Crocus sativus is the only plant species which produces apocarotenoids like crocin, picrocrocin and safranal in significant amounts. These compounds impart organoleptic properties to saffron (dried stigmas of Crocus flower) making it world’s costliest spice. Crocus apocarotenoids have tremendous medicinal properties as well. Effect of endophytes on Crocus apocarotenoid production and the molecular mechanism involved has not been reported so far. Here we studied the effect of an oleaginous fungal endophyte, Mortierella alpina CS10E4 on Crocus growth, apocarotenoid metabolism and tolerance to corm rot disease. The results demonstrated that there was a significant improvement in many morphological and physiological traits in endophyte treated Crocus plants including total biomass and size of corms, stigma biomass, number of apical sprouting buds, and number of adventitious roots. The endophyte also shifted metabolic flux towards enhanced production of apocarotenoids by modulating the expression of key pathway genes. Further, M. alpina CS10E4 enhanced tolerance to corm rot disease by releasing arachidonic acid which acts as conserved defense signal and induces jasmonic acid production in endophyte treated Crocus corms. This is first report on effect of a fungal endophyte on Crocus apocarotenoid metabolism and stress tolerance.

Crocus sativus stigmas form rich source of apocarotenoids like crocin, picrocrocin and saffranal which besides imparting color, flavour and aroma to saffron spice also have tremendous pharmacological properties. Inspite of their importance, the biosynthetic pathway of Crocus apocarotenoids is not fully elucidated. Moreover, the mechanism of their stigma specific accumulation remains unknown. Therefore, deep transcriptome sequencing of Crocus stigma and rest of the flower tissue was done to identify the genes and transcriptional regulators involved in the biosynthesis of these compounds. Transcriptome of stigma and rest of the flower tissue was sequenced using Illumina Genome Analyzer IIx platform which generated 64,604,402 flower and 51,350,714 stigma reads. Sequences were assembled de novo using trinity resulting in 64,438 transcripts which were classified into 32,204 unigenes comprising of 9853 clusters and 22,351 singletons. A comprehensive functional annotation and gene ontology (GO) analysis was carried out. 58.5 % of the transcripts showed similarity to sequences present in public databases while rest could be specific to Crocus. 5789 transcripts showed similarity to transcription factors representing 76 families out of which Myb family was most abundant. Many genes involved in carotenoid/apocarotenoid pathway were identified for the first time in this study which includes zeta-carotene isomerase and desaturase, carotenoid isomerase and lycopene epsilon-cyclase. GO analysis showed that the predominant classes in biological process category include metabolic process followed by cellular process and primary metabolic process. KEGG mapping analysis indicated that pathways involved in ribosome, carbon and starch and sucrose metabolism were highly represented. Differential expression analysis indicated that key carotenoid/apocarotenoid pathway genes including phytoene synthase, phytoene desaturase and carotenoid cleavage dioxygenase 2 are enriched in stigma thereby providing molecular proof for stigma to be the site of apocarotenoid biosynthesis. This data would provide a rich source for understanding the carotenoid/apocarotenoid metabolism in Crocus. The database would also help in investigating many questions related to saffron biology including flower development.

Glycosylation and deglycosylation are impressive mechanisms that allow plants to regulate the biological activity of an array of secondary metabolites. Although glycosylation improves solubility and renders the metabolites suitable for transport and sequestration, deglycosylation activates them to carry out biological functions. Herein, we report the functional characterization of CsBGlu12, a β-glucosidase from Crocus sativus. CsBGlu12 has a characteristic glucoside hydrolase 1 family (α/β)₈ triose-phosphate isomerase (TIM) barrel structure with a highly conserved active site. In vitro enzyme activity revealed that CsBGlu12 catalyzes the hydrolysis of flavonol β-glucosides and cello-oligosaccharides. Site-directed mutagenesis of any of the two conserved catalytic glutamic acid residues (Glu²⁰⁰ and Glu⁴¹⁴) of the active site completely abolishes the β-glucosidase activity. Transcript analysis revealed that Csbglu12 is highly induced in response to UV-B, dehydration, NaCl, methyl jasmonate, and abscisic acid treatments indicating its possible role in plant stress response. Transient overexpression of CsBGlu12 leads to the accumulation of antioxidant flavonols in Nicotiana benthamiana and confers tolerance to abiotic stresses. Antioxidant assays indicated that accumulation of flavonols alleviated the accretion of reactive oxygen species during abiotic stress conditions. β-Glucosidases are known to play a role in abiotic stresses, particularly dehydration through abscisic acid; however, their role through accumulation of reactive oxygen species (ROS) scavenging flavonols has not been established. Furthermore, only one β-glucosidase 12 homolog has been characterized so far. Therefore, this work presents an important report on characterization of CsBGlu12 and its role in abiotic stress through ROS scavenging.

Plants are exposed to a diverse range of biotic stressors, including fungi, bacteria, nematodes, insects and viruses. To combat these enemies, plants have developed an arsenal of defense mechanisms over time, among which secondary metabolites are the most effective. Moreover, to overcome the negative impact of chemical pesticides, the plant’s secondary metabolites can be harnessed to develop novel disease management strategies. Alkaloids, flavonoids, terpenes and essential oils are major pathogen/pest-responsive secondary metabolite classes in plants. Among these, saponins have shown significant potential in suppressing a wide range of plant pathogens. However, they are yet to be explored thoroughly compared to other secondary metabolites in plant defense, and therefore, a low number of disease control agents exist in agri-markets based on saponins. Thus, this review aims to rectify this bias by identifying and acknowledging the significance of saponins as being on par with other classes of secondary metabolites in plant defense systems. It also provides the first holistic review on the role of saponins with known mechanisms against all of the major plant pathogens/pests. Furthermore, this review discusses the potential of saponin-rich crops in providing eco-friendly pest/pathogen management products for integrated pest management (IPM) and prospectives on the potential of saponin derivatives in developing novel biocides for sustainable agriculture.

Crocus sativus is a triploid sterile plant with long red stigmas which form commercial saffron. Saffron is the site for synthesis and accumulation of apocarotenoids like crocin, picrocrin and safranal which are responsible for its color, flavour and aroma making it world’s most expensive spice. These compounds are formed by oxidative cleavage of zeaxanthin by carotenoid cleavage dioxygenases. Although the biosynthetic pathway of apocarotenoids is known to a considerable extent, the mechanism that regulates its tissue and developmental stage specific expression is not known. In the present work, we identified, cloned and characterized ultrapetala transcription factor called CsULT1 from Crocus. The gene contains an 80 amino acid long conserved SAND domain. The CsULT1 transcript was more abundant in stigma and showed increase in expression from pre anthesis stage till anthesis and decreased in post anthesis stage which corroborated with the accumulation pattern of crocin indicating its possible role in regulation of apocarotenoid biosynthesis. CsULT1 was found to be transcriptionally active and localized in nucleus. Its expression is induced in response to phytohormones like auxin, methyljasmonate and salicylic acid. Overexpression of CsULT1 in Crocus calli resulted in enhanced expression of key pathway genes like phytoene synthase (PSY), phytoene desaturase (PDS), beta carotene hydroxylase (BCH) and carotenoid cleavage dioxygenases (CCDs) indicating its role in regulation of apocarotenoid biosynthesis. This work presents first report on isolation and characterization of ultrapetala gene from Crocus. Our results suggest that CsULT1 is a novel regulator of Crocus apocarotenoid biosynthesis. We show for the first time involvement of plant SAND domain proteins in regulating secondary metabolic pathways.