ICAR AIEEA PG Plant Science 14 July 2023 Shift-1 Paper

Geographical evolutionary origin of Ralstonia solanacearum:

• Ralstonia solanacearum Phylotype II (historically Race 3 Biovar 2), the causal agent of brown rot / bacterial wilt of potato, co-evolved with native Andean potato landraces in the cool highlands of the South American Andes.
• Through global dissemination of infected seed tubers, this cold-tolerant lineage spread worldwide across Europe, Africa, and Asia.
• Xanthomonas oryzae originated in Asia; Xanthomonas axonopodis pv. punicae originated in the Indian subcontinent.

Historic epidemic of French viticulture:

• In the mid-19th century (1860s-1870s), the French wine industry was devastated by the aphid-like sap-sucking root insect Phylloxera vastatrix (Daktulosphaira vitifoliae), accidentally introduced from North America.
• The pest destroyed European grapevines (Vitis vinifera) until resistant North American rootstocks (Vitis riparia, Vitis rupestris) were introduced for grafting.
• Plasmopara viticola causes downy mildew; Uncinula necator causes powdery mildew.

Agrobacterium tumefaciens infection biology:

• Wounding is strictly obligatory for infection: wounded plant tissues exude phenolic signal compounds (acetosyringone) that activate the bacterial VirA/VirG two-component regulatory system: Statement (D) is true.
• Tumorigenesis is mediated by the excision and transfer of a specific single-stranded segment of the Ti plasmid (T-DNA) through a Type IV secretion system into the plant nuclear genome: Statement (C) is true.
• The bacterium does NOT multiply within intracellular plant cells to cause the physical swelling; instead, the integrated T-DNA encodes oncogenes (iaaM, iaaH, ipt) that overproduce auxin and cytokinin, inducing host cell hypertrophy and hyperplasia.