The Arabidopsis ACT7 Actin Gene Is Expressed in Rapidly Developing Tissues and Responds to Several External Stimuli

Abstract

ACT7 encodes one of the six distinct and ancient subclasses of actin protein in the complex Arabidopsis actin gene family. We determined the sequence and structure of the Arabidopsis tbaliana ACT7 actin gene and investigated its tissue-specific expression and regulation. The ACT7 mRNA levels varied by 128-fold among severa1 different tissues and organs. The highest levels of ACT7 mRNA were found in rapidly expanding vegetative organs, the lowest in pollen. A translational fusion with the 5 ' end of ACT7 (1.9 kb) joined to the P-glucuronidase reporter gene was strongly and preferentially expressed in all young, developing vegetative tissues of transgenic Arabidopsis plants. ACT7 was the only Arabidopsis actin gene strongly expressed in the hypocotyl and seed coat. Although no p-glucuronidase expression was seen in developing ovules or immature seeds, strong expression was seen in dry seeds and immediately after imbibition in the entire seedling. ACT7was the only Arabidopsis actin gene to respond strongly to auxin, other hormone treatments, light regime, and wounding, and may be the primary actin gene responding to externa1 stimuli. The ACT7 promoter sequence contains a remarkable number of motifs with sequence similarity to putative phytohormone response elements.Cellular morphogenesis in plants is constrained by rigid cell walls that limit expansion, restrict shape, and prevent migration. Thus, morphogenesis is in part a function of asymmetric cell division and expansion. A number of studies have suggested that these processes are controlled by a dynamic cytoskeleton that consists of dozens of interacting proteins and responds to both developmental and environmental cues (Lloyd, 1991). Actin is a major component of the plant cytoskeleton and is thought to be required for correct cell-division plate alignment and synthesis, cellshape determination, cell-polarity establishment, cytoplasmic streaming, organelle movement, and tip growth (Staiger and Schliwa, 1987;Staiger and Lloyd, 1991;Meagher and Williamson, 1994

Distribution of actin gene isoforms in the Arabidopsis leaf measured in microsamples from intact individual cells

“…Consequently GUS activity constitutes only an indirect measure of gene activity. Patterns of organ and tissue expression derived from direct measurements of mRNA abundance may differ from those based on actin promoter-GUS constructs (An et al 1996a, b;McDowell et al 1996a) because GUS protein may still be active long after levels of actin mRNAs have fallen below the detection limit.…”

“…McDowell et al (1996a) reported GUS activity as ''blue patches'' in young leaves, the staining tending to diffuse and disappear in older leaves. It is possible, but unlikely, that all our epidermal and mesophyll samples taken were from ''white'' patches.…”

“…The actin gene isoforms are particularly well suited for such a study, as the Arabidopsis genome potentially encodes 10 isoforms, of which 8 are expressed at detectable levels McKinney and Meagher 1998). Information on the distribution of the isoforms is already available following RT-PCR analysis of excised tissues and in vivo expression of a b-glucuronidase (GUS) reporter gene linked to specific actin promoters Huang et al 1996Huang et al , 1997McDowell et al 1996a). Recently, Kandasamy et al (1999) used class-specific antibodies to visualize the distribution of the protein.…”

Changes in gene expression during drying and imbibition of desiccation sensitive Magnolia ovata (A. St.-Hil.) spreng. seeds

“…These results are partially in accordance with the expression of ACT7, a gene from the same family, in Arabidopsis seeds, where no expression was detected in developing embryos but high expression at seed maturity after drying. This high expression persisted during imbibition (McDowell et al, 1996). Changes in expression of ACT2 during seed development have not been reported so far.…”

Microarrays (DNA chips) for the classroom laboratory

“…This gene not only serves to illustrate experimental design concepts but also shows that expression of some genes depends on the developmental stage. Actin is the positive control gene, which is constitutively expressed in seedlings grown in each condition [11]. The positive control spot indicates that the chip printing, hybridization, and scanning functioned properly.…”