The Plant Cell, Vol. 2, 731 -739, August 1990 O 1990 American Society of Plant Physiologists

abaA Controls Phialide Differentiation in Aspergí//us nídula ns

Tommy C. Sewall," Charles W. Mims," and William E. Timberlakea7bs'

a Department of Plant Pathology, University of Georgia, Athens, Georgia 30602 Department of Genetics, University of Georgia, Athens, Georgia 30602

Aspergillus nidulans is an ascomycetous fungus that reproduces asexually by forming multicellular conidiophores and uninucleate spores called conidia. Loss of function mutations in the abacus A (abaA) regulatory locus result in formation of aberrant conidiophores that fail to produce conidia. Wild-type conidiophores form two tiers of sterigmata. The first tier, metulae, divide to produce the second tier, phialides. Phialides are sporogenous cells that produce conidia through a specialized apical budding process. We have examined conidiophore development in an abaA- strain at the ultrastructural level. The results showed that in the mutant metulae produce supernumerary tiers of cells with metula-like, rather than phialide-like, properties. Temperature shift experiments with an abaA 14'" strain demonstrated that abaA' function induced phialide formation by the aberrant abacus cells and was continuously required for maintenance of phialide function. In the absence of abaA' activity, metulae simply proliferated and later developmental steps never occurred. We conclude that abaA' directs the differentiation of phialides and is continuously required for maintenance of their function.

INTRODUCTION

The brlA, abaA, and wetA genes of Aspergillus nidulans make up a central regulatory pathway that controls asex- ual development or conidiogenesis (Mirabito et al., 1989). Ordered expression of these three sporulation-specific genes is necessary for proper formation of the complex conidiophore and for conidium production in this fungus (Boylan et al., 1987; Mirabito et al., 1989). Early conidio- phore formation begins when regions of vegetative hyphae differentiate into thickened foot cells that branch to form aerial conidiophore stalks (Mims et al., 1988). brlA, the first gene expressed in the central regulatory pathway (Adams et al., 1988), mediates the transition of the stalk from polarized apical growth to conidiophore vesicle formation (Clutterbuck, 1969; Martinelli and Clutterbuck, 1971). Next, a layer of primary sterigmata, or metulae, bud from the conidiophore vesicle. Each metula gives rise to one or more phialides that, in turn, differentiate into spore-pro- ducing (conidiogenous) cells. Messenger RNA from abaA, the intermediate gene in the pathway, accumulates during the time when metulae and phialides are formed (Boylan et al., 1987). Mutation of the aba4 gene results in the repetition of phialide-like "abacus" structures rather than chains of conidia. This result has led to the concept that expression of the abaA gene is not required for phialide formation but is necessary for phialide function, namely the repeated production of conidia (Martinelli and Clutter-

' To whom correspondence should be addressed.

buck, 1971 ; Oliver, 1972; Boylan et al., 1987; Mirabito et al., 1989). Because conidiophores of abaA mutants have not been examined in detail at the ultrastructural level, specific cellular effects of abaA expression are unknown. Expression of the last gene in the pathway, wetA, requires abaA expression (Boylan et al., 1987; Mirabito et al., 1989) and is responsible for maturation and maintenance of conidium dormancy (Martinelli and Clutterbuck, 1971 ; Sewall et al., 1990).

In this study, conidiophore development was compared in wild-type and aba4 mutant strains of A. nidulans to determine the precise nature of the defects induced by mutations in the abaA gene. This information, in conjunc- tion with molecular genetic analysis of abaA, is expected to increase our understanding of this pivotal regulatory gene. Structural observations using fluorescence light mi- croscopy and scanning and transmission electron micros- copy (SEM and TEM) demonstrated that aba4 expression was not necessary for metula formation and subsequent budding from metulae. Metulae of abaA mutants produced abacus structures that closely resembled metulae in their wall structure and nuclear behavior. However, these cells could differentiate into either conidiogenous cells or vege- tative cells if abaA' function were restored. We propose that abaA is responsible for directing phialide differentia- tion. In the absence of ab&+ function, buds produced by metulae continue metular activities, such that the previous developmental step is reiterated. Thus, long chains of

732 The Plant Cell

Figure 1. Scanning Electron Micrographs of Conidiophores of the abaAl Strain and Wild-Type A. nidulans.

