Systems biology of resurrection plants.

Plant species that exhibit vegetative desiccation tolerance can survive extreme desiccation for months and resume normal physiological activities upon re-watering. Here we survey the recent knowledge gathered from the sequenced genomes of angiosperm and non-angiosperm desiccation-tolerant plants (resurrection plants) and highlight some distinct genes and gene families that are central to the desiccation response. Furthermore, we review the vast amount of data accumulated from analyses of transcriptomes and metabolomes of resurrection species exposed to desiccation and subsequent rehydration, which allows us to build a systems biology view on the molecular and genetic mechanisms of desiccation tolerance in plants.

First report of Cucurbit aphid-borne yellows virus causing yellowing disease on pumpkin (Cucurbita pepo) and cucumber (Cucumis sativus) in Bulgaria.

Cucurbit aphid-borne yellows virus (CABYV) was first reported in France in 1992 but has since been observed worldwide (Lecoq et al. 1992; Choi and Choi 2016; Buzkan et al. 2017; Zindovic et al. 2017; Vidal et al. 2018; Khanal and Ali, 2018). This virus has caused severe losses to different crops especially to the members of Cucurbitaceae and yield losses can reach up to 40-50% if infection occurs at early stages (Lecoq et al. 1992). In July 2017, leaf samples showing virus-like symptoms were collected from five pumpkin (Cucurbita pepo L. var. Clypeata Alefield) and two cucumber (Cucumis sativus L. cv. Azuma matsunari) plants, growing in а field near Sadovo, Bulgaria. Nearly all plants in the field were affected and displayed green or yellow mosaic, interveinal yellowing, blisters, and leaf deformation (Fig. 1). The collected samples were all symptomatic and were subjected to double antibody sandwich (DAS) or triple antibody sandwich (TAS) enzyme-linked immunosorbent assay (ELISA) to determine the viral agent(s). Specific monoclonal antibodies (Leibniz institute DSMZ, Germany) raised against Cucumber leaf spot virus, Cucurbit chlorotic yellows virus, Cucurbit yellow stunting disorder virus, Cucumber mosaic virus (CMV), Melon necrotic spot virus, Beet western yellows virus (BWYV), CABYV, Watermelon mosaic virus (WMV), Zucchini yellow mosaic virus (ZYMV), and Cucumber green mottle mosaic virus, were used. The total number of tested samples was seven (n=5 from pumpkin and n=2 from cucumber). All of them displayed positive signals for CABYV and BWYV, both belonging to genus Polerovirus, family Luteoviridae. In addition, ZYMV and/or WMV were detected in pumpkins while CMV and/or WMV were detected in cucumber samples, respectively. To confirm the presence of CABYV and/or BWYV, total RNA was isolated from all seven samples by TRI Reagent® (Sigma, St. Louis, USA) and converted to cDNA with First Strand cDNA Synthesis Kit, Thermo Scientific™. Reverse transcription (RT)-polyemerase chain reaction (PCR) was performed using two pairs of primers (CABYV1FW: 5'-TTATCAGGGGACTATGTTTA-3' and CABYV14REV: 5'-GAGGGGATTTTAACTGACTG-3', and BWYV1FW: 5'-AGTAAGTCCTCCCCAACTGA-3' and BWYV2REV: 5'-CTACCCACGACCGTATTCAT-3'), specifically designed to detect CABYV and BWYV, respectively. Amplicons with expected sizes of 1,930 bp were obtained only with CABYV primers for all samples while no fragments were amplified with BWYV primers. The obtained products from two samples (pumpkin and cucumber) were purified and sent to Macrogen Inc., South Korea, for direct sequencing in both directions. High quality nucleotide sequences were submitted to GenBank We have evaluated the quality of the sequencing and trimmed those parts that did not comply the needed quality. The obtained smaller fragments Nucleotide sequences were submitted to GenBank with accession numbers MK671010 (656 bp) and MK671014 (712bp). These sequences contained ORFs encoding CABYV P1-P2 fusion proteins as determined by Blastp analysis (https://blast.ncbi.nlm.nih.gov/Blast.cgi?PAGE=Proteins). A phylogenetic tree constructed by the Neighbor-joining method using 18 CABYV accessions and Potato leafroll virus as an outlier (Fig. 2) showed that the closest accessions to MK671010 and MK671014 were NC003688 (France) and EU636992 (China) with respective nucleotide identity of 98% and 99%. In 2019, another outbreak was observed in the same field near Sadovo and in a field near Plovdiv planted with pumpkins. Nearly 30% of the plants showed leaf yellowing typical for Polerovirus infection. Screening of collected samples (n=17) by RT-PCR confirmed CABYV presence in 15 samples. Based on available reports and according to our knowledge this is the first report of CABYV in Bulgaria.

