Nuclear Behavior During Basidiospore Germination in Cronartium Quercuum F. Sp. Fusiforme

Nuclear behavior during basidiospore germination in Cronartium quercuum f. sp. fusiforme

P. C. Spaine methods. In this paper, we report on a subsequent 1 ISLIA Forest Service, Southfrn Rrsrarrh Station, 320 mitotic division in C. quercuum f. sp. fusifbrvne basid- (hen Stwet, Athens, Geor@a 30602 iospores that occurred following meiosis. This addi- tional mitotic division was not found to be associated Shigeru Kaneko with different types of basidiospore germination. Laboratory qf Forest Pathology, Forestry and Forest Products Resmnh Institute, Kukizaki, Inashiki-gun, Previous studies of nuclear behavior in basidio- Ibaraki 305, /aFan spores of other rust fungi have reported a binucleate stage occurring after meiosis and before spore rc- lease from sterigmata (Anikster, 1983; Kaneko, 1975). Abstract: Nuclear behavior during basidiospore ger- Kaneko (1975) examined the nuclear behavior of te- mination in Cronartiunz quercuum f. sp. fusiforme was liospores and sporidia in Coleosporium petasitis Cooke examined on glass slides and host seedlings using 4, during indirect and direct germination, and found 6-diamidino-2-phenylindole staining. Mononucleate 97% of basidiospores were binucleate O-2 h after re- basidiospores of Cronartium quercuum f. sp. fusiforme lease from sterigmata. Similar nuclear behavior also normally were produced following meiosis in the te- was observed in C. helianthi and C. vernonia~ by Olive liospore. However, a subsequent mitotic division of- (1942) and in C. sidae by Sanwal (1953). Bauer ten occurred within each basidiospore resulting in a (1986) and Bauer and Oberwinkler (1988) found ba- short-lived binucleate condition. Within 1 h after sidiospores of several rust species, including Cronar- spores were released from the basidium, one of the tiuvn asclepiadeum, (Wilde) Fr., Gymnosporangium cla- two nuclei in most basidiospores began to degener- uarizforme (Pers.) DC., Puccinia maluacearum Bert., ate. Ninety-five percent of basidiospores directly cast Phragmidium uiolaceum (C. F. Shultz) Wint., and (:O- from telia onto seedlings of Pinus taeda germinated leosporium tussilaginis (Pers.) Lev., were binucleate directly forming thin germ tubes. More than 93% of until they formed a hypha, appressorium, or second- germ tubes were mononucleate. On glass slides, how- ary basidiospore, at which time one nucleus degen- ever, 79% of basidiospores germinated indirectly, erated. This condition also was observed in Gymno- forming a secondary basidiospore. During indirect sporangium juni@ri-vircginianae Schwein (Mims and germination, degeneration of the second nucleus oc- Richardson 1989, 1990). curred quickly without exception. No differences in In a previous study of Cronartium quercuum f. sp. nuclear behavior were found between direct and in- fusiforme, Spaine and Kaneko (1993) found that direct germination on host plants or on glass slides. more than 95% of washed basidiospores cast onto However, nuclear movement from basidiospores into water agar germinated directly, forming thin germ germ tubes was faster on the pine seedlings than on tubes. Unwashed basidiospores germinated indirect- glass slides. ly, forming secondary basidiospores. The objective of Key Words: direct germination, indirect germi- this study was to determine whether germination nation, nuclear degeneration, Pinus taeda, Quewus type, direct or indirect, influences or relates to nu- rubra, rust fungus clear behavior.

MATERIALS AND METIIODS

Fusiform rust disease is a major disease in pine plan- Leaves of Quercus rubra I,. were inoculated with ae- tations of the southeastern United States. Under- ciospores (mixed gall collection, Clarke County, standing the basic biology of the pathogen, Cronar- Georgia) of Cronartium quercuum f. sp. fusiforme. tium quercuum (Berk.) Miyabe ex Shirai f. sp. fusifor- Leaves bearing telia were collected 3 wk after inocu- me, is essential to understanding the disease cycle and lation. To stimulate both germination types, directly the development of successful biological control cast and washed basidio-spores were used (Spaine and Kaneko, 1993). In direct-cast spore treatments, oak leaves with telia

