Expression of HpaGXoo, a bacterial type-III effector, in transgenic plants induces disease resistance. Resistance also can be elicited by biocontrol bacteria. In both cases, plant growth is often promoted. Here we address whether biocontrol bacteria and HpaGXoo can act together to provide better results in crop improvement. We studied effects ofPseudomonas cepacia on the rice variety R109 and the hpaGXoo-expressing rice line HER1. Compared to R109, HER1 showed increased growth, grain yield, and defense responses toward diseases and salinity stress. Colonization of roots byP. cepacia caused 20% and 13% increase, in contrast to controls, in root growth of R109 and HER1. Growth of leaves and stems also increased in R109 but that of HER 1 was inhibited. WhenP. cepacia colonization was subsequent to plant inoculation withRhizoctonia solani, a pathogen that causes sheath blight, the disease was less severe than controls in both R109 and HER1; HER1, nevertheless, was more resistant, suggesting thatP.cepacia and HpaGXoo cooperate in inducing disease resistance. Several genes that critically regulate growth and defense behaved differentially in HER1 and R109 while responding toP. cepacia. In R109 leaves, theOsARF1 gene, which regulates plant growth, was expressed in consistence with growth promotion byP. cepacia. Inversely,OsARF1 expression was coincident with inhibition in growth of HER1 leaves. In both plants, the expression ofOsEXP1, which encodes an expansin protein involved in plant growth, was concomitant with growth promotion in leaves instead of roots, in response toP. cepacia. We also studiedOsMAPK, a gene that encodes a mitogen-activated protein kinase and controls defense responses toward salinity and infection by pathogens in rice. In response toP. cepacia, an early expression ofOsMAPK was coincident with R109 resistance to the disease, while HER1 expressed the gene similarly whetherP. cepacia was present or not. Evidently,P. cepacia and GXoo-gene mediated resistance may act differently in rice growth and resistance. Whereas combinative effectsof P. cepacia and HpaGXoo in disease resistance have a great potential in agricultural use, it is interesting to study mechanisms that underlie interactions involving biocontrol bacteria, type-III effectors and pathogens.
Volume 44 | Issue 5
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