Date Published: January 13, 2010
Publisher: Public Library of Science
Author(s): Amadou Diallo, Ndjido Kane, Zahra Agharbaoui, Mohamed Badawi, Fathey Sarhan, Samuel P. Hazen. http://doi.org/10.1371/journal.pone.0008690
Abstract: The vernalization gene 2 (VRN2), is a major flowering repressor in temperate cereals that is regulated by low temperature and photoperiod. Here we show that the gene from Triticum aestivum (TaVRN2) is also regulated by salt, heat shock, dehydration, wounding and abscissic acid. Promoter analysis indicates that TaVRN2 regulatory region possesses all the specific responsive elements to these stresses. This suggests pleiotropic effects of TaVRN2 in wheat development and adaptability to the environment. To test if TaVRN2 can act as a flowering repressor in species different from the temperate cereals, the gene was ectopically expressed in the model plant Arabidopsis. Transgenic plants showed no alteration in morphology, but their flowering time was significantly delayed compared to controls plants, indicating that TaVRN2, although having no ortholog in Brassicaceae, can act as a flowering repressor in these species. To identify the possible mechanism by which TaVRN2 gene delays flowering in Arabidopsis, the expression level of several genes involved in flowering time regulation was determined. The analysis indicates that the late flowering of the 35S::TaVRN2 plants was associated with a complex pattern of expression of the major flowering control genes, FCA, FLC, FT, FVE and SOC1. This suggests that heterologous expression of TaVRN2 in Arabidopsis can delay flowering by modulating several floral inductive pathways. Furthermore, transgenic plants showed higher freezing tolerance, likely due to the accumulation of CBF2, CBF3 and the COR genes. Overall, our data suggests that TaVRN2 gene could modulate a common regulator of the two interacting pathways that regulate flowering time and the induction of cold tolerance. The results also demonstrate that TaVRN2 could be used to manipulate flowering time and improve cold tolerance in other species.
Partial Text: In temperate regions, low temperature (LT) constitutes a major factor that regulates flowering time and many developmental transitions such as germination, bud dormancy and bursting . In response to LT-conditions, plants cold acclimate and vernalize to prevent the sensitive floral meristem from freezing damages during the winter by postponing flowering . The ability of plants to switch from vegetative to reproductive phase after a long period of cold, a process known as vernalization , allows plants to promote flowering early in the spring. During this cold exposure period, LT-responsive genes CBFs (C-repeat binding factor) and COR (Cold Regulated) are activated, allowing plants to increase their tolerance to cold and survive the winter (). Cereals are classified into spring and winter growth habit according to their vernalization requirement , . Spring varieties do not respond to vernalization and flower rapidly whereas winter varieties have a quantitative vernalization requirement. Therefore, winter varieties require vernalization to accelerate flowering and complete their life cycle. Understanding the genetic/molecular basis of both LT-responsive pathways (cold acclimation and vernalization), can help to better manipulate, the two important agronomical traits flowering and freezing tolerance.
Molecular characterization of VRN2 in hexaploid wheat demonstrates sequence variations between the homologous TaVRN2 genes. TaVRN-A2 and TaVRN-B2 exhibit marked sequence variation and higher level of transcript accumulation. This could be explained by the fact that the A, B and D genomes evolved at different rates . This variation also supports the idea that various events of sequence changes have occurred in the evolutionary history of wheat homeologous genomes that diverged about 5.0–6.9 million years ago . Indeed, since the polyploidization event of wheat, significant sequence changes have occurred that might cause mutation or lost of TaVRN2 in modern (spring) wheat. Alignment of the CCT domain of TaVRN-A2 and TaVRN-B2 shows a point mutation at position 35 of the CCT domain, where a tryptophan (W) substitutes for an arginine (R) amino acid. Mutation within this domain results in that TaVRN-A2 encodes a non-functional protein as shown previously . This observation may explain why TaVRN-A2 is expressed at a very low level to the extent that we could not clone it from our cDNA libraries. It is not known, however, if the gene is expressed into non-functional proteins or is not translated at all.