Effects Of Auxin On The Coordinate Regulation Of Plant Growth And Systemic Acquired Resistance

Auxin is a classic phytohormone involved in plant development, and this kind of small non-protein molecule plays an important role in a myriad of developmental process, including embryo patterning, cell division and elongation, vascular differentiation, lateral root initiation, gravitropism, and phototropism. Meanwhile, auxin also regulates the expression of numerous genes. In recent years, especially with the found of the auxin receptor (transport inhibitor response protein 1, TIR1) and the possible receptor (auxin binding protein 1, ABP1), the auxin signaling has been clearly and simply demonstrated. Most recent study shows that SA causes global repression of auxin-related genes, and this inhibitory effect on auxin signaling is a part of the SA-mediated disease-resistance mechanism. This indicates that auxin also plays key role in plant defense. However, how this seemingly simple pathway controls the myriad of plant developmental processes, and how auxin coordinate regulates the plant growth and defense, these questions are still to be answered.1. Constitutive expression of systemic acquired resistance in a tobacco dwarf mutant is regulated by an auxin-repressed proteinSystemic acquired resistance (SAR) in plants is induced by various elicitors. In response to an elicitor, the ankyrin NPR1 functions to regulate expression of plant defense response genes, including SAR genes and the hypersensitive response-associated genes, like hsr203 and hin1. These genes encode defensive compounds, such as pathogenesis-related (PR) proteins and chitinase (Chia) or other enzymes that can attack pathogens. The products cooperate to prevent pathogens from infection and pathogenicity, leading to resistance phenotype in plants. Thus, induced expression of defense response genes and systemic resistance are considered as main characters of SAR. Under particular circumstances when plant genetic properties are modified, SAR characters may become constitutive and may be hereditable through plant reproduction. In this case, constitutive SAR (cSAR) may be transferred to sexual progenies of plants, and defense response genes can express without requirement for the presence of elicitors.Multiple approaches have been used to modify plant genetic properties and alter SAR traits. A method is plant engineering with defense regulatory genes fused to a constitutive promoter, such as 35S from cauliflower mosaic virus. Genetic mutagenicity is widely used to tag plant genes involved in positive and negative regulations of defense responses. A conventional mutagenicity performs via somaclonal variations during asexual reproduction of plant germplasm, typically like cell or tissue culture and plant regeneration. The process may stabilize plant genetic variations caused by SAR elicitors and alter heredity modes of SAR traits. In the consequence, induction treatment sometimes causes modifications of plant genomes.To study coordinate regulation of plant growth and systemic acquired resistance (SAR), we applied a somaclonal variation protocol to tobacco (Nicotiana tabacum) and identified the mutant constitutive expresser of SAR 1-1 (ces1-1). Compared to wild-type (WT) plant, ces1-1 dwarfs but resists infection by a fungal pathogen. The mutant constitutively accumulates transcripts of plant defense response genes but fails to express expansin genes involved in the plant growth pathway. Genetic analyses of the backcross reveal that ces1-1 represents a disruption at a single gene and is dominant in heredity over the WT locus. The profile of differentially expressed cDNA shows that a transcript highly identical with the NtARP1 gene encoding an auxin-repressed protein is present in ces1-1 but absent in WT. We cloned NtARP1 by rapid amplification of cDNA end and tested effects of auxin on expression of the gene and production of NtARP1 protein in the plant. NtARP1 is expressed and NtARP1 localizes to nuclei in ces1-1 rather than WT. NtARP1 expression and NtARP1 production are both inhibited by treating ces1-1 with the auxin indole-3-acetic acid (IAA). We further characterized the functions of NtARP1 in regulation of tobacco growth and defense responses. When NtARP1 is silenced, ces1-1 grows like WT but loses constitutive SAR characters. IAA content is lower in ces1-1 than in WT, whereas, external application of IAA partially restores WT characters to ces1-1. Our results suggest the presence of an auxin signaling pathway that oppositely regulates growth and defense in the plant. 2. The production and identification of the RfBP transgenic ArabidopsisRiboflavin (vitamin B₂) participates in many physiological processes in organisms. The vitamin often is phosphorated and combined with nucleotides to form flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), important components of the respiratory electron-transmission chain, which affects antioxidation through the role of vitamin C. FMN and FAD often serve as coenzyme of many enzymes (flavoproteins), which control many physiological reactions, and particularly production of reactive oxygen species (ROS). ROS are essential for the hypersensitive cell death and growth, defense against pathogens and insects, and many aspects of responses to environmental stress. Levels of riboflavin in plants are believed to be important to these processes. As such, if riboflavin levels could be manipulated, the important processes could be controlled as a consequence. This would result in great improvement of plant defense and productivity. This also could provide us with a particular insight into studying coordination of plant defense with growth regulation, which may be affected by levels of riboflavin.Our lab has cloned a riboflavin receptor protein (riboflavin binding protein, RfBP) gene from soft-shelled turtle (Trionyx sinensis japonicus). To study the localization of RfBP protein in plants and the function of RfBP protein in plant growth and defense, we transformed RfBP gene to Arabidopsis. The RfBP gene from Trionyx sinensis japonicus was fused to the Aequoria victoria green-fluorescence protein (GFP) gene and cloned into eukaryotic expression and transformation pBI121 vector. Resulting unit was transferred into Agrobacterium tumefaciens strain EHA105. Then, the recombinant unit was introduced into Arabidopsis Clo-0. Kanamycin-resistant green shoots was obtained. PCR, Southern blot, Western blot, RT-PCR and the fluorescent detection have confirmed successful integration of the RfBP gene into the genomes of Arabidopsis. These results suggest that increase in endogenous riboflavin can enhance both plant growth and disease resistance. The presence and function of RfBP in plants is an absolute new area.