootstock Influence on Growth and Mineral Content of Citrus limon and Citrus sinensis cv. Valencia Inoculated with Candidatus Liberibacter Asiaticus

“…On the contrary, the concentration of this mineral was higher (20%) in infected plants of C. sinensis and there were no variations in the content of C. limon. In this same species, a sulfur deficiency was observed; it should be clarified that other works do not report deficiencies of this mineral but it is an essential nutrient in foliar nutrition programs to counteract the adverse effects of HLB for its antibacterial activity [56,57]. Some of the results of this study differ with those reported in the literature, this is because different species were evaluated.…”

“…Like the aforementioned nutrients, HLB also causes Fe deficiencies [42,56]. On the contrary, the concentration of this mineral was higher (20%) in infected plants of C. sinensis and there were no variations in the content of C. limon.…”

Utilization of somatic fusion techniques for the development of HLB tolerant breeding resources employing the Australian finger lime (Citrus australasica)

“…Several genes are differentially expressed between susceptible and tolerant citrus following infection by Ca Las [ 30 , 63 , 64 ]. Additionally, several ion transport genes are differentially expressed between Ca Las - infected and healthy citrus trees [ 65 ]. The expression levels of PR1 and PR2 genes have long been used to gauge plant defense responses [ 66 , 67 ].…”

“…Auxins are known to alter the expression of various genes [ 76 ] and play a role in plant defense [ 77 ]. Enhanced auxin levels can sometimes facilitate pathogenesis [ 78 ] but the selected gene (ABF3) evaluated in this study was downregulated in HLB-infected plants [ 65 ]. The CAM8 evaluated in this study was highly upregulated in the FL mesophyll parent and somatic hybrid line 1.…”

<strong>Identification of huanglongbing Based on Visual Symptoms: a Grower’s Diagnostic Toolkit</strong>

“…Before the really visible symptoms appear, initiation of root dieback observed, the proportion of root-shoot ratio decreases over time in HLB affected trees (Johnson et al, 2014). One thing is clarified by (Shahzad et al, 2020) that health tress is a higher amount of biomass for root, shoot, and lead compared to the HLB affected trees, and data revealed remarkably that when affected with HLB, nearly 40 percent to 50 percent less root biomass is recorded compared to healthy trees.…”