Abstract
Phytoplasmas are obligate, phloem-limited bacterial pathogens that reprogram host development through secreted effector proteins. The effectors characterized to date, including SAP11, SAP54/PHYL1, SAP05 and SWP1, act principally by binding and destabilizing host transcription factors. Although α/β-hydrolase-fold phytoplasma proteins with catalytic activity toward lipid substrates have recently been described, no phytoplasma effector has been shown to possess enzymatic activity toward a plant hormone. The effector RY378 from rice yellow dwarf (RYD) phytoplasma induces excessive tillering and dwarfing in rice and alters strigolactone- and auxin-responsive gene expression; its predicted α/β-hydrolase domain was proposed to mediate these effects, but the protein was never expressed, purified or biochemically characterized, leaving the mechanism of this effector class unresolved. From the closed genome of 'Ca. P. sacchari' isolate SCGS, associated with sugarcane grassy shoot disease, we identified SGP-7, an orthologue of RY378 that is conserved across at least seven 16Sr phytoplasma groups. SGP-7 adopts a canonical α/β-hydrolase fold with a Ser116-Asp229-His261 catalytic triad, and its predicted substrate pocket is comparable in volume to that of the Arabidopsis strigolactone receptor AtD14. Recombinant SGP-7 is a bona fide esterase with a clear preference for short-chain acyl esters, and hydrolyses both the fluorogenic strigolactone probe Yoshimulactone Green (apparent Km 3.55 uM; kcat 9.38 x 10^-3 s^-1) and the strigolactone analogue rac-GR24. Both activities are abolished by substitution of the catalytic serine (SGP-7S116A). Transient expression of SGP-7 in Nicotiana benthamiana reduced plant height by 46%, increased axillary bud outgrowth approximately fourfold, and lowered transcript levels of the strigolactone-responsive marker NbBRC1, reproducing in a dicot host the architectural phenotype reported for RY378 in rice. Together, these results define the biochemical function of a conserved phytoplasma effector family and identify strigolactone-like molecules as direct enzymatic targets of a bacterial plant pathogen.