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The ASK1 gene regulates B function gene expression in cooperation with UFO and LEAFY in Arabidopsis

¹ Department of Biology and the Life Sciences Consortium, 504 Wartik Laboratory, and
² The Intercollegiate Graduate Program in Plant Physiology, Pennsylvania State University, University Park, PA 16802, USA

View larger version (64K): [in a new window]   Fig. 1. The phenotypes of Arabidopsis wild-type and mutant mature flowers. All photographs are at the same magnification. One sepal was removed to show the interior organs except for flowers in A, B, E, F and T. (A) A wild-type flower showing sepals (s), petals (p), stamens (st), and carpels (c). (B) An ask1-1 flower with 3 sepals; two of them are small and one is fused with stamen tissues (chimeric petal, cp). The stamens are shorter than normal. (C) A ufo-2 flower with abnormal organs interior to whorl one, including sepals (s), sepal-like organs (sl), one petal-like organ (pl) and one normal stamen. (D.) A ufo-2 ask1-1 flower showing sepal, sepal-like, filament (f) and stamen-like (stl) organs. (E) A ufo-6 (weak allele) flower with normal petals, stamens, and one petal fused with stamen tissues. (F) A ufo-6 flower with a normal petal, two chimeric petals and normal stamens. (G) A ufo-6 ask1-1 flower showing a stamen-like organ with carpel tissues (arrow). (H) A ufo-6 ask1-1 flower with sepals and filaments interior to whorl one. (I) An ap3-1 flower with four sepals in whorl two and stamens in whorl three. (J) An ap3-1 ask1-1 flower showing sepals and filament fused with carpel tissues (arrow) interior to whorl one. (K) An ap3-1 ask1-1 flower with sepals and carpel-like organs (arrow) interior to whorl one. (L) An ap3-3 flower with sepals, filaments and carpel-like organs interior to whorl one. (M) An ap3-3 ask1-1 flower with a similar phenotype to the ap3-3 but with fewer sepals. (N) An ap3-3 ask1-1 flower showing two small sepal-like organs (arrows). (O) An ap3-3 ufo-2 ask1-1 triple mutant flower with only one sepal between whorls one and four. (P) An ap3-3 ufo-2 ask1-1 flower showing no organs between whorls one and four. (Q) A pi-1 single mutant flower. (R) A pi-1 ask1-1 flower with fewer sepals than the pi-1. (S) A pi-1 ufo-2 ask1-1 triple mutant flower showing only one filament-like organ (arrow) and carpels fused with ovule-like tissues (arrowhead). (T) A sup-1 single mutant flower showing 11 stamens. (U,V) Two sup-1 ask1-1 double mutant flowers showing a reduced number of stamens and an increase in carpel structure. The flower in U has fewer, smaller stamens and more carpel-like organs than the sup-1 (Arrow indicates a small petal, which is often found in the ask1-1 mutant flower). The flower in V has normal carpels in the center and seven stamens with short filaments, as in the ask1-1 flower. (W) sup-1 ufo-2 flower with a reduced number of stamens (arrow points to a normal stamen with pollen grains) and a carpel structure larger than that in sup-1. (X) A sup-1 ufo-2 ask1-1 triple mutant flower showing a large gynoecium in the center and a near absence of organs between the outer sepals and the central gynoecium.

