Witches’ broom disease of lime contributes to phytoplasma epidemics and attracts insect vectors

An insect-transmitted phytoplasma causing Witches’ Broom Disease of Lime (WBDL) is responsible for the drastic decline in lime production in several countries. However, it is unclear how WBDL phytoplasma (WBDLp) induces witches’ broom symptoms and if these symptoms contribute to the spread of phytoplasma. Here we show that the gene encoding SAP11 of WBDLp (SAP11WBDL) is present in all WBDLp isolates collected from diseased trees. SAP11WBDL interacts with acid lime (Citrus aurantifolia) TCP transcription factors, specifically members of the TB1/CYC class that have a role in suppressing axillary branching in plants. Sampling of WBDLp-infected lime trees revealed that WBDLp titers and SAP11WBDL expression levels were higher in symptomatic leaves compared to asymptomatic sections of the same trees. Moreover, the witches’ brooms were found to attract the vector leafhopper. Defense genes that have a role in plant defense responses to bacteria and insects are more downregulated in witches’ brooms compared to asymptomatic sections of trees. These findings suggest that witches’ broom-affected parts of the trees contribute to WBDL epidemics by supporting higher phytoplasma titers and attracting insect vectors.

Micropropagation of Mountain Mulberry (Morus bombycis Koidz.) ‘Kenmochi’ on Cytokinin-Free Medium

The aim of the study was to compare two methods of micropropagation of mulberry: single-node culture (“SNC”), and axillary-branching (“AxB”). The experiments were carried out on in vitro cultures for 6 successive passages. The “AxB” cultures were propagated on modified MS medium (+ 25% Ca2+ and Mg2+), supplemented with WPM vitamins, sucrose (30 g L−1), and BA (1.5 mg l-1). The “SNC” cultures were grown on cytokinin-free 1/2 MS (macro- and micronutrients) medium supplemented with WPM vitamins, IBA (0.05 mg l-1), and sucrose (15 g l-1). Both media (pH 5.8) were solidified with agar (7.0 g l-1). Initiation of in vitro cultures from explants taken from adult trees and young, potted plants was feasible on both media. Cultures were established from about 1 cm long nodal explants. Generally “SNC” cultures formed one well rooted, significantly longer axillary shoot with bigger leaves than “AxB” cultures, which developed significantly more shoots and big callus at the explant base. All shoots collected from “SNC” and “AxB” cultures rooted in vivo in peat mixture and developed into similar plantlets. The single-node method based on application of cytokinin-free medium is a good alternative for the axillary-branching method for micropropagation of mountain mulberry (Morus bombycis) ‘Kenmochi”’.

Micropropagation of Scutellaria baicalensis Georgi

Plant propagation via nodal explants of <em>Scutellaria baicalensis</em> Georgi (Lamiaceae) was studied. Murashige and Skoog (MS) medium supplemented either with NAA and kinetin or NAA and BA in different concentrations was used for this purpose. The most favourable for axillary branching was MS medium supplemented with 0.5 µM NAA and 2.5 µM kinetin. The number of regenerated shoots per explant reached 3.8 ± 1.3. To increase the multiplication rate a two stage procedure of propagation was also elaborated. First, an induction of organogenic calli was achieved using thidiazuron (TDZ) as a sole growth regulator in the nutrient medium (concentration range: 0.1-0.5 µM). Next, regeneration of shoots on the medium containing 2.2 µM BA was performed. Over 20 shoots per explant could be obtained following the described procedure. The regenerated shoots rooted easily on a modified MS medium (1/2 macronutrients) and were successfully adapted to growth in pots.