Leaf Primordia-Free Shoot Apical Meristem Culture: A New Method for Production of Viroid-Free Plants

The Elimination of Viroids through In Vitro Thermotherapy and a Meristem Tip Culture from a New Limonime Hybrid (Citrus x limon var. limon (L.) Burm. f. x Citrus latifolia var. latifolia)

Viruses and viroids pose a significant challenge in citriculture, and their control is crucial for plant health. This study evaluated the effectiveness of in vitro thermotherapy combined with a meristem tip culture for eliminating citrus exocortis viroid (CEVd) and hop stunt viroid (HSVd) from a new limonime hybrid (Citrus x limon var. limon x Citrus latifolia var. latifolia). The elimination success was confirmed by RT-PCR assays. The in vitro elimination rate for CEVd during the shoot proliferation stage (43%) was higher than for HSVd (21%). Accordingly, in the subsequent rooting stage, the in vitro elimination rate for CEVd (50%) was higher than for HSVd (33%). Successful CEVd and HSVd eradication at a 100% rate was confirmed in the ex vitro acclimatized plants in the greenhouse. The study also established an efficient micropropagation protocol. The optimal treatment for in vitro shoot induction was 0.5-2 mg L-1 benzyladenine (BA) + 0.5 mg L-1 gibberellic acid (GA3) + 0.25 mg L-1 naphthalene acetic acid (NAA), while for shoot elongation, it was 0.5 mg L-1 BA + 0.5 mg L-1 kinetin (KIN) + 0.5 mg L-1 GA3 + 0.25 mg L-1 NAA. Rooting was best promoted by 1 mg L-1 NAA. This study provides valuable insights for the mass production of viroid-free propagation material in this new lemon x lime hybrid, contributing to the conservation of genetic resources in citrus breeding programs through the combined application of in vitro thermotherapy and an in vitro meristem tip culture, a novel and highlighted achievement reported for the first time in this study.

Micrografting: An Old Dog Plays New Tricks in Obligate Plant Pathogens

Micrografting, which was developed almost 50 years ago, has long been used for virus eradication, micropropagation, regeneration, rejuvenation, and graft compatibility. Recently, micrografting has been used for studies of long-distance trafficking and signaling of molecules between scions and rootstocks. The graft transmissiveness of obligate plant pathogens, such as viruses, viroids, and phytoplasmas, facilitated the use of micrografting to study biological indexing and pathogen transmission, pathogen-induced graft incompatibility, and screening for the pathogen resistance during the past 20 years. The present study provides comprehensive information on the latter subjects. Finally, prospects are proposed to direct further studies.

Recovery of virus-free Almond (Prunus dulcis) cultivars by somatic embryogenesis from meristem undergone thermotherapy

One of the world's main horticulture problems is the contamination of fruit trees with a variety of plant diseases, especially viral and pseudo-viral diseases. Due to the non-sexual propagation of the trees, these diseases have been transmitted to different parts of the world. The main aim of this study was to obtain a new effective method for virus elimination from almond cultivars, which was performed in two phases. In the first phase, we tested various almond cultivars with ELISA and RT-PCR. The results showed the infection of mother plantlets. So, three types of in vitro thermotherapy treatments were performed on infected plants to make them virus-free. The plantlets obtained from 0.5 mm meristem treated with the first type of thermotherapy (TH1: 8 h at 27 °C and 16 h at 38 °C for 18 days) showed the highest percentage of elimination of ApM, ACLS and TRS viruses. In the second phase, meristems were cultured on MS medium containing 0, 0.5, 1 and 2 mg/L 2,4-D with 1 mg/L TDZ and after two weeks, thermotherapy treatments were performed. The results showed, combining three methods of thermotherapy (TH1), meristem culture and somatic embryogenesis induction from meristem on MS medium supplemented with 0.5 mg/L 2,4-D and 1 mg/L TDZ is the most effective and safe technique for virus eradication without meristem size challenges. The samples that were diagnosed as virus-free were proliferated in temporary immersion bioreactor systems, and rooted to be used for later propagation and establishment of mother healthy orchards.

Chrysanthemum Stunt Viroid Resistance in Chrysanthemum

Chrysanthemum stunt viroid (CSVd) is one of the most severe threats in Chrysanthemum morifolium production. Over the last decade, several studies have reported the natural occurrence of CSVd resistance in chrysanthemum germplasms. Such CSVd-resistant germplasms are desirable for the stable production of chrysanthemum plants. Current surveys include finding new resistant chrysanthemum cultivars, breeding, and revealing resistant mechanisms. We review the progress, from discovery to current status, of CSVd-resistance studies, while introducing information on the improvement of associated inoculation and diagnostic techniques.

Tobacco streak virus (strain dahlia) suppresses post-transcriptional gene silencing of flavone synthase II in black dahlia cultivars and causes a drastic flower color change

Tobacco streak virus suppressed post-transcriptional gene silencing and caused a flower color change in black dahlias, which supported the role of cyanidin-based anthocyanins for black flower appearance. Black flower color of dahlia (Dahlia variabilis) has been attributed, in part, to the high accumulation of cyanidin-based anthocyanins that occurs when flavone synthesis is reduced because of post-transcriptional gene silencing (PTGS) of flavone synthase II (DvFNS). There are also purple-flowering plants that have emerged from a black cultivar 'Kokucho'. We report that the purple color is not caused by a mutation, as previously thought, but by infection with tobacco streak virus (TSVdahlia), which suppresses the PTGS of DvFNS. When TSVdahlia was eliminated from the purple-flowering 'Kokucho' by leaf primordia-free shoot apical meristem culture, the resulting flowers were black. TSVdahlia-infected purple flowers had lower numbers of siRNAs to DvFNS than black flowers, suggesting that TSVdahlia has a silencing suppressor. The graft inoculation of other black cultivars with TSVdahlia altered their flower color drastically except for 'Fidalgo Blacky', a very deep black cultivar with the highest amount of cyanidin-based anthocyanins. The flowers of all six TSVdahlia-infected cultivars accumulated increased amounts of flavones and reduced amounts of cyanidin-based anthocyanins. 'Fidalgo Blacky' remained black despite the change in pigment accumulation, and the amounts of cyanidin-based anthocyanins in its TSVdahlia-infected plants were still higher than those of other cultivars. We propose that black flower color in dahlia is controlled by two different mechanisms that increase the amount of cyanidin-based anthocyanins: DvFNS PTGS-dependent and -independent mechanisms. If both mechanisms occur simultaneously, the flower color will be blacker than if only a single mechanism is active.