A Change in the Plan of Con­struc­tion

by Roberto

On oc­ca­sion you might come across an odd-look­ing plant where the flower or­gan has been re­placed by a leafy struc­ture. To the ama­teur gar­dener this will seem a re­mark­able cur­iosi­ty, to the pro­fes­sio­nal plant grower it will sig­ni­fy sub­stan­tial eco­no­mic los­ses. To Jo­hann Wolf­gang von Goe­the such sights meant the plant had suf­fered a change in its "plan of con­struc­tion." He re­ferred to this ab­nor­mal growth as "meta­mor­pho­sis." Based on these and other ob­ser­va­tions Goethe devel­oped the con­cept of ho­mol­ogy in his 1790 es­say Ver­such die Meta­mor­phose der Pflanzen zu er­klären. To­day we know the ab­nor­mal de­vel­op­ment of flower or­gans into leaf-look­ing struc­tures as phyl­lody. What might be the cause of such a de­vel­op­men­tal aber­ra­tion?

Typ­i­cal symp­toms in a phy­to­plasma-in­fec­t­ed hy­drangea (right), called phyl­lody. A healthy flower is shown on the left. Source

Of­ten phyl­lody is the re­sult of a bac­te­r­ial in­fec­tion. And therein lies yet an­other no­table evo­lu­tion­ary tale, the story of the Phy­to­plasma. These yet-to-be-cul­ti­vated bac­te­ria, mem­bers of the wall-less Tener­i­cutes phy­lum, are ob­lig­ate par­a­sites that also re­quire an in­sect vec­tor for their trans­mis­sion from plant to plant. Al­though Phy­toplasma all have very small genomes, they pre­serve a func­tional pro­tein se­cre­tion sys­tem and en­code numer­ous se­creted ef­fec­tors that af­fect their host's phys­i­ol­ogy. One par­tic­u­larly in­ter­est­ing fam­ily of ef­fec­tors, termed the phyl­lo­gens, ren­der some plant tran­scrip­tion fac­tors un­sta­ble. The re­sult is a de­vel­op­men­tal path­way gone awry, with leaves ap­pear­ing where flow­ers should be. Why should this be se­lected for in evolu­tion? It turns out, the in­sects that serve as vec­tors are par­tic­u­larly at­tracted to the phyl­lody fea­tures of the in­fected plant! A most clever strat­egy in­deed. In an en­gag­ing re­view by Melissa Tom­kins et al. from the John Innes Cen­tre, Nor­wich, UK, the au­thors make the anal­ogy be­tween Phy­toplasma and mal­ware: "phy­to­plasma ef­fec­tors ap­pear to re­pro­gram plant de­vel­op­ment and de­fence to lure in­sect vec­tors, sim­i­larly to so­cial en­gi­neer­ing mal­ware, which em­ploys tricks to lure peo­ple to in­fected com­put­ers and web­pages." Won­der­ful anal­ogy for won­der­ful bi­ol­ogy!