Vegetative desiccation tolerance in the resurrection plant Xerophyta humilis has not evolved through reactivation of the seed canonical LAFL regulatory network.

It has been hypothesised that vegetative desiccation tolerance in resurrection plants evolved via reactivation of the canonical LAFL (i.e. LEC1, ABI3, FUS3 and LEC2) transcription factor (TF) network that activates the expression of genes during the maturation of orthodox seeds leading to desiccation tolerance of the plant embryo in most angiosperms. There is little direct evidence to support this, however, and the transcriptional changes that occur during seed maturation in resurrection plants have not previously been studied. Here we performed de novo transcriptome assembly for Xerophyta humilis, and analysed gene expression during seed maturation and vegetative desiccation. Our results indicate that differential expression of a set of 4205 genes is common to maturing seeds and desiccating leaves. This shared set of genes is enriched for gene ontology terms related to abiotic stress, including water stress and abscisic acid signalling, and includes many genes that are seed-specific in Arabidopsis thaliana and targets of ABI3. However, while we observed upregulation of orthologues of the canonical LAFL TFs and ABI5 during seed maturation, similar to what is seen in A. thaliana, this did not occur during desiccation of leaf tissue. Thus, reactivation of components of the seed desiccation program in X. humilis vegetative tissues likely involves alternative transcriptional regulators.

Comparative Analysis of ROS Network Genes in Extremophile Eukaryotes.

The reactive oxygen species (ROS) gene network, consisting of both ROS-generating and detoxifying enzymes, adjusts ROS levels in response to various stimuli. We performed a cross-kingdom comparison of ROS gene networks to investigate how they have evolved across all Eukaryotes, including protists, fungi, plants and animals. We included the genomes of 16 extremotolerant Eukaryotes to gain insight into ROS gene evolution in organisms that experience extreme stress conditions. Our analysis focused on ROS genes found in all Eukaryotes (such as catalases, superoxide dismutases, glutathione reductases, peroxidases and glutathione peroxidase/peroxiredoxins) as well as those specific to certain groups, such as ascorbate peroxidases, dehydroascorbate/monodehydroascorbate reductases in plants and other photosynthetic organisms. ROS-producing NADPH oxidases (NOX) were found in most multicellular organisms, although several NOX-like genes were identified in unicellular or filamentous species. However, despite the extreme conditions experienced by extremophile species, we found no evidence for expansion of ROS-related gene families in these species compared to other Eukaryotes. Tardigrades and rotifers do show ROS gene expansions that could be related to their extreme lifestyles, although a high rate of lineage-specific horizontal gene transfer events, coupled with recent tetraploidy in rotifers, could explain this observation. This suggests that the basal Eukaryotic ROS scavenging systems are sufficient to maintain ROS homeostasis even under the most extreme conditions.

The window of desiccation tolerance shown by early-stage germinating seedlings remains open in the resurrection plant, Xerophyta viscosa.

Resurrection plants are renowned for their vegetative desiccation tolerance (DT). While DT in vegetative tissues is rare in angiosperms, it is ubiquitous in mature orthodox seeds. During germination, seedlings gradually lose DT until they pass a point of no return, after which they can no longer survive dehydration. Here we investigate whether seedlings of the resurrection plant Xerophyta viscosa ever lose the capacity to establish DT. Seedlings from different stages of germination were dehydrated for 48 hours and assessed for their ability to recover upon rehydration. While a transient decline in the ability of X. viscosa seedlings to survive dehydration was observed, at no point during germination was the ability to re-establish DT completely lost in all seedlings. Pre-treatment of seedlings with PEG or sucrose reduced this transient decline, and improved the survival rate at all stages of germination. Additionally, we observed that the trait of poikilochlorophylly (or loss of chlorophyll) observed in adult X. viscosa leaves can be induced throughout seedling development. These results suggest that the window of DT seen in germinating orthodox seeds remains open in X. viscosa seedlings and that vegetative DT in Xerophyta species may have evolved from the ability to retain this program through to adulthood.