While HLB is a devastating citrus disease, improved tolerant cultivars, like Sugar Belle (SB) mandarin, have been identified. To understand the responses that HLB-affected SB undergoes, we compared 14CO2 fixation, carbohydrate export, phloem callose accumulation, relative expression of plant defense activators, and anatomical changes between healthy and infected SB trees versus susceptible Pineapple (PA) sweet orange. Eight to 10-weeks-old leaves of infected SB showed a 2.5-fold increase in 14CO2 fixation and a 13% decrease in 14C-carbohydrate export whereas HLB-affected PA presented a decrease of 33% and 50%, respectively. Mean distance of a callose deposit to its closest neighbor was 36% smaller in infected SB versus healthy while in HLB-affected PA it was 33% higher. Expression of papain-like cysteine proteases (PLCPs) was upregulated in SB but downregulated in PA. Infected SB showed minor alterations in the number of xylem vessels, 16% larger xylem vessel’s lumen area, and a 14% increase in the proportional area of the xylem. In contrast, PA showed a 2.4-fold increase in xylem vessel’s number and a 2% increase in the proportional xylem area. Three complementary mechanisms of tolerance in SB are hypothesized: i) increased carbohydrate availability induced by greater CO2 fixation, mild effect in carbohydrate export, and local accumulation of callose in the phloem, ii) activation of defense response via upregulation of PLCPs, and iii) increased investment in xylem structure. Thus, phloem and xylem modifications seem to be involved in SB tolerance.

Growth of aloe vera (Aloe barbadensisMiller

Plant bacterial pathogens transmitted by Hemipteran vectors pose a large threat to the agricultural industry worldwide. While virus-vector relationships have been widely investigated, a significant gap exists in our understanding of the molecular interactions between circulative bacteria and their insect vectors, mainly leafhoppers and psyllids. In this review, we will describe how these bacterial pathogens adhere, invade, and proliferate inside their insect vectors. We will also highlight the different transmission routes and molecular factors of phloem-limited bacteria that maintain an effective relationship with the insect host. Understanding the pathogen-vector relationship at the molecular level will help in the management of vector-borne bacterial diseases.