A Conserved Homeobox Transcription Factor Htf1 Is Required for Phialide Development and Conidiogenesis in Fusarium Species

Wenhui Zheng,Huanchen Zhai,Xu Zhao,Zonghua Wang,Qingping Huang,Qiurong Xie,Liping Xu,Chengkang Zhang,Guodong Lu,Won-Bo Shim

doi:10.1371/journal.pone.0045432

Abstract

Conidia are primary means of asexual reproduction and dispersal in a variety of pathogenic fungi, and it is widely recognized that they play a critical role in animal and plant disease epidemics. However, genetic mechanisms associated with conidiogenesis are complex and remain largely undefined in numerous pathogenic fungi. We previously showed that Htf1, a homeobox transcription factor, is required for conidiogenesis in the rice pathogen Magnaporthe oryzae. In this study, our aim was to characterize how Htf1 homolog regulates common and also distinctive conidiogenesis in three key Fusarium pathogens: F. graminearm, F. verticillioides, and F. oxysporum. When compared to wild-type progenitors, the gene-deletion mutants in Fusarium species failed to form conventional phialides. Rather, they formed clusters of aberrant phialides that resembled elongated hyphae segments, and it is conceivable that this led to the obstruction of conidiation in phialides. We also observed that mutants, as well as wild-type Fusaria, can initiate alternative macroconidia production directly from hyphae through budding-like mechanism albeit at low frequencies. Microscopic observations led us to conclude that proper basal cell division and subsequent foot cell development of macroconidia were negatively impacted in the mutants. In F. verticillioides and F. oxysporum, mutants exhibited a 2- to 5- microconidia complex at the apex of monophialides resulting in a floral petal-like shape. Also, prototypical microconidia chains were absent in F. verticillioides mutants. F. graminearum and F. verticillioides mutants were complemented by introducing its native HTF1 gene or homologs from other Fusarium species. These results suggest that Fusarium Htf1 is functionally conserved homeobox transcription factor that regulates phialide development and conidiogenesis via distinct signaling pathways yet to be characterized in fungi.

Highlights

Asexual sporulation is the preferred mode of reproduction in most pathogenic fungi [1,2]
When we compared the predicted protein sequence of F. graminearum FGSG_07097 gene, F. verticillioides FVEG_08072 gene, F. oxysporum FOXG_01706 gene, and M. oryzae MGG_00184 gene, identity was greater than 95% at the protein level within the homeodomain amongst Fusarium species, and MoHtf1 homeodomain shared 60% identity with Fusarium counterparts (Figure 1B)
We found that HTF1 is essential for conidiation in M. oryzae

Summary

Introduction

Asexual sporulation is the preferred mode of reproduction in most pathogenic fungi [1,2]. These asexual spores, commonly known as conidia, are used as a primary dissemination tool as well as for initiating infection [3,4,5,6,7]. Further genetic and biochemical studies led to the discovery of abaA and wetA [13,14] These three genes (brlA-abaA-wetA) have been proposed to constitute a central regulatory pathway that acts in concert with other genes to control conidiation in Aspergillus [15,16]. We need to recognize that different fungal species may have developed different regulatory mechanisms for producing various types of conidia

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Conclusion