Regulation of development

The production of developmental structures requires the temporal and spatial action of a number of proteins which have to be strictly regulated dependent on environmental factors. One of the most important regulators for A. nidulans development is light. For proper light response, the fungus requires a set of different receptor proteins (Bayram & Braus, 2012). The red light receptor is encoded by fphA

which displays similarity to bacterial phytochromes. It plays a role in the repression of sexual reproduction in red light (Blumenstein et al., 2005). LreA and LreB function in the detection of blue light (Purschwitz et al., 2008). In addition the only cryptochrome/photolyase protein of A. nidulans, CryA, acts as blue light sensor.

Deletion of cryA leads to Hülle cell formation in vegetative culture conditions (Bayram et al., 2008a, Bayram & Braus, 2012).

A well-studied protein for the connection of light sensing, development and secondary metabolism is VeA (Bayram & Braus, 2012, Purschwitz et al., 2009, Yager, 1992). VeA interacts with FphA which in turn is connected to LreA and LreB (Purschwitz et al., 2008) (Figure 8). Additionally, CryA regulates veA expression (Bayram et al., 2008a). VeA shows light-dependent localization: in light, VeA is present in the cytoplasm and the nucleus, while in darkness it accumulates in the nucleus. The transport is mediated by the importin KapA (Figure 8) (Stinnett et al., 2007). Deletion of veA results in a block of sexual development, whereas overexpression leads to the production of high numbers of cleistothecia even in liquid culture and a reduced amount of asexual structures (Kim et al., 2002).

Additionally, the veA deletion strain showed a reduction in the production of sterigmatocystin and penicillin (Kato et al., 2003).

VeA forms a dimer with the velvet-like protein VelB in the cytoplasm. in darkness, VelB is moved into the nucleus with VeA. Inside the nucleus the so-called velvet complex is formed between VeA, VelB and the methyltransferase LaeA (Bayram et al., 2008b) (Figure 8). LaeA is an important regulator for the production of secondary metabolites, as for example sterigmatocystin and penicillin, but is also important for proper light regulation (Bayram et al., 2008b, Bayram & Braus, 2012, Bok & Keller, 2004). Deletion of laeA causes a reduction of conidiospore formation as well as an increase of sexual development in strains with an intact veA gene.

Additionally, laeA deletion results in a reduction of cleistothecia size due to a lack of nursing Hülle cells (Sarikaya Bayram et al., 2010). Deletion of velB leads to defects in sexual fruit body production and an altered secondary metabolism (Bayram et al., 2008b). In vegetative hyphae and during darkness, VelB forms a dimer with VosA (viability of spores A) in the nucleus. This interaction is required for spore viability and the repression of asexual development (Sarikaya Bayram et al., 2010).

Introduction

The regulation of sexual development additionally includes a MAP kinase cascade with AnFus3. VeA becomes phosphorylated by AnFus3. In addition, AnFus3 promotes the interaction of VeA with VelB (Bayram et al., 2012).

Figure 8: Light dependent regulation of development.

In darkness, the VelB/VeA dimer is imported into the nucleus, mediated by KapA. In the nucleus, it can form the velvet complex with the methyltransferase LaeA. This complex is involved in the regulation of development and secondary metabolism. VelB can also form a dimer with VosA which is important for the repression of asexual development and spore viability. VeA interacts with the red light receptor FphA which interacts with the blue light receptors LreA and LreB. VeA might undergo a posttranslational modification (PM). Light promotes asexual development by decreasing the levels of VosA/VelB. Modified from (Bayram & Braus, 2012).

VeA also has an influence on the balance of cellular oxilipin levels by affecting the expression of the dioxygenase like enzyme PpoA. PpoA, PpoB and PpoC are involved in the biosynthesis of hormone-like signal molecules, also known as psi (precocious sexual inducer) factors. The ratio of these factors determines the induction of development. In the veA deletion strain, the ppoA expression is drastically reduced (Bayram & Braus, 2012, Tsitsigiannis et al., 2004). PpoA and PpoC are additionally regulated in a CSN-dependent manner (Nahlik et al., 2010, Tsitsigiannis et al., 2004).

Several proteins have been identified which function in the signal transduction pathways of sexual reproduction. Among them NsdD (never in sexual

development D), SteA (sterile 12-like A) and RcoA. Deletion of nsdD results in a block of cleistothecia formation. In contrast, overexpression increases the number of fruit bodies and induces Hülle cell formation under vegetative growth conditions (Han et al., 2001). Also deletions in steA and rcoA lead to a block in sexual development (Todd et al., 2006, Vallim et al., 2000).

BrlA (bristle A) and AbaA (abacus A) are two transcription factors involved in the regulation of asexual development (Clutterbuck, 1969). BrlA is a zinc-finger protein important for the switch from apical growth to vesicle and sterigmata formation. brlA mutants are impaired in asexual development, as stalk formation is not affected but neither sterigmata nor spores are produced (Prade & Timberlake, 1993). Overexpression of brlA leads to conidiophore development under vegetative growth conditions (Adams et al., 1988). BrlA itself activates several additional genes which are indispensable for conidiophore formation. Among those are yA and abaA (Timberlake & Clutterbuck, 1994). AbaA is important for phialide differentiation (Sewall et al., 1990) and potentiates its own transcription in a positive feedback-loop.

Additionally, it induces the expression of wetA. In return, brlA transcription is repressed (Han & Adams, 2001). WetA is a regulator for spore-specific genes. For example, it is involved in the production of cell wall components (Marshall &

Timberlake, 1991). Asexual development is regulated by a number of proteins.

Several factors have been identified that are involved in the activation of conidiation and the transcription of brlA. FluG is important for the inhibition of vegetative growth and the induction of brlA activation (Park & Yu, 2012). Also VosA, which interacts with VelB in response to light, functions in asexual development. Deletion of vosA leads to constitutive conidiospore formation, even in liquid culture.

Furthermore, the expression of brlA, yA and wA is altered in the vosA deletion strain.

VosA is also required for the synthesis of threalose which is important for the survival of spores (Ni & Yu, 2007).