892

SPAINE AND KANEKO: C RONARTZUM N UCLEAR B EHA VIOR 893 were hydrated for 12 h in darkness at 20 C in a petri RESULTS dish containing moist filter paper. This procedure initiated the formation of basidiospores. Leaves were Each cell of a basidium contained one nucleus (FIG. then suspended for 1 h directly over either glass I), but mitosis was observed in immature basidio- slides sprayed with distilled H,O or needles and spores still attached to sterigmata. The binucleate stems of 3-mo-old Pinus taeda seedlings that had condition was short-lived, with most spores becoming been cut off at the stem collar. The P hour incuba- mono-nucleate before germination. Washing spores tion represented spores harvested after a 30-min cast- apparently hastened this process. Nuclear behavior ing period. All other incubation periods followed a was similar during both types of germination, direct, l-h casting period. Slides and seedlings were incu- i.e., formation of a thin germ tube, or indirect, i.e., bated in a petri dish with moistened filter paper for by repeated secondary basidiospore formation. 0, 1, 2, 4, or 6 h. FIG URES 1-8 illustrate the nuclear conditions ob- Washed spores were prepared by a method similar served in the basidium and basidiospores during in- to the one described by Miller (1970). Oak leaves direct and direct germination. Each cell of the basid- were hydrated as described above. Leaves were then ium was mononucleate (FIG. 1). When basidiospores hung for 1 h over a petri dish containing H,O ad- were first cast from the basidium, the majority of justed to pH 2.2 with HCl. Discharged basidiospores spores were binucleate (FIG. 2). However, after a l- were collected on an Advantec’ millipore filter (3.0 2 h period, one of the two nuclei was degenerating km) (Toy0 Roshi, Ltd., Japan), and washed with dis- and fading (FIG. 3). When degeneration of the sec- tilled H,O. Suspensions of collected basidiospores in ond nucleus was complete, basidiospores remained distilled H,O were sprayed onto seedlings of Pinus mononucleate (FIG. 4). In basidiospores that germi- taeda or glass slides in petri dishes containing moist- nated indirectly, the nucleus often was seen migrat- ened filter paper. ing into the sterigma (FIG. 5). The nucleus then mi- All petri dishes of slides and seedlings were kept at grated into the secondary basidiospore where it un- 20 C in darkness for 0, 1, 2, 4, and 6 h. Five slides derwent a second mitotic division (FIG. 6). The bi- and five seedlings were prepared for washed and un- nucleate condition in the secondary basidiospore washed spores for each incubation period. Immedi- also was short-lived, and one nucleus degenerated ately after removal from the petri dishes, slides were quickly leaving a mononucleate spore. During direct air-dried at 50 C. To observe germination of basidi- germination, typically a single nucleus migrated into ospores on seedlings, the needles and stem of a seed- the germ tube (FIG. 7). However, occasionally a bi- ling were placed on a glass slide previously sprayed nucleate condition was observed in the germ tube with distilled H,O, and the basidiospores were (FIG. 8). washed with distilled H,O from the seedlings. These Effects of substrate and of basidiospore washing on slides also were air-dried at 50 C. Five slides were pre- the frequency of nuclear conditions during basidio- pared for unwashed basidiospores, and five for spore germination of C. quercuum f. sp. fusiforme are washed basidiospores. To determine the nuclear be- summarized in FIG. 9. When basidiospores were cast havior within basidiospores, the nuclear conditions in directly onto glass slides, after 30 min in the casting both germination types were examined by 4, 6-diam- chamber and O-h further incubation, 85% of the idino-2-phenylindole (DAPI) staining methods. spores had two nuclei of equal size (FIG. 9, a). How- Before DAPI staining, all dried slides were fixed in ever, after l-h in the casting chamber with O-h further a solution of absolute ethanol and acetic acid (3:l) incubation, only 19% of the spores were binucleate for 10 min, washed in distilled H,O for 10 min five (FIG. 9, a), 76% contained one normal-sized nucleus, times, and air-dried. Slides were stained in DAPI fol- and one smaller nucleus (FIG. 9, b); and 5% were lowing the method described by Kuroiwa and Suzuki mononucleate (FIG. 9, c). Mononucleate basidio- (1980), but changing the DAPI concentration to 0.5 spores increased to 73% after a 1 h incubation period kg/mL. Observations were made with an Olympus (FIG. 9, c). BH-2 epifluorescence microscope. Spores (50 on Basidiospores directly cast onto glass slides began each of five slides, 250 total) were examined for each to develop germlings after a l-h incubation period. of the 25 treatments. Nuclear conditions in basidia However, 65% of the spores germinated indirectly, developed from teliospores were examined by stain- producing secondary basidiospores within a 6h in- ing germinating telia with DAPI. cubation (FIG. 9, i-q) period. Only one nucleus was observed (FIG. 5) in 98% of primary spores or secondary sterigmata from the I- to 6-h incubation pe- 1 The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Depart- riod (FIG. 9, l-o). This suggests nuclear degeneration ment of Agriculture of any product or service. occurred rapidly during indirect germination. After 894

