Conserved Expression of the Arabidopsis ACT1 and ACT3 Actin Subclass in Organ Primordia and Mature Pollen

Abstract

We have proposed that ancient and divergent classes of plant actin genes have been preserved throughout vascular plant evolution, because they have distinct patterns of gene regulation. The hypothesis was explored for ACT1 and ACT3, which represent one of the six ancient subclasses in the Arabidopsis actin gene family. Comparison of ACT1 and ACT3 cDNA and genomic sequences revealed highly divergent flaking and intron sequences, whereas they encoded nearly identical proteins. Quantification of their level of divergence suggests that they have not shared a common ancestor for 30 to 60 million years. Gene-specific RNA gel blot hybridization and reverse transcriptase-polymerase chain reaction analyses demonstrated that the distribution of ACT1 and ACT3 mRNAs was very similar: both preferentially accumulated at high levels in mature pollen and at very low levels in the other major organs. The 5' flanking regions of both genes, including the promoter, leader exon and intron, and the first 19 condons, were fused to the beta-glucuronidase (GUS) reporter gene. The expression of these reporter fusions was examined in a large number of transgenic Arabidopsis plants. Histochemical assays demonstrated that both ACT1-GUS and ACT3-GUS constructs were expressed preferentially in pollen, pollen tubes, and in all organ primordia, including those in roots shoots, and the inflorescence. Comparison of the 5' flanking regions of ACT1 and ACT3 revealed a number of short conserved sequences, which may direct their common transcriptional and post-transcriptional regulation. The expression patterns observed were distinct from those of any other other Arabidopsis actin subclass. The conservation of their expression pattern and amino acid sequences suggests that this actin subclass plays a distinct and required role in the plant cytoskeleton.

The late pollen actins are essential for normal male and female development in Arabidopsis

“…3A). The T-DNA knockout mutants act1-1 and act3-6 and likely knockdown mutant act1-3 each showed significant reductions in LPA expression down to 50-60% of wildtype levels, consistent with previous data suggesting ACT1 and ACT3 were the most highly expressed LPAs (An et al 1996a;Huang et al 1996). The act1-3 mutant with an insertion in the 3¢UTR of ACT1 showed a similar dramatic reduction in LPA expression.…”

“…ACT11 encodes a distinct reproductive actin that is expressed in pollen, ovules, developing embryos, endosperm, and some early seedling vegetative structures (Huang et al 1997). The other four actin genes, ACT1, ACT3, ACT4, and ACT12, encode the late pollen actins (LPAs) that are highly expressed in mature pollen, pollen tubes, and a few cells within the ovule (An et al 1996a;Huang et al 1996;Meagher et al 1999a;Kandasamy et al 2002). Compared to their levels in pollen, the LPA RNAs are expressed at 1,000-fold lower levels in most vegetative organs including leaf, stem, and root, and LPA protein cannot be detected in these organs.…”

Root cytoskeleton: its role in perception of and response to gravity

“…With respect to the absence of F-actin cables, a situation similar to that in statocytes occurs in germinating zygotes of brown algae, which show predominantly diuse actin (Kropf et al 1989), and in budding yeast cells, which display only actin patches beneath the growing portions of plasma membrane (Mulholland et al 1994). In this respect, the ®nding that the actin cytoskeleton is composed of unique actin isoforms in the root cap cells (MacLean et al 1990;An et al 1996) is both interesting and perhaps of some relevance.…”

Functional Divergence in the Glutathione Transferase Superfamily in Plants

“…Finally, in view of the known inducibility of many plant GSTs to auxin treatment (3), the cultures were also exposed to 2,4-dichlorophenoxyacetic acid and NAA. To ensure comparability in the results, the intensity of the reverse transcriptase-PCR-amplified products was normalized against an internal standard, namely the PCR product derived from the actin1 gene whose mRNA is reportedly constitutively expressed in Arabidopsis (25). In each case, significant induction was assumed when PCR indicated at least a doubling in AtGST transcript abundance (Fig.…”