Agrobacterium tumefaciens as an (2003)

Twenty-six years ago it was found that the commonsoil bacterium Agrobacterium tumefaciens is capable ofextraordinary feats of interkingdom genetic transfer.Since this discovery, A. tumefaciens has served as amodel system for the study of type IV bacterialsecretory systems, horizontal gene transfer and bac-terial–plant signal exchange. It has also been modiﬁedfor controlled genetic transformation of plants, a coretechnology of plant molecular biology. These areashave often overshadowed its role as a serious, wide-spread phytopathogen – the primary driver of the ﬁrst80 years of Agrobacterium research. Now, the diverseareas of A. tumefaciens research are again convergingbecause new discoveries in transformation biology andthe use of A. tumefaciens vectors are allowing thedevelopment of novel, effective biotechnology-basedstrategies for the control of crown gall disease.Members of the genus Agrobacterium are ubiquitouscomponents of the soil microﬂora, the vast majority ofwhich are saprophytic, surviving primarily on decayingorganic matter. However, several species of agrobacteriacause neoplastic diseases in plants, including Agrobacter-ium rhizogenes (hairy root disease), Agrobacterium rubi(canegalldisease),Agrobacteriumtumefaciens(crowngalldisease) and Agrobacterium vitis (crown gall of grape).During the past 26 years, it has become apparent thatAgrobacterium pathogenesis is a unique and highlyspecialized process involving bacterium–plant interking-domgenetransfer[1].Crowngallandhairyroothavebeendescribedasaformof‘geneticcolonization’[2]inwhichthetransfer and expression of a suite of Agrobacterium genesin a plant cell causes uncontrolled cell proliferation andthe synthesis of nutritive compounds that can be metab-olized speciﬁcallybytheinfecting bacteria.Thus,infectioneffectively creates a new niche speciﬁcally suited toAgrobacterium survival. This article focuses speciﬁcallyon crown gall, the most agriculturally signiﬁcant diseasecausedbyagrobacteria,andthestateofcurrentandfuturestrategies for crown gall disease control.Fridiano Cavara ﬁrst identiﬁed a ﬂagellate, bacilloidbacterium (termed Bacillus ampelopsorae) as the causalagent of crown gall of grape in 1897 [3]. This organism,now called A. vitis, causes the growth of neoplastic tumorson the stem and crown of grapevines and induces necroticlesions on grape roots [4]. A. vitis can survive in planta inthe intercellular spaces of grape tissue without causingdisease but will initiate tumorigenesis upon tissuewounding (most commonly frost injury) [4]. Erwin Smithand C.O. Townsend [5] reported 10 years after thediscovery of A. vitis that Bacterium tumefaciens (nowAgrobacteriumtumefaciens)wasthecausalagentofcrowngall disease in Paris daisy. This organism is capable ofinducing tumors at wound sites on the stems, crowns androots of hundreds of dicots, although root necrosis is notcharacteristic of A. tumefaciens-mediated crown galldisease [6]. The pioneering work of Cavara and Smithand Townsend ushered in a century of study of Agrobac-terium as an agent ofdisease, amodel system ofhorizontalgene transfer and atool for plant transformation(Table1).Although crown gall disease is not generally fatalunless infection occurs in young plants, crown gall relatedreduction in crop yield and/or vigor can be signiﬁcant inmany perennial horticultural crops [7], such as grape [8],apple [9] and cherry [10]. The decreased productivity ofgalled plants is probably caused by several factors,including decreased water and nutrient ﬂow owing todamaged or constricted vasculature at the site of galldevelopment, and signiﬁcant water and nutrient allo-cation to the rapidly dividing but unproductive gall sink[11–13]. In addition, crown galls are sites for secondaryinfection by other phytopathogens (e.g. Pseudomonassyringae and Armillarea mellea) or pests (insect borers),and can increase plant susceptibility to abiotic stresses[6,8,12]. Finally, in planta populations of tumorigenicagrobacteria can negatively affect graft take, becausetumor tissue developing at the graft union prevents fusionof stock and scion tissues [8].Disease process – transformation and tumorigenesisAgrobacterium pathogenesis requires two basic elements:(1) delivery of tumorigenic DNA into the plant genome(transformation); and (2) the resultant alteration of plantcell metabolism, resulting in cell proliferation and thesynthesis of nutritive compounds that provide a selectiveadvantage for Agrobacterium (tumorigenesis). The focushereisentirelyonthegall-formingagrobacteria;seeRef.[14]for a review of A. rhizogenes and hairy root disease.From chemotaxis to integrationA detailed treatment of Agrobacterium transformation is