Ethylene regulates a multitude of plant processes, ranging from seed germination to organ senescence. Of particular economic importance is the role of ethylene as an inducer of fruit ripening. Ethylene is synthesized from S-adenosyl-L-methionine via 1-aminocyclopropane-1-carboxylic acid (ACC). The enzymes catalyzing the two reactions in this pathway are ACC synthase and ACC oxidase. Environmental and endogenous signals regulate ethylene biosynthesis primarily through differential expression of ACC synthase genes. Components of the ethylene signal transduction pathway have been identified by characterization of ethylene-response mutants in Arabidopsis thaliana. One class of mutations, exemplified by etr1, led to the identification of the ethylene receptors, which turned out to be related to bacterial two-component signaling systems. Mutations that eliminate ethylene binding to the receptor yield a dominant, ethylene-insensitive phenotype. CTR1 encodes a Raf-like Ser/Thr protein kinase that acts downstream from the ethylene receptor and may be part of a MAP kinase cascade. Mutants in CTR1 exhibit a constitutive ethylene-response phenotype. Both the ethylene receptors and CTR1 are negative regulators of ethylene responses. EIN2 and EIN3 are epistatic to CTR1, and mutations in either gene lead to ethylene insensitivity. Whereas the function of EIN2 in ethylene transduction is not known, EIN3 is a putative transcription factor involved in regulating expression of ethylene-responsive genes. Biotechnological modifications of ethylene synthesis and of sensitivity to ethylene are promising methods to prevent spoilage of agricultural products such as fruits, whose ripening is induced by ethylene.

SummaryPreviously, we identi®ed a novel rice gene, GROWTH-REGULATING FACTOR1 (OsGRF1), which encodes a putative transcription factor that appears to play a regulatory role in stem elongation. We now describe the GRF gene family of Arabidopsis thaliana (AtGRF), which comprises nine members. The deduced AtGRF proteins contain the same characteristic regionsÐthe QLQ (Gln, Leu, Gln) and WRC (Trp, Arg, Cys) domainsÐas do OsGRF1 and related proteins in rice, as well as features indicating a function in transcriptional regulation. Most of the AtGRF genes are strongly expressed in actively growing and developing tissues, such as shoot tips,¯ower buds, and roots, but weakly in mature stem and leaf tissues. Overexpression of AtGRF1 and AtGRF2 resulted in larger leaves and cotyledons, as well as in delayed bolting of the in¯orescence stem when compared to wild-type plants. In contrast, triple insertional null mutants of AtGRF1±AtGRF3 had smaller leaves and cotyledons, whereas single mutants displayed no changes in phenotype and double mutants displayed only minor ones. The alteration of leaf growth in overexpressors and triple mutants was based on an increase or decrease in cell size, respectively. These results indicate that AtGRF proteins play a role in the regulation of cell expansion in leaf and cotyledon tissues.

Ethylene influences a number of developmental processes and responses to stress in higher plants. The molecular basis for the action of ethylene was investigated in mutants of Arabidopsis thaliana that have altered responses to ethylene. One mutant line, which has a dominant mutation at a locus designated etr, lacks a number of responses to ethylene that are present in the wild-type plant. These include inhibition of cell elongation, promotion of seed germination, enhancement of peroxidase activity, acceleration of leaf senescence, and feedback suppression of ethylene synthesis by ethylene. These diverse responses, which occur in different tissues of Arabidopsis, appear to share some common element in their transduction pathways-for example, a single receptor for ethylene. Results of ethylene binding experiments in vivo indicate that this receptor may be affected by the etr mutation.

Previously, we described the AtGRF [Arabidopsis thaliana growthregulating factor (GRF)] gene family, which encodes putative transcription factors that play a regulatory role in growth and development of leaves and cotyledons. We demonstrate here that the C-terminal region of GRF proteins has transactivation activity. In search of partner proteins for GRF1, we identified another gene family, GRF-interacting factor (GIF), which comprises three members. Sequence and molecular analysis showed that GIF1 is a functional homolog of the human SYT transcription coactivator. We found that the N-terminal region of GIF1 protein was involved in the interaction with GRF1. To understand the biological function of GIF1, we isolated a loss-of-function mutant of GIF1 and prepared transgenic plants subject to GIF1-specific RNA interference. Like grf mutants, the gif1 mutant and transgenic plants developed narrower leaves and petals than did wild-type plants, and combinations of gif1 and grf mutations showed a cooperative effect. The narrow leaf phenotype of gif1, as well as that of the grf triple mutant, was caused by a reduction in cell numbers along the leaf-width axis. We propose that GRF1 and GIF1 act as transcription activator and coactivator, respectively, and that they are part of a complex involved in regulating the growth and shape of leaves and petals.

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