Molecular interactions between ethylene and gibberellic acid pathways in plants

Flooding avoidance in deepwater rice is characterised by rapid growth of the youngest internode which allows the plant to keep part of its foliage above the surface of raising flood waters. The primary signal triggering internodal elongation is the phytohormone ethylene which accumulates as a result of increased ethylene biosynthesis and entrapment. Through unknown signalling components, ethylene increases the level of bioactive gibberellins (GA) and responsiveness of the tissue to GA. GA is the hormone ultimately responsible for induction of internodal growth. I report the identification of ACC-induced (aci) genes through subtractive hybridisation of cDNA libraries constituted from internodes incubated with the ethylene precursor ACC. Two aci genes, aci7 and Osaci3-1 were shown to be regulated by ethylene but not by GA in planta. Sequence comparison and domain searches provided a likely function for ACI7 in the MTA recycling pathway which is linked to ethylene biosynthesis. Expression of Osaci3-1 was induced by ethylene prior to increase in GA content, which made of Osaci3-1 a putative candidate gene for ethylene to gibberellin signalling. Ataci3-1 is the closest homologue of Osaci3-1 in Arabidopsis thaliana. Like Osaci3-1, expression of Ataci3-1 correlated to elevated growth rates during vegetative growth. Sequence analysis of OsACI3-1 and AtACI3-1 revealed homology to MIP1, a MADS-box interacting protein from Antirrhinum majus likely involved in transcriptional regulation. With the recent finding that several MADS-box transcription factors regulate expression levels of GA biosynthetic genes, these results pinpoint a possible role for OsACI3-1 and AtACI3-1 in co-operating with MADS-box proteins to regulate GA biosynthesis

Functional Analysis of Methylthioribose Kinase Genes in Plants

Through a biochemical and a genetic approach, we have identified several plant genes encoding methylthioribose (MTR) kinase, an enzyme involved in recycling of methionine through the methylthioadenosine (MTA) cycle. OsMTK1, an MTR kinase from rice (Oryza sativa), is 48.6 kD in size and shows cooperative kinetics with a V(max) of 4.9 pmol/min and a K(0.5) of 16.8 μm. MTR kinase genes are the first genes to be identified from the MTA cycle in plants. Insertional mutagenesis of the unique AtMTK gene in Arabidopsis (Arabidopsis thaliana) resulted in an inability of plants to grow on MTA as a supplemental sulfur source. MTK knock-out plants were not impaired in growth under standard conditions, indicating that the MTA cycle is a nonessential metabolic pathway in Arabidopsis when sulfur levels are replete. In rice, OsMTK genes were strongly up-regulated in shoots and roots when plants were exposed to sulfur starvation. Gene expression was largely unaffected by lack of nitrogen or iron in the nutrient solution, indicating that OsMTK regulation was linked specifically to sulfur metabolism

PSK-alpha promotes root growth in Arabidopsis

Phytosulfokine-alpha (PSK-alpha) is a disulfated pentapeptide described to act as a growth factor in suspension cells. In this study, the involvement of PSK signaling through the PSK receptor gene AtPSKR1 in Arabidopsis root growth was assessed. Expression studies of PSK precursor genes and of AtPSKR1 were performed in roots with RT-PCR and P:GUS analyses. Root elongation, lateral root formation, cell production and root cell elongation were analyzed in wild-type (wt) and in the receptor knockout mutant Atpskr1-T treated with or without synthetic PSK-alpha. Phytosulfokine and AtPSKR1 genes are differentially expressed in roots. PSK-alpha induced root growth in a dose-dependent manner without affecting lateral root density. Kinematic analysis established that enhancement of root growth by PSK-alpha was mainly caused by an increase in cell size. In Atpskr1-T, the primary roots were shorter as a result of reduced mature cell size and a smaller root apical meristem composed of fewer cells than in wt. The results indicate that PSK-alpha signaling through AtPSKR1 affects root elongation primarily via control of mature cell size. Root organogenesis, on the other hand, is not controlled by PSK-alpha

OsMTN Encodes a 5′-methylthioadenosine Nucleosidase that is Up-Regulated During Submergence-Induced Ethylene Synthesis in Rice (\u3cem\u3eOryza sativa\u3c/em\u3e L.)

Methylthioadenosine (MTA) is released as a by-product of S-adenosylmethionine (AdoMet)-dependent reactions central to ethylene, polyamine, or phytosiderophore biosynthesis. MTA is hydrolysed by methylthioadenosine nucleosidase (MTN; EC 3.2.2.16) into adenine and methylthioribose which is processed through the methionine (Met) cycle to produce a new molecule of AdoMet. In deepwater rice, submergence enhances ethylene biosynthesis, and ethylene in turn influences the methionine cycle through positive feedback regulation of the acireductone dioxygenase gene OsARD1. In rice, MTN is encoded by a single gene designated OsMTN. Recombinant OsMTN enzyme had a KM for MTA of 2.1 mM and accepted a wide array of 5′ substitutions of the substrate. OsMTN also metabolized S-adenosylhomocysteine (AdoHcy) with 15.9% the rate of MTA. OsMTN transcripts and OsMTN-specific activity increased slowly and in parallel upon submergence, indicating that regulation occurred mainly at the transcriptional level. Neither ethylene, MTA, nor Met regulated OsMTN expression. Analysis of steady-state metabolite levels showed that MTN activity was sufficiently high to prevent Met and AdoMet depletion during long-term ethylene biosynthesis