(A) Early abaAl conidiophore with stalk (S), conidiophore vesicle (CV), a layer of metulae (M), and numerous abacus structures (A).Magnification X1700.

Aspergillus nidulans Conidiogenesis 733

morphologically abnormal metulae make up the abacus structures for which the mutant was named.

RESULTS

Comparison of abaA 1 mutant conidiophores with wild-type conidiophores in Figure 1 demonstrated clearly that the effect of the abaA7 mutation was to disrupt the differentia- tion of phialides. The stalk, vesicle, and metulae of a mutant conidiophore (Figure 1 A) were morphologically nor- mal, but abacus structures rather than functional phialides were formed as a result of budding from metulae (Figure 1 B). Abacus structures of the abaA1 mutant strain (Figure 1B) differed from wild-type phialides (Figure 1C) in that they produced additional abacus structures rather than conidia and were capable of lateral branching. Apical growth and lateral branching of abacus structures (Figure 1 D) eventually resulted in an aberrant conidiophore (Figure 1 E).

TEM of freeze-substituted abaA7 conidiophores, shown in Figure 2, confirmed that the initial budding and com- partmentation of metulae (Figures 2A and 28) as well as formation of the first abacus structure (Figure 2C) were comparable with wild-type metula and phialide formation. Each metula was separated from the first abacus structure by a septum with a central pore and Woronin bodies (Figures 2B and 20) and each compartment was uninu- cleate. Abacus structures had structural similarities to both metulae and phialides and were uninucleate. Figures 2A to 2C represent the same developmental stage as shown in Figure 1 B. Each abacus structure was separated from its parent cell by a septum that had a central pore and Woronin bodies (Figure 20). Staining for carbohydrates containing vicinal- hydroxyl residues demonstrated that the necks of abacus structures did not have the extra phialide wall layer that contributes to the formation of the inner wall of the conidium initial in wild-type strains (Figures 2E and 2F).

Abacus structures were somewhat varied in their ap- pearance, as shown in Figure 3, but could be described as flask-shaped (phialide-like, Figure 3A). Each abacus structure contained a radially expanded region, in which the nucleus was located, and an apically growing tubular extension. Fluorescence staining for nuclei with 4' ,&diam-

idino-2-phenylindole (DAPI) (Figures 3B and 3C) demon- strated that although most abacus structures were unin- ucleate, severa1 were binucleate. Although not illustrated here, nuclear division figures were occasionally observed in sections of freeze-substituted abacus structures, indi- cating that the expanded region was the site of mitosis. Each tubular extension of abacus structures had an ac- cumulation of vesicles at its apex, indicating that the apex is a site of cell growth.

Temperature-shift experiments, illustrated in Figure 4, were performed with the aba414 temperature-sensitive mutant strain to help clarify the relationship between aba- cus structures, metulae, and phialides. Conidiophores of the aba474 strain grown at the restrictive temperature (38OC) produced only abacus structures, whereas those grown at the permissive temperature (22OC) produced normal phialides and conidia. Conidiophores grown initially at the restrictive temperature and then shifted to the permissive temperature for 24 hr produced conidia at the most distal tips of the conidiophore. Young conidiophores containing one or two levels of abacus structures produced structurally and functionally normal phialides at the distal ends of the abacus chains (Figure 4A). Few abnormal phialides were produced by young temperature-shifted conidiophores, and the production of conidia appeared normal. The older a particular conidiophore was before it was shifted to the permissive temperature, the more likely it was either to produce abnormal phialides or fail to produce conidia. The converse experiment was performed by growing abaA74 conidiophores at the permissive tem- perature and then shifting to the restrictive temperature for 24 hr. In this case, conidium production stopped and conidiophores gave rise to abacus structures (Figure 46). Abacus structures could be produced on branches from metulae or at the tips of phialides. Often, when conidia were present on a phialide (Figure 4C), a change in the appearance of the outer layer of the neck was apparent (Figure 4D). Abacus structures were transferred to drop- lets of culture medium and incubated at 22OC or 38°C for 8 hr to 24 hr and then assayed for vegetative growth by using differential interference microscopy (DIC). Individual or short chains of abacus structures were not capable of germination or vegetative growth. However, the most dis- tal abacus structures of intact conidiophores were capable of converting to apical vegetative growth in submerged culture at either 22OC or 38°C (Figure 4E).

~~

Figure 1. (continued).

