Differences in stylet sheath occurrence and the fibrous ring (sclerenchyma) between xCitroncirus plants relatively resistant or susceptible to adults of the Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae)

Abstract

The Asian citrus psyllid (ACP, Diaphorina citri, Hemiptera: Liviidae), is the principal vector of the phloem-limited bacteria strongly associated with huanglongbing (HLB), the world's most serious disease of citrus. Host plant resistance may provide an environmentally safe and sustainable method of controlling ACP and/or HLB. Two xCitroncirus accessions (hybrids of Poncirus trifoliata and Citrus spp.), that are relatively resistant (UN-3881) or relatively susceptible (Troyer-1459) to ACP adults with regard to adult longevity, were compared in relation to ACP feeding behavior and some structural features of the leaf midrib. The settling (putative feeding/probing) sites of ACP adults on various parts of the leaf were not influenced primarily by plant accession. However, fewer ACP stylet sheaths were found in the midrib and fewer stylet sheath termini reached the vascular bundle (phloem and/or xylem) in UN-3881 compared to Troyer-1459 plants. Furthermore, in midribs of UN-3881 leaves the fibrous ring (sclerenchyma) around the phloem was significantly wider (thicker) compared to that in midribs of Troyer-1459 leaves. Our data indicate that feeding and/or probing by ACP adults into the vascular bundle is less frequent in the more resistant (UN-3881) than in the more susceptible (Troyer-1459) accessions. Our results also suggest that the thickness of the fibrous ring may be a barrier to stylet penetration into the vascular bundle, which is important for successful ACP feeding on the phloem and for transmitting HLB-associated bacteria. These results may help in the development of citrus plants resistant to ACP, which in turn could halt or slow the spread of the HLB-associated bacteria by this vector.

Figure 1

A, Clip cages used for confining adults of D. citri on xCitroncirus leaves. B–F, Epifluorescence micrographs showing stylet sheaths (ss) of D. citri in cross sections of the midrib in leaves of the relatively susceptible (Troyer-1459) plants. In B–F, the upper (adaxial) leaf side is up, and the lower (abaxial) side is down; unlabeled single arrows indicate smaller gaps in the fibrous ring; double arrows indicate wider gaps in this ring at the sides of the vascular bundle. Abbreviations: cc, central canal in the stylet sheath; co, core cells; fr, upper fibrous ring; frl, lower fibrous ring; gp, ground/spongy parenchyma; pc, palisade parenchyma cells; ph, upper phloem; phl, lower phloem; ss, stylet sheath; xy, upper xylem; xyl, lower xylem. Scale bars = 50 µm.

Figure 2. Epifluorescence micrographs showing stylet sheaths (ss) of D. citri in cross sections of the midrib in leaves of the relatively resistant (UN-3881) plants.

In A–E, the upper (adaxial) leaf side is up, and the lower (abaxial) side is down; unlabeled single arrows indicate smaller gaps in the fibrous ring; double arrows indicate wider gaps in this ring at the sides of the vascular bundle. Abbreviations: cc, central canal in the stylet sheath; co, core cells; fr, upper fibrous ring; frl, lower fibrous ring; gp, ground/spongy parenchyma; pc, palisade parenchyma cells; ph, upper phloem; phl, lower phloem; ss, stylet sheath; xy, upper xylem; xyl, lower xylem. Scale bars = 50 µm.

Figure 3. Epifluorescence micrographs showing stylet sheaths (ss) of D. citri in cross sections of the midrib (A) or leaf blade (B) of the relatively resistant (UN-3881) plants.

Abbreviations: frl, lower fibrous ring; gp, ground/spongy parenchyma; pc, palisade parenchyma cells; phl, lower phloem; ss, stylet sheath; ssb, part of stylet sheath branching inside the fibrous ring; xyl, lower xylem. Scale bars = 50 µm.

Figure 4. Stylet sheath interactions with the ground parenchyma and fibrous ring in cross sections, stained with propidium iodide, in the midrib of D. citri-resistant Poncirus trifoliata (A & B) or the relatively susceptible xCitroncirus (C & D) plants.

A & C, epifluorescence microscopy images; B & D, confocal laser scanning images of the same or adjacent sections as those in A and C, respectively; differential interference contrast was also used in B–D to show cell boundaries. Unlabeled single arrows indicate smaller gaps in the fibrous ring; double arrows indicate wider gaps in this ring at the sides of the vascular bundle. Abbreviations: cc, central canal inside sheath; fl, stylet sheath flange; fr, upper fibrous ring; frl, lower fibrous ring; gp, ground/spongy parenchyma; lb, larger blebs (secretion bursts) in the stylet sheath; pc, palisade parenchyma cells; phl, lower phloem; ss, stylet sheath; ssf, part of stylet sheath going around fibrous ring; xyl, lower xylem. Scale bars = 50 µm.