N-terminus Region Of Harpin Hpa1 And Plant Gibberellin Coincidently Regulate Hpa1-induced Growth And Disease Resistance In Plants

Harpin proteins produced by Gram-negative plant-pathogenic bacteria can activate plant defense responses and also promote plant growth.Harpin proteins have been identified in different bacterial species and they share common set of effects in a variety of plants.However,molecular mechanisms that govern the different effects of harpins have not been fully understood,especially for the regulation of plant growth enhancement in relation to drought tolerance.Hpal is a harpin protein produced by PX099^A,a causal agent of rice bacterial blight.A Hpal variation called Hpa1?NT,which is lack of the 53 amino acids in the N-terminus of Hpal,almost cancel all the biological functions compared to the full-length Hpa1.According to the above result we can gain the point that the N-terminus is a must for Hpal exerting its biological function when being used to plants.And the point may help us to define the acting site to the N-terminus,and may also be a great enlightenment for us to find the acting site in plant.In addition,due to the important role the plant hormones play in the regulation of plant growth and development,we links plant growth and growth-related gene expression stimulated by Hpal to gibberellin.And find that gibberellin does matter in these effects caused by Hpal.And the conclusion may facilitate us to further understand the mechanism how harpin proteins work.1.The harpin protein Hpal needs its N-terminus to induce plant growth and resistance to diseaseHpal is a harpin protein and it can promote plant growth,enhance the resistance to pathogen and drought.But in which manner it functions remains unknown.Previous researches demonstrate that the N-terminus of Hpal is essential for it eliciting hypersensitive response in tobacco.Here we we generated an inactive variant protein Hpal ANT,which is lack of the 53 amino acids in the N-terminus of Hpa1,and compared Hpal ?NT with Hpal in terms of the influences on plant resistance to bacteria,plant growth and related physiological responses in Arabidopsis.When Arabidopsis was sprayed by Hpa1,it showed obvious resistance to Pst DC3000 and also enhanced vegetative growth without its floral development being changed.Enhanced plant growth was accompanied by higher expression level of growth-related genes and cell growth in plant leaves.The growth promotion activity of Hpa1 was further associated with a physiological consequence shown as promotions in stomatal and mesophyll's conductance to CO₂ and leaf photosynthesis.When it comes to Hpa1?NT,plant resistance to pathogen,growth and growth-related gene expression level,and the physiological consequence changed little.The two distinct phenomena suggest us that the N-terminus is required for Hpal to fulfill its biological function,like activating plant defense responses and facilitating CO₂ transport inside leaf cells and promoting leaf photosynthesis and vegetative growth of the plant.2.Gibberellin plays a role in Hpal-induced plant growthThe phytohormones regulate many aspects of plant development,from seed germination to flowering and fruit production till plant decay to death.A mass of studies have shown that even in the plant growth stimulated by harpin,plant hormones still play important roles.Previous studies showed that de novo expressed or externally applied Hpal in plants performed similarly to promote the vegetative growth and growth-related gene expression.It was unclear whether this role of Hpal is subject to regulation by any phytohormones.Based on molecular,genetic,and pharmacological analyses,gibberellin was engaged in plant growth enhancement by Hpal.In Hpal-treated plants,gibberellin concentrations were increased significantly compared to the steady-state level detected in control plants.This induction of gibberellin was correlated with enhanced plant growth.However,Hpal failed to induce plant growth and growth-related genes expression in the loss-of-function mutant ga5-1.The promoting effect of Hpal on plant growth was also significantly impaired,despite not fully canceled,by a specific inhibitor of gibberellin biosynthesis present in Hpal treatment.Considerately,gibberellin plays a regulatory role in plant growth promotion and growth-related gene expression stimulated by Hpal.