Plant Growth Enhancement And Associated Physiological Responses Are Coregulated By Ethylene And Gibberellin In Response To Harpin Protein HPA1

Harpin proteins produced by Gram-negative plant pathogenic bacteria have multiple effects in plants, for example, eliciting defense responses and promoting growth and development. Phytohormones ethylene and gibberellin play a key role in regulating cell wall losing, cell expansion, and therefore modulating the growth of different organs or tissues and the growth of plants as a whole. Hpal is a harpin protein secreted by Xanthomonas oryzae pv. oryzae, a pathogen that causes bacterial blight of rice. Like other harpins, Hpal is able to induce ethylene signaling, promote photosynthesis, induce the expression of expansion (EXP) genes, and thereby enhance plant growth. This study was aimed at analyzing functional relationships between those Hpal-induced responses with the key finding that the multiple effects of Hpal were executed by elevating the net photosynthesis rate and inducing the expression of EXP genes under coregulation by ethylene and gibberellin.1. The molecular and physiological basis of Hpal-induced plant growth enhancementFollowing treatment with Hpal, Arabidopsis, tobacco, and rice plants exhibited the phenotype of enhanced growth. This phenotype was correlated with several physiological responses, inducing increasese in the content of total nitrogen, concentrations of chlorophyll a and b, and chlorophyll a/b ratio in particular. These responses, especially the chlorophyll a/b ratio, was thought to indicate a role of Hpal In promoting photosynthesis, and this notion was confirmed as the net photosynthesis rate was increased in all plants treated with Hpal. Following the treatment, meanwhile, the expression of 11 EXP genes was induced; four of these genes were related to Hpal-activated phytohormone signaling pathways; and additional four of the EXP genes were newly characterized in regard to the specificity of foliar expression in either tobacco or rice. In the independent experiments, concentrations of ethylene and gibberellin were found to be increased in Hpal-treated plants of the three species. These resulets suggest that the molecular and physiological fundamentals for the growth enhancement effect of Hpal include increased production of ethylene and gibberellin, improved photosynthesis-related responses, and induced expression of growth-promoting genes.2. Ethylene and gibberellin coregulate Hpal-induced plant growth enhancement and associated physilolgial responsesPrevious studies showed that harpin enhances plant growth through activating ethylene signaling and gibberellin signaling pathways, which otherwise regulate the expression of EXP genes. The present study demonstrated that the Hpal treatment upregulated expression levels of ethylene signaling regulatory genes and ethylene responsive genes as well. To determine whether both hormones act together to regulate the effects of Hpal on plant growth and associated physiological responses, Arabidopsis mutants etrl and ga5-1, which are defected in ethylene perception and gibberellin biosynthesis, respectively, were investigated in comparison with the wide-type (WT) plant, regarding their responses to Hpal treatment. Extents by which Hpal performed to enhance plant growth, induce EXP expression, and promot photosynthesis were partially impaired, but not cancelled, in both etrl and ga5-1 mutants. Similar results were observed in WT plants treated separately with the ethylene signaling inhibitor 1-MCP and the gibberellin biosynthesis inhibitor PBZ. However, the effects of Hpal were cancelled by the application of 1-MCP to ga5-1, by the applicationg of PBZ to etrl, or by a combinative application of 1-MCP and PBZ to WT plants. Clearly, ethylene and gibberellin coregulate the effects of Hpal on plant growth and related physiological responses.