Abstract
In phytopathogenic fungi the establishment and maintenance of polarity is not only essential for vegetative growth and differentiation, but also for penetration and colonization of host tissues. We investigated orthologs of members of the yeast polarity complex in the grey mould fungus Botrytis cinerea: the scaffold proteins Bem1 and Far1, the GEF (guanine nucleotide exchange factor) Cdc24, and the formin Bni1 (named Sep1 in B. cinerea). BcBem1 does not play an important role in regular hyphal growth, but has significant impact on spore formation and germination, on the establishment of conidial anastomosis tubes (CATs) and on virulence. As in other fungi, BcBem1 interacts with the GEF BcCdc24 and the formin BcSep1, indicating that in B. cinerea the apical complex has a similar structure as in yeast. A functional analysis of BcCdc24 suggests that it is essential for growth, since it was not possible to obtain homokaryotic deletion mutants. Heterokaryons of Δcdc24 (supposed to exhibit reduced bccdc24 transcript levels) already show a strong phenotype: an inability to penetrate the host tissue, a significantly reduced growth rate and malformation of conidia, which tend to burst as observed for Δbcbem1. Also the formin BcSep1 has significant impact on hyphal growth and development, whereas the role of the putative ortholog of the yeast scaffold protein Far1 remains open: Δbcfar1 mutants have no obvious phenotypes.
Highlights
The grey mould fungus Botrytis cinerea is a broad host range plant pathogen that is able to infect different tissues of more than 200 plants, causing serious crop losses worldwide [1]
B. cinerea Bem1 ortholog (BcBem1) shows 32%, 55%, 60% and 63% to the characterized orthologs of S. cerevisiae (Bem1p), A. nidulans (BemA), E. festucae (BemA) and N. crassa (Bem-1) and contains the typical Bem1 features: two SH3 (Src homology 3; IPR001452) domains that may mediate protein-protein interactions and binding to Cdc42, one PX domain known to mediate phosphoinositide binding and one PB1 (Phox/Bem1p; IPR000270) domain that has been shown to interact with the PB1 domain of Cdc24 in yeast [19] (Fig. S1A)
As in N. crassa but unlike A. nidulans, in B. cinerea the conidial germination is impaired in the deletion mutants of bcbem1 (Dbcbem1) mutant
Summary
The grey mould fungus Botrytis cinerea is a broad host range plant pathogen that is able to infect different tissues of more than 200 plants, causing serious crop losses worldwide [1]. The conserved components may have different importance and functions, e.g. in contrast to yeast Dcdc mutants of filamentous fungi are not lethal and not even severely impaired in polar growth, as was recently shown in B. cinerea [14]. This holds true for the central scaffold protein Bem, which in filamentous fungi - in contrast to yeast - does not seem to be essential but has varying impact on hyphal growth [15,16,17]. We confirm that - in spite of obviously not being essential for growth and polarity, in this system Bem interacts with the ortholog of a putative complex partner, Cdc, and we present for the first time functional analyses data for the B. cinerea orthologs of the yeast Bem complex partners Cdc24p, Far1p, and Bni1p
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