(B) Higher magnification of aba47 conidiophore showing metulae (M) and two levels of abacus structures (A1 and A2, arrows). Unlike wild-type phialides, the first layer of abacus structures (A l ) frequently produced lateral buds (single arrowheads). The second layer of abacus structures initially were spherical but later elongated and formed a tubular apical extension (double arrowheads). Magnification X4500. (C) Wild-type (aba4) conidiophore with phialides (P), conidium initial (CI), and maturing conidia (C). Magnification x5800. (D) Branched chains (arrows) of abacus structures (A). Magnification x2800. (E) A mature abaA7 conidiophore with the stalk (S), conidiophore vesicle (CV), metulae (M), and chains of branched abacus structures (A). Magnification Xl100.

734 The Plant Cell

Figure 2. Transmission Electron Micrographs of Metulae and Abacus Structures of the abaA1 Mutant and Wild-Type strains ofA. nidu/ans.

Samples in (A) to (D) were prepared by freeze substitution. Samples in (E) and (F) were chemically fixed and stained for carbohydratescontaining vicinal- hydroxyl groups by using the periodic acid, thiocarbohydrazide, silver protein method.(A) Section through abaA 1 conidiophore vesicle (CV), metula (M), and the first abacus structure (A1). Metula is separated from conidiophorevesicle by a septum (S) and contains a single nucleus (N), mitochondria (Mt), and endoplasmic reticulum (ER). The first abacus structureis continuous with the metula and contains cytoplasmic organelles but not a nucleus. Magnification xSOOO.

Aspergillus nidulans Conidiogenesis 735

DISCUSSION

The abaA gene has been the subject of detailed studies to determine its function and analyze molecular interactions between it and the brlA and wetA genes (Boylan et al., 1987; Adams et al., 1988; Timberlake and Marshall, 1988; Mirabito et al., 1989). The abaAf allele has been cloned, sequenced, and shown to code for a putative 90-kD pro- tein containing a motif resembling a “leucine zipper” (Boy- lan et al., 1987; Landschulz et al., 1988; Mirabito et al., 1989), supporting the hypothesis that abaA encodes a DNA-binding protein. brlA mutations are epistatic to abaA and wetA mutations, and aba4 mutations are epistatic to wetA mutations at both the morphological and molecular levels (Boylan et al., 1987). abaA, in concert with brlA and wetA, regulates the expression of numerous sporulation- specific genes (Mirabito et al., 1989). The abaA gene product has been presumed to affect some aspect of phialide differentiation because abaA mRNA accumulates at the approximate time of metula and phialide formation and because abaA mutants fail to form morphologically discernible phialides (Boylan et al., 1987). A limited ultra- structural study of the aba4 mutant strain (Oliver, 1972), as well as light microscopic observations of abaA mutant conidiophores (Clutterbuck, 1969; Martinelli and Clutter- buck, 1971) and SEM (Boylan et al., 1987), showed that abaA mutants produce chains of abacus structures rather than conidia. Although these studies equated abacus structures with phialides and stated that the abaA gene was not necessary for phialide formation, they did not address the ultrastructural and cytological effects that the abaA mutation might have on different stages of conidio- phore formation.

Severa1 possible functions of the abaA gene could ex- plain the morphology of the abaA mutant conidiophore. First, abaA expression could be necessary for metulae to produce phialides. In this case, abaA- mutants might pro- duce additional metulae rather than phialides. Alternately, abaA expression could be required after phialide formation for differentiation of immature phialides into conidiogenous

cells. Comparisons of the ontogeny, nuclear behavior, and developmental fate of metulae, phialides, and abacus structures in wild-type and abaA mutant strains of A. nidulans were used to distinguish between these possible functions of the abaA gene.

Metulae can be functionally defined as support cells for the phialides. They are formed when the wall of the coni- diophore vesicle thins and buds at numerous discrete points (Oliver, 1972; Mims et al., 1988). After a period of apical growth, a single nucleus migrates into each metula and a septum forms to delimit the metula from the coni- diophore vesicle (Mims et al., 1988). Each metula apically and laterally buds, undergoes a limited series (one to four) of nuclear divisions, and forms septa that delimit metulae from the apically growing buds (immature phialides in wild- type conidiophores; Oliver, 1972; Mims et al., 1988). For- mation of uninucleate compartments by apical budding associated with this pattern of nuclear division and migra- tion is referred to as acropetal development. The result of acropetal development is that the most dista1 compartment contains an actively dividing nucleus, whereas proximal compartments do not. Some fungi produce conidia by this mechanism. No differences could be detected between abaA- mutant and wild-type conidiophores through this developmental stage.

