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Phytosulfokine-á (PSK-á) 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-á. Phytosulfokine and AtPSKR1 genes are differentially expressed in roots. PSK-á induced root growth in a dose-dependent manner without affecting lateral root density. Kinematic analysis established that enhancement of root growth by PSK-á 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-á signaling through AtPSKR1 affects root elongation primarily via control of mature cell size. Root organogenesis, on the other hand, is not controlled by PSK-á.

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.