The plant actin cytoskeleton is involved in a range of cellular processes, including stress response (reviewed in Hussey et al., 2006; Staiger and Blanchoin, 2006; Drobak et al., 2004). Intracellular actin filament activity is modulated by a number of actin binding proteins such as profillin, actin depolymerizing factor (ADF)/cofilin, myosin, fibrin and villin. Plant ADFs with low molecular weight (16–20 kD) can act synergistically with profillin to increase the turnover rates and sever actin filaments (Staiger et al., 1997). The interaction between actin and ADF is regulated by reversible phosphorylation, pH, and specific phosphoinositides (Allwood et al., 2002; Smertenko et al., 1998).
The temporal and spatial expression of higher-plant ADFs has gradually been deciphered, but not with rice OsADF gene family. In Arabidopsis, ADF gene expression can be separated into vegetative- and reproductive-specific classes (Ruzicka et al., 2007). In cotton, GhADF6 and GhADF8 express mainly in petals, whereas GhADF7 expression is anther specific (Li et al., 2010). In lily and maize, LiADF1 and ZmADF1/2 accumulate solely in pollen, whereas ZmADF3 is expressed differentially in vegetative tissues (Jiang et al. 1997). The subcellular localization of various AtADFs was intensively studied by histochemical staining of AtADF::GUS fusion genes. Two classes of AtADFs may co-evolve in a tissue and developmental-specific manner and mediate distinct functions (Ruzicka et al., 2007). As well, intron-mediated enhancement of ADF gene expression was reported in vascular bundle tissue of Arabidopsis (AtADF1) and petunia (PhADF1) (Mun et al., 2002; Jeong et al., 2009).
Little is known about the precise physiological function and role of members of the plant ADF gene family. Specific members are important for plant growth, development and viability. ADFs are involved in pollen tube growth with dynamic cytoskeleton rearrangement (Allwood et al., 2002; Lopez et al., 1996). The moss Physcomitrella patens contains only a single essential ADF gene, and loss of PpADF led to inhibited tip growth (Augustine et al., 2008). In Arabidopsis, the AtADF9 mutant, which is moderately expressed in the shoot apical meristem, shows few lateral branches, reduced callus formation, early flowering, associated with less active chromatin state of F lowering L ocus C (Burgos-Rivera et al., 2008). The downregulation of GhADF1 expression affected cotton fiber properties by increasing fiber length and strength (Wang et al., 2009).
Recently, plant actin cytoskeleton had been shown to play an important role in response to plant hormones and biotic or abiotic stresses (Solanke and Sharma, 2008; Drobak et al., 2004). ADFs from Arabidopsis (AtADF2 and AtADF4) and barley were found related to plant resistance to various pathogens (Clement et al., 2009; Miklis et al. 2007; Tian et al., 2009). Alteration in the core amino acid residue in moss ADF (ADF-V69A) allowed the plant to grow at a permissive temperature (20°C to 25°C) but not a restrictive temperature (32°C; Vidali et al., 2009). Heat stress induced depolymerization of actin microfilaments and changed endoplasmic reticulum morphologic features in tobacco BY2 cultured cells (Malerba et al., 2010). Moreover, in winter oilseed rape suspension cells, freezing-induced depolymerization of actin microfilaments was sensitive in the cell growth phase (Egierszdorff and Kacperska, 2001). During cold acclimation, TaADF accumulated to higher levels in freezing-tolerant but not -sensitive wheat cultivars. This ADF was specifically induced by low temperature but not salt or heat (Ouellet et al., 2001). However, Basisakh and Subudhi (2009) identified an ADF gene in smooth cordgrass (Spartina alterniflora L.) that was highly induced with salt and heat stress in leaf and shoot but only heat stress in root. Proteomic analysis revealed induction of OsADF in vegetative-stage rice leaves of an upland cultivar CT9993 under drought stress that disappeared on re-watering but remained unaffected in the lowland cultivar IR62266 (Salekdeh et al. 2002ab). The expression of OsADF3 (GeneBank: AC104433) was induced by drought and osmotic stresses but not salt, cold or ABA in the leaf sheath of rice seedlings (Oryza sativa L. cvs. Nipponbare and Zhonghua 8) (Ali and Komatsu, 2006). However, OsADF3 protein was induced by salt stress in Nipponbare root (Yan et al., 2005). OsADF3 protein could be induced by exogenous ABA and may be involved in altering the morphologic features of Taichung native 1 (TCN1) rice root growth and development (Chen et al., 2006). Interestingly, cDNA-amplified fragment length polymorphism analysis revealed induced expression of OsADF2 (GeneBank: AC084320) under drought stress in the seminal root of the upland rice cultivar Azucena (Yang et al., 2003).
Investigating abiotic stress associated OsADF expression is important to determine whether the genes are involved in abiotic stress tolerance in rice. Nevertheless, a comprehensive analysis of gene expression patterns of the rice ADF gene family has not been performed yet. To reveal the physiological role of OsADFs, we characterized the temporal and spatial gene expression patterns of the OsADF gene family in different tissues, growth stages and under various abiotic stresses of rice. We determined the subcellular localization and promoter activity of OsADF1 and OsADF3 genes. We also overexpressed OsADF3 in Arabidopsis to provide further evidence of the OsADF3 function in enhancing drought/osmotic stress tolerance of transgenic Arabidopsis by modulating several downstream abiotic stress-responsive target genes related to drought responses.