FIGS. 1-8. Fluorescence micrographs of Cronartium quewuum f. sp. f usif orme illustrating different nuclear conditions observed in the basidium and basidiospores during indirect and direct germination. P = primary basidiospore; S = secondary basidiospore. Bar = 20 pm for all figures. 1. Basidium with one nucleus in each cell. 2. Binucleate basidiospores. 3. Binucleate basidiospores, with one nucleus degenerating. 4. Degeneration of second nucleus nearly complete. 5. Indirect germination with one nucleus in secondary sterigma. 6. Indirect germination, with mitotic division in secondary spore. 7. Direct germination, with one nucleus in germ tube. 8. Direct germination showing two nuclei in germ tube.

secondary spores were produced, mitotic division oc- 3% were binucleate (FIG. 9, h). These results show curred in these spores (FE. 6; FIG. 9, p-q). At the that nuclear degeneration occurred in the course of 6-h incubation period, the number of secondary direct germination. spores with unequal-sized nuclei (FIG. 9, q) increased, suggesting nuclear degeneration in the sec- DISCUSSION ondary spores. Twenty-one percent of spores on glass slides ger- Our results indicated that nuclear behavior was in- minated directly (Frc;. 7). For these spores, 96% of dependent of germination type in C. quercuum f. sp. the germ tubes contained one nucleus (FIG. 9, g), fusiforme basidiospores. Most washed basidio-spores and 4% of the germ tubes contained two nuclei (FIG. germinated directly (96%)) regardless of substrate 9, h). This suggests that nuclear degeneration also (Frc;. 9). This observation supported previous find- occurred during direct germination within the first ings (Spaine and Kaneko 1993). Comparison of the 6-h incubation period. 30 min/l h casting period with no further incubation Germination appeared to occur earlier in spores period demonstrates how rapidly nuclear degenera- directly cast onto seedlings than in those cast onto tion occurred. At 30 min casting period, 19% of the glass slides. After a l-h casting period, with O-h fur- ungerminated washed basidiospores on glass were ther incubation, 30% of the basidiospores already mononucleate. After 1 h casting time and 1 h incu- had germinated. As time passed, the ratio of unger- bation time, this had increased to 60%. After the 2 minated mononucleate spores (FE. 9, c) to binucle- h incubation period, 66% of ungerminated spores ate spores increased. This was the result of one of the were mononucleate. The frequency of ungerminated two nuclei degenerating. On seedlings, 95% of the mononucleate basidiospores increased in the longer spores germinated directly. This was compared to incubation periods on both glass slides and seedlings. 21% of the spores on glass slides. Of these, 97% of This indicates that nuclear degeneration also oc- the germ tubes were mononucleate (FIG. 9, g), and curred in the washed basidiospore treatment, usually SPAINE AND K ANEKO: CRONARTIUM N UCLEAR B EHAVIOR 895

Ungerminated Direct Germination Indirect Germination Spores

(4 (b) (4 (4 (4 (0 Cd (h) 0) 0) 64 (1) 04 0-4 (0)

