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.

OsGRF1 (Oryza sativa GROWTH-REGULATING FACTOR1) is a rice gene encoding a putative novel transcriptional regulator. We identified and characterized eleven homologs of OsGRF1 in the rice genome. All twelve OsGRF proteins have two highly conserved regions, the QLQ (Gln, Leu, Gln) and WRC (Trp, Arg, Cys) domains, and sequences reminiscent of transcription factors. OsGRF genes were preferentially expressed in young and growing tissues, and applied gibberellic acid (GA3) enhanced the expression of seven OsGRF genes. In situ hybridization showed high levels of OsGRF1 transcripts in the shoot apical meristem and in cells surrounding the vasculature of the intercalary meristem. In a GAL4-based yeast assay, the C-terminal region of OsGRF1 was found to have transactivation activity. These results indicate that OsGRF1 acts as a transcriptional activator. Based on the in situ expression pattern of OsGRF1, we postulate that it may be involved in regulating vegetative growth in rice.

To investigate the in vivo functions of expansins, we generated transgenic rice plants that express sense and antisense constructs of the expansin gene OsEXP4 . In adult plants with constitutive OsEXP4 expression, 12% of overexpressors were taller and 88% were shorter than the average control plants, and most overexpressors developed at least two additional leaves. Antisense plants were shorter and flowered earlier than the average control plants. In transgenic plants with inducible OsEXP4 expression, we observed a close correlation between OsEXP4 protein levels and seedling growth. Coleoptile and mesocotyl length increased by up to 31 and 97%, respectively, in overexpressors, whereas in antisense seedlings, they decreased by up to 28 and 43%, respectively. The change in seedling growth resulted from corresponding changes in cell size, which in turn appeared to be a function of altered cell wall extensibility. Our results support the hypothesis that expansins are involved in enhancing growth by mediating cell wall loosening.

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Expansins were originally identified as cell wall‐loosening proteins. The existence and various roles of expansins have been discovered in many plants. Expansins are encoded by a superfamily of genes comprised of subfamilies that evolved from a common ancestor and encode the α‐expansins (EXPAs), the β‐expansins (EXPBs), the expansin‐like A (EXLA), and expansin‐like B (EXLB) proteins. Several expansin‐like genes have also been identified in non‐plant organisms (e.g. a slime mold, fungi, nematodes, and a mollusk). Localization of EXPA and EXPB in the cell wall was confirmed by immunogold electron microscopy. Studies using transgenic plants provided evidence for a broad range of biological roles of expansins in diverse aspects of plant growth and development, such as cell wall extension, fruit softening, abscission, floral organ development, symbiosis, and the response to environmental stresses.