Compartments that bud from the metulae of a b a - mutants have been defined as phialides on the basis of either their position or morphology (flask shape), rather than on the basis of their structure and function. Metulae of abaA mutants clearly produced abacus structures rather than phialides. Phialide differentiation requires the forma- tion of an additional wall layer at the apex during budding and a change in the pattern of nuclear behavior (Oliver, 1972; Mims et al., 1988; Sewall et al., 1990). Each wild- type phialide has a single neck region from which conidia bud. As each conidium initial buds from the phialide, vesi- cles are deposited in the phialide neck and radially in the conidium initial to form the inner conidium wall (Sewall et al., 1990). Carbohydrate staining confirmed that abacus structures lacked the characteristic additional wall layer always present in necks of wild-type phialides. Abacus

Figure 2. (continued). (6) Similar section through the conidiophore vesicle (CV), metula (M), and first abacus structure (Al). Nuclear division and migration has occurred and the first abacus structure and its nucleus (N) are delimited from the metula by a septum (S) with a central pore (arrowhead). Endoplasmic reticulum (ER), Golgi cisternae (G), and numerous mitochondria (Mt) are present. Magnification ~18 ,700 . (C) Section through a second abacus structure (A2) being produced from the first abacus structure (Al). A single nucleus (N) and an apical extension (arrowhead) are present. The first and second abacus structures are separated by a septum (S) with a central pore (not shown in this section) similar to those separating metulae and the first abacus structures. The first abacus structure has branched in this section (double arrowheads). Magnification ~23,800. (D) A typical septum delimiting abacus structures with a central pore (arrowhead) and one or more Woronin bodies (W). Magnification X30,900. (E) Carbohydrate staining of the junction between two abacus structures showing the continuity of the outer (Ll) and inner (L2) wall layers. Magnification ~76,100. (F) Carbohydrate staining of the neck of an a b a 7 phialide (P) with a conidium initial (CI) showing the discontinuity between the inner wall layer (L2) of the phialide and conidium initial. Outer layer was continuous. Magnification x22,500.

736 The Plant Cell

Figure 3. Structural Analysis and Nuclear Behavior in Abacus Structures of the abaA1 Mutant Strain of A. nidulans.

(A) Section through a chemically fixed abaA1 conidiophore showing the conidiophore vesicle (CV), metulae (M), and three levels of abacusstructures (A1 to A3). Note the variability in the shape of abacus structures depending on the distance away from the conidiophorevesicle. Magnification x8800.(B) Fluorescent nuclei of abaA1 conidiophore stained with DAPI. Note that several abacus structures contained two nuclei (arrowheads).Magnification x870.(C) Bright-field light micrograph of the same abaA 1 conidiophore. Stalks (S), conidiophore vesicle (CV), metulae (M), and several layersof abacus structures (A) were visible. Arrowheads mark the same abacus structures in which two nuclei were visible with DAPI staining.Magnification x870.

Aspergillus nidulans Conidiogenesis 737

DFigure 4. abaA14K Conidiophores Subjected to Changes in Growth Conditions.

(A) Section through a conidiophore grown at the restrictive temperature (38°C), then shifted to the permissive temperature (22°). Ametula (M), abacus structure (A1), and phialide (P) with a conidium initial (Cl) and conidium (C) are shown. Magnification x9200.(B) Section through a conidiophore grown at the permissive temperature, and then shifted to the restrictive temperature. Metulae (M)and phialides (P) with a differentiated neck region (arrowheads) are shown. One phialide produced a series of abacus structures (A1 toA3), whereas another had a single conidium initial (Cl). Normal conidia (C) were frequently present. Magnification X5600.(C) A conidium initial (Cl) formed on an abaA14 phialide (P) grown at the permissive temperature and shifted to the restrictive temperature.The outer region of the phialide neck was much more electron-opaque (arrowheads) than during normal conidium formation. MagnificationX24.500.(D) An abaA14 conidiophore produced dt restrictive temperature was transferred to liquid culture medium and incubated for 8 hr at therestrictive temperature. Abacus structures underwent vegetative growth seen at the arrowheads. Magnification x280.