Treatment

0= 84.8 12.8 2.4 0 0 0 0 0 0 0 00 0 0 0 0 0 100 0 18.8 76.0 5.2 0 0 0 0 0 0 0 00 0 0 0 0 0 100 Direct cast on I 2.4 22.4 73.2 0 0 0 0.4 0 0 0 1.6 0 0 0 0 0 0 100 glass slides 2 0.8 10.8 64.8 0 0.4 4.8 0.4 0 0 0.8 17.2 0 0 0 0 0 0 100 4 0.4 7.2 14.8 1.2 1.6 13.6 3.2 0 0.4 0.8 33.2 0 4.0 1.2 2.0 11.2 5.2 100 6 0.8 5.2 18.0 0.4 0 5.2 5.2 0.4 0 0.4 20.4 0 19.2 0.4 9.2 6.0 9.2 100

0 6.8 19.6 44.0 2.4 1.6 19.2 1.6 0 0 0 4.8 0 0 0 0 0 0 100 1 5.6 8.4 40.0 1.6 2.4 30.0 9.2 0 0 0 2.0 0 0 0 0 0.4 0.4 100 Direct cast on 2 0.4 0.8 22.0 0 0.4 37.2 36.0 1.6 0 0 1.6 0 0 0 0 0 0 100 seedlings 4 0.4 0 6.0 0.4 0.8 39.6 52.8 0.4 0 0 0 0 0 0 0 0 0 100 6 0 1.6 10.8 0 0.4 4.8 72.8 3.6 0 0 1.2 0.4 3.2 0 0.8 0.4 0 100

Oa 11.2 69.6 19.2 0 0 0 0 0 0 0 00 0 0 0 0 0 100 0 25.6 60.0 14.4 0 0 0 0 0 0 0 00 0 0 0 0 0 100 Washedon I 12.8 27.6 58.8 0 0 0.8 0 0 0 0 0 0 0 0 0 0 0 100 glass slides 2 5.2 16.4 65.6 0 1.2 9.6 0 0 0 0 2.0 0 0 0 0 0 0 100 4 1.2 7.2 22.0 0.4 4.0 48.8 12.8 0.8 0 0.4 1.6 0 0 0 0 0 0.8 100 6 1.6 7.2 22.0 0 1.2 33.2 29.6 2.0 0 0 2.0 0 0.8 0 0.4 0 0 100

1 6.0 20.8 60.4 3.6 0 8.0 0 0 0 0.4 0.8 0 0 0 0 0 0 100 Washed on 2 2.8 11.2 34.4 2.8 4.4 28.8 12.4 0.8 0 0 1.2 0 0 0 0.4 0 0.4 100 seedlings 4 1.6 6.0 21.2 2.0 6.0 30.0 28.4 2.8 0 0 1.6 0 0 0 0.4 0 0.4 100 6 0 0 7.2 0 0.8 3.2 84.4 3.2 0 0 0.4 0 0.4 0.4 0 0 0 100