View larger version (164K): [in a new window]   Fig. 2. Morphology of early flowers of wild-type and mutant plants carrying ap3, pi, and sup mutations. In all flowers, the whorl one sepals or sepal primordia were removed. (A) A wild-type bud at stage 6 showing six stamen primordia (st), carpel primordia (gynoecium, g), and a tiny petal primordium (p). (B) A wild-type bud at stage 8 with clearly visible stamens and fused carpels. (C) A wild-type bud at stage 9 showing petal primordia becoming more prominent than before. (D) A stage 6 ap3-1 floral bud with sepal primordia and stamen primordia that are smaller than the normal. (E) An ap3-1 bud at about stage 10 with sepals (s), stamens (st) and filaments (f). (F) A stage 6 ap3-1 ask1-1 bud showing no stamen primordia but with sepal primordia and the central gynoecium primordium that had become flattened and enlarged at the periphery. (G) An ap3-1 ask1-1 bud at about stage 7 showing sepal, filament primordia (f) and enlarged carpel primordia. (H) An ap3-1 ask1-1 bud at about stage 10 showing sepals and filaments. (I) A late stage 6 ap3-3 bud without stamen primordia, but with sepal primordia and a gynoecium primordium that is enlarged and misshapen. (J) An ap3-3 bud at about stage 10 showing sepals and filamentous organs. (K) A stage 6 ap3-3 ask1-1 bud with sepal primordia which are fewer than in ap3-3, and the enlarged gynoecium primordium is similar to ap3-3. (L) An ap3-3 ask1-1 bud at about stage 10 with one small sepal-like (s) organ but no filaments. (M) A stage 6 ap3-3 ufo-2 ask1-1 triple mutant bud showing enlarged carpel primordia but no sepal primordia. (N) An ap3-3 ufo-2 ask1-1 bud at about stage 10 having only the gynoecium interior to the removed whorl one sepals. (O) A pi-1 bud at stage 6 showing sepal primordia and anenlarged gynoecium primordium. (P) A pi-1 bud at about stage 10 showing sepals and filaments. (Q) A stage 6 pi-1 ask1-1 bud showing enlarged carpel primordia but no sepal primordia. (R) A pi-1 ask1-1 bud at about stage 10 with filaments but without sepals. (S) A pi-1 ufo-2 ask1-1 bud at about stage 10 with one filament but no sepals. (T) A stage 6 sup-1 bud that has six stamen primordia and is beginning to form two more stamen primordia (arrows). (U) A sup-1 bud at stage 9 showing 6 large stamens and two small stamens, but no detectable carpel structure. (V) A sup-1 ask1-1 bud at late stage 6 that has six stamen primordia and is begimming to form one more stamen primordium (arrow). (W) A sup-1 ask1-1 bud at stage 8 showing six stamens and fused carpels (g). (X) A sup-1 ufo-2 ask1-1 triple mutant bud at late stage 6 showing a phenotype similar to that of the sup-1 ask 1-1 bud with filament primordia and fewer stamen primordia. The gynoecium primordium was also larger than that of the sup-1 ask1-1. Scale bars, in A (A,T,V,X) 10 µm; in D (D,F,G,I,K,M,O,Q) 10 µm; in B (B,C,H,L,N,R,S,W) 50 µm; in E (E,J,P,U) 50 µm.

View larger version (40K): [in a new window]   Fig. 3. Mature floral phenotypes of mutants with lfy-6 or lfy-5 alleles. (A) A lfy-6 flower with leaf-like (ll) and carpelloid (co) organs. (B) A lfy-6 ask1-1 flower showing a similar phenotype to the lfy-6 mutant. (C) A lfy-6 ufo-2 flower with the similar phenotype to lfy-6 and lfy-6 ask1-1 flowers. (D) A lfy-6 ufo-2 ask1-1 flower, similar to lfy-6, lfy-6 ask1-1, and lfy-6 ufo-2 flowers. (E) A lfy-5 flower showing sepals (s), petal (p), petal-like organ (pl), stamens (st), and carpels (c), unlike the lfy-6 flower. (F) A lfy-5 ask1-1 flower with a phenotype similar to that of lfy-6, and much more severe than that of lfy-5. (G) A lfy-5 ufo-2 flower exhibiting a phenotype similar to those of lfy-6 and lfy-5 ask1-1 flowers. (H) A lfy-5 ufo-2 ask1-1 triple mutant flower, showing a similar phenotype to those of lfy-6, lfy-5 ask1-1, lfy-5 ufo-2, lfy-6 ufo-2, lfy-6 ask1-1, and lfy-6 ufo-2 ask1-1 flowers. All photographs were taken at the same magnification.

