plant surface microbiology.pdf

2.2 Expression of Actin-Encoding Genes
18 Mycorrhizal Development and Cytoskeleton 297
Arabidopsis is known to have ten actin genes, two of which are pseudogenes
(Meagher et al. 1999). Out of the eight expressed actin genes ACT2, ACT8,and
ACT7 are named Arabidopsis vegetative actin genes due to the accumulation
pattern of transcripts. ACT2 is expressed in young and old vegetative tissue,
but ACT8 only in a subset of the organs and tissues expressing ACT2 (An et al.
1996a). ACT7 is expressed in young expanding vegetative tissues and is also
involved in phytohormone responses (Kandasamy et al. 2001). The rest of the
Arabidopsis actin genes appear to be associated with reproductive processes
(An et al. 1996b; Huang et al. 1997; Meagher et al. 1999). In spite of the high
number of actin genes in most plant species and the important cell biological
functions of the actin cytoskeleton, only a few analyses about the expression
of different actin genes in root tissue have been performed (McLean et al.
1990) and the effect of mycorrhiza on the expression of actin genes at mRNA
level has not been investigated.
One-dimensional analyses of actin expression at the protein level have
been performed in Pinus ectomycorrhiza. The similar mobility of plant and
fungal actins in the immunoblots of 1-D gels from P. sylvestris- S. bovinus
ectomycorrhiza made it difficult to record the contribution of each symbiotic
partner to the actin signal (Timonen et al. 1993). The presence of the actin signal
throughout the different developmental stages of ectomycorrhiza was suggested
to be a reliable marker for still metabolically active symbiosis (Timonen
et al. 1996).
In the immunoblots of 2-D gels four actin polypeptides were detected in P.
sylvestris radicles, main roots and first order laterals and in short roots. The
polypeptides were also detected in P. sylvestris- S. bovinus young and dichotomous
mycorrhizal short roots. In fully mature coralloid mycorrhiza only two
actin polypeptides occurred. Whether they represented plant or fungal actin
or both was not possible to conclude. The presence of four actin polypeptides
in Pinus root tissues is in agreement with the occurrence of a similar number
of actin polypeptides in Vicia faba roots (Janssen et al. 1996), while in the
roots of Phaseolus vulgaris one and two actin polypeptides were detected in
symbiotic root nodules and in uninfected roots, respectively (Pérez et al.
1994).
In the immunoblots of 2-D gels of the vegetative hyphae of S. bovinus two
actin polypeptides occurred that were also detected in ectomycorrhiza (Niini
et al. 1996). Recently, two actin-encoding genes, Sbact1 and Sbact2,were isolated
from S. bovinus nonsymbiotic hyphae (Tarkka et al. 2000). Northern
hybridization with specific probes for each actin gene of S. bovinus indicated
that both actins are expressed in vegetative hyphae and ectomycorrhiza. In
vegetative hyphae, the expression rate and protein level of Sbact1 was tenfold
higher than that of Sbact2. A ten times higher accumulation of Sbact1 than
Sbact2 mRNA was also observed in ectomycorrhizal hyphae, although the