738 The Plant Cell

structures gave rise to additional abacus structures by budding, followed by apical extension, nuclear division and migration, and septation. Unlike wild-type phialides, but like metulae, abacus structures could form lateral branches.

Each phialide nucleus undergoes a directed nuclear division with one daughter nucleus remaining in the phial- ide, where it continues to divide, while the other nucleus migrates into the conidium initial and arrests in the G1 phase of the cell cycle until germination (Mims et al., 1988; Timberlake and Marshall, 1988). This process of conidium formation, whereby nuclear division occurs only in a basal cell (phialide) and the oldest conidium is the most distal to the phialide, is referred to as basipetal development (Cole, 1986). In abaA- mutants, the most distal abacus structure was the site of nuclear division and apical growth. After nuclear division and subsequent migration of a daughter nucleus, a septum that had a pore and associated Woronin bodies delimited the most distal abacus structure. This observation indicates that the basic pattern of conidio- phore development, that is, repetition of growth followed by delimitation of a compartment, is maintained in the absence of abaA expression. However, aba4 mutants fail to switch from acropetal to basipetal development. Thus, the pattern of nuclear behavior and wall formation during budding in abacus structures was more similar to metulae than to phialides of wild-type A. nidulans.

From ultrastructural observations alone it was not pos- sible to determine conclusively whether abacus structures potentially were conidiogenous cells (phialides). Manipula- tion of the abaA 74 temperature-sensitive strain allowed abaA expression to be selectively turned on or off by growth at the permissive temperature or restrictive tem- perature. The most distal abacus structures (abaA74, re- strictive) that had not budded and presumably had not undergone nuclear division could be converted to a func- tional conidiogenous cell by switching to the permissive temperature. There were no readily detectable differences between wild-type phialides and phialides produced by this temperature-shift experiment. The converse temperature- shift experiment indicated that loss of abaA expression caused a rapid cessation of conidium production and a shift to the formation of abacus structures. It is not likely that abacus structures are aberrant conidia because only abacus structures on intact conidiophores, and not individ- ual abacus structures, could undergo indeterminant apical growth when placed in nutrient medium. These experi- ments demonstrated that aba4 expression is not required for metula formation or budding. In the absence of abaA expression, the pattern of acropetal development contin- ues indefinitely. Although immature abacus structures are potentially conidiogenous cells, they more closely resemble metulae when abaA expression is absent. The abaA gene mediates the switch from acropetal development to basi- peta1 development (spore production). We conclude that aba4 expression is required at the cellular leve1 for the

formation and maintenance of the phialide wall layer in- volved in the formation of the inner conidium wall and, also, to regulate nuclear behavior in phialides. The devel- opmental fate of the structures that are produced by metulae in both wild-type and abaA mutant strains is dependent on expression of abaA+. Although abacus structures produced in the absence of abaA expression function as metulae, they have the potential to differentiate into conidiogenous cells. This indicates that abaA gene expression is necessary to induce conidium production any time after metulae formation. Furthermore, abaA expression is required to maintain conidium production.

METHODS

Strains, Media, and Growth Conditions

Aspergillus nidulans FGSC4 (Glasgow wild type) and abacus (abaA7 and abaA74) mutant strains were grown on minimal medium supplemented with 0.22 g/L L-arginine, 2.5 pg/L biotin, and 1 O0 pg/L para-aminobenzoic acid (Kaffer, 1977). Cultures were grown in the dark at 38OC except for strains carrying the temperature-sensitive abaA74 allele, which were grown at 38OC (restrictive) and 22OC (permissive). Conidia and abacus structures from wild-type or abaA74 mutant strains were obtained by lightly scraping the surface of 4-day-old to 6-day-old agar plate cultures that had been flooded with liquid culture medium. Conidia, coni- diophores, and abacus structures obtained in this way were incubated in 50-pL droplets of defined medium to assay for germination or vegetative growth (Sewall and Pommerville, 1987). For electron microscopy, 5-mm segments of 0.5 mm aluminum wire were dipped in molten culture medium and placed in the path of colony growth on the surface of solid culture medium in Petri plates. Growth of colonies was monitored by microscopic obser- vation until the appropriate developmental stages were present on the wires.