View larger version (180K): [in a new window]   Fig. 4. Early floral morphology of mutants with lfy-6 or lfy-5 alleles. (A) A lfy-6 bud at about late stage 6 showing leaf-like primordia (ll). (B,C) Two lfy-6 flowers with leaf-like organs. The flower in B is younger than the flower in C. (D) A lfy-5 bud at about late stage 6 showing stamen primordia (st) and carpel primordia (g). (E) A stage 9 lfy-5 bud with obvious stamens (st), petals (p) and carpels (g) in the center. (F) A stage 11 lfy-5 bud with normal stamens, petal and two curled carpels. (G) A lfy-5 ask1-1 bud at about stage 6 showing spiral leaf-like primordia that were similar to those in the lfy-6 bud at the same stage. (H,I) Two lfy-5 ask1-1 flowers showing a similar phenotype to that of the lfy-6 flower. The flower in H is younger than the flower in I. Scale bars, A, 10 µm; B (B,H), 50 µm: C (C,F,I), 50 µm; D (D,G), 10 µm; E, 50 µm.

View larger version (148K): [in a new window]   Fig. 5. AP3 and PI expression in wild-type and mutant flowers. Sections of inflorescences in A-H were hybridized with an AP3 probe and in I-P were hybridized with a PI probe. All photographs are at the same magnification. The numbers indicate the bud stage; im, inflorescence meristem. (A) Wild-type showing AP3 RNA expression in the floral meristem at a high level at stages 3 and 4 and in whorl two and three in an old flower. (B) ask1-1 exhibiting AP3 RNA expression at the same position and nearly the same level as in the wild-type flower. (C,D) lfy-5 has a similar expression pattern of AP3 RNA to that in the wild type, but the expression level is slightly reduced. (E,F) lfy-6 showing no or very low AP3 RNA expression (arrows) in a young and an old bud, respectively. (G,H) lfy-5 ask1-1 double mutant showing that AP3 RNA expression is much reduced and largely undetectable. Only very low AP3 RNA expression in limited areas was observed in some buds (arrows). (I) Wild-type showing that PI RNA is present at a high level in a stage-3 floral meristem and in whorl two and three of old flowers. (J) ask1-1; the PI RNA signal shows a similar pattern to that in the wild-type flower. (K,L) lfy-5; the PI RNA expression pattern is similar to that in the wild type, but the level of expression is slightly lower than the normal. (M,N) lfy-6 showing very limited PI RNA expression in a floral bud (arrow). (O,P) lfy-5 ask1-1 ; PI RNA expression is also much reduced. Only an occasional small region of PI expression could be observed in some buds (arrows).

View larger version (18K): [in a new window]   Fig. 6. Models for regulation of AP3 and PI expression by LFY, ASK1 and UFO. (A) Model I. ASK1 and UFO facilitate the degradation of a negative modulator (X) of LFY protein activity. In the wild type, the level of X is low due to the function of ASK1, UFO and other ASK and F-box proteins, together with cullin, the third subunit of SCF. In the ask1-1 mutant, the level of X may increase slightly but other ASK genes can still provide some needed function. Wild-type LFY protein is not obviously affected by the slight increase of X. In the lfy-5 mutant, the mutant protein has reduced activity, leading to decreased AP3 and PI transcription. In the lfy-5 ask1-1 double mutant, the combination of a weak LFY protein and an increased level of X causes a dramatic reduction of AP3 and PI expression. If LFY activity is completely eliminated, as in lfy-6 (not shown here), then there is little AP3 and PI expression regardless of the level of X. (B) Model II, ASK1 and UFO facilitate the degradation of a transcriptional repressor (Y) of AP3 and PI genes. Again, the combination of ask1-1 and lfy-5 mutations would result in both an increase in the Y repressor and a decrease in the LFY activator, and a severe reduction in AP3 and PI expression, whereas either single mutation would have less pronounced effects. In the absence of LFY activator (lfy-6), the Y repressor would have no effect.