Abstract
Many species of plant-pathogenic gram-negative bacteria deploy the type III (T3) secretion system to secrete virulence components, which are mostly characteristic of protein effectors targeting the cytosol of the plant cell following secretion. Xanthomonas oryzae pv. oryzae (Xoo), a rice pathogen causing bacterial blight disease, uses the T3 accessory protein HrpE to assemble the pilus pathway, which in turn secretes transcription activator-like (TAL) effectors. The hrpE gene can execute extensive physiological and pathological functions beyond effector secretion. As evidenced in this study, when the hrpE gene was deleted from the Xoo genome, the bacteria incur seriouimpairments in multiplication, motility, and virulence. The virulence nullification is attributed to reduced secretion and translocation of PthXo1, which is a TAL effector that determines the bacterial virulence in the susceptible rice varieties. When the HrpE protein produced by prokaryotic expression is applied to plants, the recombinant protein is highly effective at inducing the defense response. Moreover, leaf photosynthesis efficiency is enhanced in HrpE-treated plants. These results provide experimental avenues to modulate the plant defense and growth tradeoff by manipulating a bacterial T3 accessory protein.
Highlights
Bacteria adopt different life styles for adapting to their surroundings; alone, cooperative or synergetic, commensal and parasitic
8 of 17 8 of 17 compared to PXO99A and ΔhrpE/hrpE (Figure 3C). These results demonstrated that hrpE is involved in PXTOo9e9vAalvuirautelewnchee.ther HrpE is able to elicit hypersensitive response (HR), Nicotiana benthamiana leaves were infiltrated with ∆hrpET,oPeXvOal9u9aAt,eawndhe∆thhreprEH/hrrppEE.iTsoabbalecctooleaves infiltrated with PXO99A and ∆hrpE/hrpE showed HR, while ∆hrpE failed to elicit HR (Figure 3D), indicating that HrpE induces HR in N. benthamiana
Significant increases in the stomatal conductance (Figure 8B), photRoiscyenltehaevteics wacetriveiptyre(‐Ftriegautreed8wAit)haHndrptEraonrsRpiFrPatpiornoteraintes,(aFnigdutrheen8Cin)owcuelraeteodbsweirtvhePdXiOn 9P9XA,Oa9n9dA leinafveecstepdrlee‐atrveeastepdrew-triethatwedatwerithaltohneeHwrperEecuosmedpaarseda wcoitnhtrtohlo. sPehiontoocsuylnatehdetwicitehffPicXieOn9c9yAoaflothnee,pwlahnitcsh wisacsodmetpearrmabinledtoathdoisfefetrreenattetidmweiitnhtewravtaelrs.aSloignnei.ficant increases in the stomatal conductance (Figure 8B), photosynthetic activity (Figure 8A) and transpiration rate (Figure 8C) were observed in PXO99A infected leaves pre‐treated with the HrpE compared with those inoculated with PXO99A alone, which is comparable to those treated with water alone
Summary
Bacteria adopt different life styles for adapting to their surroundings; alone, cooperative or synergetic, commensal and parasitic. The harpins included in the hrp gene cluster are nominated as hrpA to hrpF [7] These two proteins are substrates of the T3 secretion system where they play a predominant role in the delivery of T3 proteic and toxic effectors through and towards the plasma membrane of the plant cell. In our study, we premeditated the connotation of hrpE in pathogen virulence and integrity as well as the functional description of Xoo HrpE as an elicitor of the plant immune response in rice For this purpose, we infiltrated the plant leaves with HrpE protein produced by prokaryotic expression, and studied its roles in eliciting the HR and in inducing PTI responses. This study sheds light on the role of HrpE protein in leaf photosynthesis
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