FIG. 9. Effects of substrate and basidiospore washing on the percenage frequency of nuclear conditions during various phases of basidiospore germination of Cronnrtium quercuum f. sp. fuszformr. 0” = spores harvested after a SO-min casting period; all other incubation periods followed a 1-hr casting period; N = normal nucleus; DN = degenerating nucleus; PB = primary basidiospore; GT = germ tube; SS = secondary sterigmata; and SB = secondary basidiospore. 896 MYCOLOGIA before development of germ tubes. When germ indirect germination is unclear. Mims and Richard- tubes developed but nuclei remained in the primary son (1989) suggested that it might prevent secondary spores, 93% were mononucleate condition (FIG. 9, d- spores from becoming tetranucleate because mitosis f) on glass slides and 78% were mononucleate seed- seems to be a common event during maturation of lings (FIG. 9, d-f). secondary spores. In this study, only one tetranuclea- When nuclei migrated into the developed germ te secondary basidiospore was observed. Therefore, tube, 94% were in the mononucleate condition (FIG. nuclear degeneration in the process of indirect ger- 9, g) on glass slides, and 95% on seedlings. This fre- mination of basidiospores may be a necessary event quency is similar to that observed in the direct ger- for rust fungi that have binucleate basidiospores. mination of direct-cast spores and strongly suggests that nuclear degeneration occurred regardless of ACKNOWLEDGMENTS spore washing. This research was supported, in part, by the Science and In Coleosporium petasitis, the smaller of the two nu- Technology Agency of Japan. clei in basidiospore germlings was thought to be degenerating (Kaneko, 1975). Using transmission elec- tron microscopy, Bauer and Oberwinkler (1988) and LITERATURE CITED Mims and Richardson (1989) clearly demonstrated Anikster, Y 1983. Binucleate basidiospores in Pucciniaceae the smaller nuclei were degenerating in Cronartium from the United States and Israel. (Abstract). The 3rd asclepiadeum and in Gymnosporangium juniperi-virgi- International Mycological Congress. August 28-Septem- nianae, respectively. Similar behavior also occurred ber 3. Tokyo, Japan. in Cronartium quercuum f. sp. fuszforme basidiospores. Bauer, R. 1986. Basidiosporenentwicklung und-keimung When cast onto moist glass slides, one of the two nu- bei Heterobasidiomyceten. Be?: Deutsch. Bot. Ges. 99: 67-81. clei degenerated before germ tube development. , and F. Oberwinkler. 1988. Nuclear degeneration This study showed that fundamental nuclear be- during ballistospore formation of Cronartium asclepi- havior was similar in direct and indirect germination adeum (Uredinales). Bot. Acta. 101: 272-282. on slides (non-host) and seedlings (host). However, Jewell, F. F., R. P. True, and S. L. Mallett. 1962. Histopa- the percentage direct germination was much higher thology of Cronartium fusa$rme in slash pine seedlings. in direct-cast spores on host surfaces and in washed Phytopathology 52: 850-858. spores on both host and non-host surfaces. The only -, and N. M. Walker. 1965. Normal and abnormal difference found between direct and indirect ger- mycelial characteristics of Cronartium quercuum in mination involved a few instances during direct ger- shortleaf pine. Phytopathology 55: 1325-1327. Kaneko, S. 1975. Karyological observations on germination mination in which nuclear degeneration failed to oc- of teliospores and sporidia in Coleospotium petasitis cur before germ tube development. No such in- (D.C.) Lev. Rep. Tottori Mycol. Inst. 12: 15-24. stances were observed during indirect germination. Kuroiwa, T., and T. Suzuki. 1980. An improved method for These results differ from observations in other rust the demonstration of the in situ chloroplast nuclei in species (Bauer, 1986; Kaneko, 1975; Mims and Rich- higher plants. Cell Structure and Function 5: 195-197. ardson, 1989). These earlier studies suggested no nu- Miller, T. 1970. Inoculation of slash pine seedlings with clear degeneration occurred during direct germina- stored basidiospores of Cronartium fusa$rme. Phytopa- tion. thology 60: 1773-l 774. Mims, C. W., and E. A. Richardson. 1989. Ultrastructure of Spaine and Kaneko (1993) stated that basidiospore appressorium development by basidiospore germlings exudates and other external factors affect germina- of the rust fungus Cjmnosporangium juniperi-virgini- tion type of C. quercuum f. sp. fusiformebasidiospores. anae. Protoplasma 148: 11 l-l 19. This follow-up study shows that germination type is -, and -. 1990. Ultrastructure of secondary not controlled by differences in nuclear behavior. spore formation in the rust Gymnosporangium juniperz- However, nuclear degeneration advanced more uirginianae. Mycologia 82: 236-242. quickly on host plants than on artificial surfaces. Olive, L. S. 1942. Nuclear phenomena involved at meiosis Nuclear degeneration during direct germination in Coleospotium helianthi. J. Elisha Mitchell Sci Sot. 58: may establish the way for the haploid condition need- 43-51. Sanwal, B. D. 1953. The development of the basidium in ed before spermatization. Mycelia and haustoria of Coleospotium sidae. Bull. Torrey Bot. Club 80: 205-216. Cronartium species in pine tissues have been report- Spaine, P. C., and S. Kaneko. 1993. Spore exudates and ed to be primarily mononucleate (Jewel1 et al., 1962; other factors affecting germination type of Cronartium Jewel1 and Walker, 1965). quercuum f. sp. fisiforme basidiospores. Mycologia 85: The significance of nuclear degeneration during 51-61.