Micromorphological analysis indicates reversion of sesame extrafloral nectary to flower-like structure with the onset of phytoplasma infection reflecting its evolutionary path

Abstract

Plant pathogens are known to cause diverse symptoms in plants. Candidatus Phytoplasma (Class: Mollicutes), a phloem inhabiting bacterial pathogen is transmitted by sap sucking insects and is becoming a serious concern in sesame crops grown for oilseed, especially in the Asiatic region. Sesame (Sesamum indicum L.), a member of family Pedaliaceae, bears prominent extrafloral nectaries (EFN) at each node. Our aim was to investigate the morphological and micromorphological changes in EFN development in phytoplasma infected sesame plants with phyllody disease. Scanning electron microscopy was used to study the micromorphological changes from freshly collected EFNs from healthy and phyllody diseased sesame plants. Observations revealed that the extrafloral nectary in sesame gets converted into flower (green in color) on phytoplasma infection. In diseased plants, this reversion was also seen in early stages of developing EFN primordia. Apart from the reversion, different variants were observed in EFN such as developed EFN showing indeterminacy but retaining the EFN structure or instead occasional morphological alterations were observed such as elongation or splitting of EFN tissue into halves, that often showed conversion to spatula shaped structures with leafy characters. In a few instances, inflated gynoecium like-structure emerged from the center of a split EFN. Rarely, deformed stalked flowers were also observed in place of EFN, with positional and structural abnormalities in stamens, gynoecium defects, leafiness of reduced petals and sepal narrowing cum elongation. Phytoplasma infection in sesame caused development of each EFN into a flower indicating that the EFN gland is a suppressed flower in sesame, the suppression of which is released on infection of the plant pathogen. Instead of solitary nature seen in most Sesamum indicum varieties, the cymose nature of inflorescence is thereby resumed. Phytoplasma-induced EFN conversion is a unique symptom among plant pathogens. In addition, EFNs from affected plant also showed indeterminacy and transition organs show leaf-like features indicating a shift of reproductive to vegetative developmental program on phytoplasma infection. Bushiness on affected sesame is mainly due to conversion of EFN to phylloid flowers, especially those with indeterminate nature. In addition, loss of EFN is suggestive of increased susceptibility of phyllody affected sesame to insect infestation. Further studies on anatomy of symptomatic parts will provide a deeper understanding of the conclusions derived from micromorphological data of phytoplasma associated sesame EFN reversion, presented in the current study.