Non-AUG Initiation of AGAMOUS mRNA Translation in Arabidopsis thaliana

180

110

The disparity between rates of morphological and molecular evolution remains a key paradox in evolutionary genetics. A proposed resolution to this paradox has been the conjecture that morphological evolution proceeds via diversification in regulatory loci, and that phenotypic evolution may correlate better with regulatory gene divergence. This conjecture can be tested by examining rates of regulatory gene evolution in species that display rapid morphological diversification within adaptive radiations. We have isolated homologues to the Arabidopsis APETALA3 (ASAP3͞TM6) and APETALA1 (ASAP1) floral regulatory genes and the CHLOROPHYLL A͞B BINDING PROTEIN9 (ASCAB9) photosynthetic structural gene from species in the Hawaiian silversword alliance, a premier example of plant adaptive radiation. We have compared rates of regulatory and structural gene evolution in the Hawaiian species to those in related species of North American tarweeds. Molecular evolutionary analyses indicate significant increases in nonsynonymous relative to synonymous nucleotide substitution rates in the ASAP3͞TM6 and ASAP1 regulatory genes in the rapidly evolving Hawaiian species. By contrast, no general increase is evident in neutral mutation rates for these loci in the Hawaiian species. An increase in nonsynonymous relative to synonymous nucleotide substitution rate is also evident in the ASCAB9 structural gene in the Hawaiian species, but not to the extent displayed in the regulatory loci. The significantly accelerated rates of regulatory gene evolution in the Hawaiian species may reflect the influence of allopolyploidy or of selection and adaptive divergence. The analyses suggest that accelerated rates of regulatory gene evolution may accompany rapid morphological diversification in adaptive radiations.floral regulatory loci ͉ MADS-box ͉ CAB9 ͉ Hawaiian silversword alliance R ates of morphological evolution are generally not correlated with rates of molecular evolution. This paradoxical observation was highlighted early by Wilson and coworkers (1-3), and subsequent molecular studies in species groups that have undergone recent adaptive radiations, such as African rift lake cichlids (4), columbines (5), and the Hawaiian silversword alliance (6), have documented marked incongruities in rates of morphological and molecular evolution. A proposed resolution to this paradox has been the conjecture that evolutionary changes in regulatory genes, rather than large-scale diversification in structural genes, may be responsible for interspecific variation in organismal morphologies (1)(2)(3)(7)(8)(9). This conjecture is reinforced by molecular developmental studies that indicate that dramatic shifts in organismal structure may arise from mutations at key regulatory loci (7-9). One test of this conjecture is to examine whether there is a significant acceleration in rates of regulatory gene evolution in species that display rapid morphological diversification within adaptive radiations.The Hawaiian silversword alliance (Asteraceae: Heliantheae-Madiinae) is...

Section: Regulatory and Structural Genes In The Hawaiian And North Ammentioning

confidence: 99%

Section: Regulatory and Structural Genes In The Hawaiian And North Ammentioning

confidence: 99%

Accelerated regulatory gene evolution in an adaptive radiation

Barrier

Robichaux

Purugganan

2001

180

110

“…The CBFs/DREB1s belong to the AP2/EREBP family of transcription factors (6) and bind to the cold-and dehydration-responsive DNA regulatory element designated C-repeat (CRT)/dehydration response element (DRE) (7,8). CRT/DRE elements contain the conserved CCGAC core sequence, which is sufficient to activate gene transcription under cold stress (7,8) and is present in the promoters of many cold-inducible genes (9).…”

Section: T He Identification Of the C-repeat-binding Factors (Cbf1-3)mentioning

confidence: 99%

ArabidopsisCBF1 and CBF3 have a different function than CBF2 in cold acclimation and define different gene classes in the CBF regulon

