Comparative Transcriptome Provides a New Insight into Floral Regulation and Defense Response Against Phytoplasma in Sesame (Sesamum indicum L.)

Abstract

Phyllody in sesame (Sesamum indicum L.) is caused by the mollicute, and phytoplasma is often associated with floral malformations, affecting partial to complete sterility in those plants. The molecular basis for floral malformations in sesame remains largely unexplored. A comparative transcriptome study was performed between symptomatic (phytoplasma-infected) and asymptomatic (control) sesame plants by screening with phytoplasma-specific primers. The same-aged floral buds with undifferentiated floral organs were subjected to suppression subtractive hybridization (SSH) technique and a total of 525 expressed sequence tags (ESTs) comprising of unique and differentially expressed sequences were cloned. The differential ESTs cloned from Sesamum indicum-forward subtraction (SiFS) and Sesamum indicum-reverse subtraction (SiRS) were 240 and 285, respectively. The ESTs for metabolism and cellular and membrane transport were abundant in SiRS library, suggesting their upregulation in response to phytoplasma. Likewise, ESTs for chaperones and retroelements with a role in plant defense were identified in SiRS library alone, suggesting host defense activation in response to phytoplasma. On the contrary, seven ESTs involved in flower developmental process, viz., BEL1-like homeodomain protein, zinc finger protein CONSTANS-LIKE 13, polygalacturonase-like, STS14 protein, auxin transport protein BIG, glutaredoxin, and 26S proteasome non-ATPase regulatory subunit 8 homolog-A were detected in SiFS library alone, indicating their repression in phytoplasma-infected sesame. Conversely, genes with a role in negative floral regulation, two-component response regulator-like APRR1 and squamosa promoter-binding-like protein 1, were abundant in SiRS library alone, confirming a role of phytoplasma in expression of negative regulators of flower development. Further, these findings were validated by real-time quantitative polymerase chain reactions (qPCRs).