Sample Preparation for Light Microscopy

Conidia and conidiophores were fixed for 5 min to 30 min in 2% (v/v) glutaraldehyde and 0.5% (v/v) Triton X-100 in 0.05 M Tris- maleate buffer, pH 7.4, and then briefly washed in the same buffer. Samples were examined by bright-field or differential inter- ference contrast (DIC) microscopy. Nuclei were observed with fluorescence microscopy after staining for 30 min with 5 pg of DAPl in buffer. DAPI-stained samples were examined with epiflu- orescence microscopy using a 365-nm excitation filter, a 400-nm dichroic mirror, and a 400-nm barrier filter (Nikon UV-1A filter cube). Micrographs using fluorescence, DIC, and bright-field mi- croscopy were taken on Kodak Technical Pan (2415) film at ASA 125.

Sample Preparation for Electron Microscopy

Conidiophores were chemically fixed for SEM and TEM by flooding colonies with 2% (v/v) glutaraldehyde in 0.1 mM potassium phos-

Aspergillus nidulans Conidiogenesis 739

phate buffer (KPB), pH 6.8, containing 0.5% (v/v) Triton X-1 O0 as a wetting agent. Wires bearing conidiophores were carefully dis- lodged from the agar surface and transferred to vials of fresh fixative without Triton X-100 and maintained under vacuum (<20 torr) for 1 hr at 22OC. They were then incubated at 4°C in the same solution at atmospheric pressure for 12 hr. Samples were washed three times in 0.1 M KPB and then fixed for 2 hr at 4°C in 1% (w/v)osmium tetroxide in 0.1 M KPB. Samples were washed three times in distilled water, stained overnight in 0.5% (w/v) aqueous uranyl acetate at 4OC (omitted for SEM), and dehydrated through a graded ethanol series to anhydrous 100% ethanol.

Conidiophores on aluminum wires were critical point dried for SEM with anhydrous ethanol as a transition fluid and then mounted on aluminum specimen stubs by using conductive silver paint. Samples were sputter coated with a 50-nm layer of gold- palladium and examined with a Philips 505 SEM at 15 keV.

Samples for TEM were transferred from 100% ethanol to anhydrous acetone and infiltrated with the firm formulation of Spurr's low-viscosity resin (Spurr, 1969) in 25% increments over a 4-day period using acetone as a transition fluid. After 24 hr in 100% plastic, a sharpened wooden applicator stick was used to dislodge conidiophores from wires into fresh droplets of 100% plastic on tetrafluoroethylene-coated microscope slides (Taylor, 1984). Conidiophores were flat embedded, selected, and sec- tioned as previously described (Mims et al., 1988; Sewall et al.,

Alternately, conidiophores were prepared by freeze substitution (Hoch, 1986; Mims et al., 1988). Wires containing conidiophores were plunged into liquid propane and then transferred to a -85OC substitution fluid composed of 2% (w/v) osmium tetroxide, 0.05% (w/v) uranyl acetate in anhydrous HPLC-grade acetone. Samples were held at -85OC for 2 to 3 days, then warmed to -20°C, 4OC, and 22OC in 2-hr intervals. Substitution fluid was replaced with two changes of anhydrous acetone after 30 min at 22°C. Samples were infiltrated with 10% graded steps of Epon-Araldite at 1-hr intervals and two additional changes of 100% plastic at 3-hr intervals. Conidiophores were removed from wires and flat em- bedded as described above.

Sections were cut with a diamond knife, collected on copper or gold slot grids, and dried on Formvar-coated aluminum racks (Rowley and Moran, 1975). Sections on copper grids were post- stained with saturated uranyl acetate, followed by lead citrate (Reynolds, 1963), whereas sections on gold grids were stained for carbohydrates containing vicinal- hydroxyl residues by the method of Thiéry (1 967), as modified by Silva and Macedo (1 987). Exact procedures and controls have been described (Sewall et al., 1990).

1990).

ACKNOWLEDGMENTS

We thank Stephanie Smith for her assistance with the preparation of micrographs used in this paper. This project was supported by grants from the National lnstitute of Health (GM37886 to W.E.T.), the United States Department of Agriculture, and the University of Georgia Bioresources and Biotechnology Program (to W.E.T. and C.W.M.)

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Received May 30, 1990.

DOI 10.1105/tpc.2.8.731 1990;2;731-739Plant Cell

T C Sewall, C W Mims and W E TimberlakeabaA controls phialide differentiation in Aspergillus nidulans.

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