Novillo

Medina

Salinas

2007

321

254

The C-repeat-binding factor (CBF)/dehydration-responsive element-binding factor (DREB1) proteins constitute a small family of Arabidopsis transcriptional activators (CBF1/DREB1B, CBF2/ DREB1C, and CBF3/DREB1A) that play a prominent role in cold acclimation. A fundamental question about these factors that remains to be answered is whether they are functionally equivalent. Recently, we reported that CBF2 negatively regulates CBF1 and CBF3 expression, and that CBFs are subjected to different temporal regulation during cold acclimation, which suggested this might not be the case. In this study, we have analyzed the expression of CBF genes in different tissues of Arabidopsis, during development and in response to low temperature, and characterized RNA interference (RNAi) and antisense lines that fail to accumulate CBF1 or/and CBF3 mRNAs under cold conditions. We found that CBF1 and CBF3 are regulated in a different way than CBF2. Moreover, in contrast to CBF2, CBF1 and CBF3 are not involved in regulating other CBF genes and positively regulate cold acclimation by activating the same subset of CBF-target genes. All these results demonstrate that CBF1 and CBF3 have different functions than CBF2. We also found that the CBF regulon is composed of at least two different kind of genes, one of them requiring the simultaneous expression of both CBF1 and CBF3 to be properly induced. This indicates that CBF1 and CBF3 have a concerted additive effect to induce the whole CBF regulon and the complete development of cold acclimation.freezing tolerance ͉ low temperature ͉ DREB1 ͉ abiotic stress ͉ cold signaling T he identification of the C-repeat-binding factors (CBF1-3)(1-3), also named dehydration-responsive element-binding factors (DREB1B, -1C, and -1A, respectively) (4), represented a significant step toward the understanding of how gene expression is regulated during cold acclimation, the adaptive response whereby many plants increase their freezing tolerance in response to low nonfreezing temperatures (5). The CBFs/DREB1s belong to the AP2/EREBP family of transcription factors (6) and bind to the cold-and dehydration-responsive DNA regulatory element designated C-repeat (CRT)/dehydration response element (DRE) (7,8). CRT/DRE elements contain the conserved CCGAC core sequence, which is sufficient to activate gene transcription under cold stress (7,8) and is present in the promoters of many cold-inducible genes (9). The CBF/DREB1 genes do not contain the CCGAC sequence in their promoters but are also induced by low temperature. This induction is transient and precedes that of cold-inducible genes with the CRT cis element. The expression of CBF/DREB1 genes, however, is not activated by dehydration and salt stress (2-4).An important issue in the study of CBFs is their individual function. Despite extensive research carried out on these transcriptional activators, whether they have overlapping function (s) has not yet been conclusively established. Constitutive overexpression of each CBF in Arabidopsis results in similar express...

“…In particular, a family of transcription factors in Arabidopsis known either as C-repeat-binding factor (CBF)1, CBF2, and CBF3 (3,4) or dehydration-responsive elementbinding factor (DREB)1B, DREB1C, and DREB1A (5), respectively, has been identified. These factors belong to the Apetala 2͞ethylene-responsive element-binding protein (AP2͞ EREBP) family of DNA-binding proteins (6) and bind to the cold-and dehydration-responsive DNA regulatory element (DRE) (7), also termed C-repeat (CRT) (8). CRT͞DRE elements contain the conserved CCGAC core sequence, which is sufficient to induce gene transcription under cold stress (7,8) and is present in the promoters of many cold-inducible genes (2).…”

mentioning

confidence: 99%

CBF2/DREB1C is a negative regulator ofCBF1/DREB1BandCBF3/DREB1Aexpression and plays a central role in stress tolerance inArabidopsis

Novillo

Alonso²,

Ecker³

et al. 2004

483

359

CBF͞DREB1 (C-repeat-binding factor͞dehydration responsive element-binding factor 1) genes encode a small family of transcriptional activators that have been described as playing an important role in freezing tolerance and cold acclimation in Arabidopsis. To specify this role, we used a reverse genetic approach and identified a mutant, cbf2, in which the CBF2͞DREB1C gene was disrupted. Here, we show that cbf2 plants have higher capacity to tolerate freezing than WT ones before and after cold acclimation and are more tolerant to dehydration and salt stress. All these phenotypes correlate with a stronger and more sustained expression of CBF͞DREB1-regulated genes, which results from an increased expression of CBF1͞DREB1B and CBF3͞DREB1A in the mutant. In addition, we show that the expression of CBF1͞DREB1B and CBF3͞DREB1A in response to low temperature precedes that of CBF2͞DREB1C. These results indicate that CBF2͞DREB1C negatively regulates CBF1͞DREB1B and CBF3͞ DREB1A, ensuring that their expression is transient and tightly controlled, which, in turn, guarantees the proper induction of downstream genes and the accurate development of Arabidopsis tolerance to freezing and related stresses.F reezing temperatures greatly limit the geographical distribution and growing season of plants and cause negative effects on crop quality and productivity. As a consequence, appreciable effort has been conducted to determine the adaptive mechanisms plants have evolved to survive this adverse environmental condition. Many plants, including Arabidopsis, increase their freezing tolerance in response to low, nonfreezing temperatures. This process, called cold acclimation (1), is complex and involves a number of biochemical and physiological changes, ranging from alterations in lipid composition to accumulation of sugars (2). Different studies have suggested that low-temperature-regulated gene expression is critical in plants for cold acclimation (2). Low-temperature-responsive genes encode a diverse number of proteins, including enzymes involved in respiration and metabolism of carbohydrates, lipids, phenylpropanoids and antioxidants, molecular chaperones, antifreeze proteins, among others, with a believed function in freezing tolerance (2).During the past few years, substantial progress has been made toward understanding how low temperatures regulate gene expression. In particular, a family of transcription factors in Arabidopsis known either as C-repeat-binding factor (CBF)1, CBF2, and CBF3 (3, 4) or dehydration-responsive elementbinding factor (DREB)1B, DREB1C, and DREB1A (5), respectively, has been identified. These factors belong to the Apetala 2͞ethylene-responsive element-binding protein (AP2͞ EREBP) family of DNA-binding proteins (6) and bind to the cold-and dehydration-responsive DNA regulatory element (DRE) (7), also termed C-repeat (CRT) (8). CRT͞DRE elements contain the conserved CCGAC core sequence, which is sufficient to induce gene transcription under cold stress (7,8) and is present in the promoters of many cold-induc...