Alfalfa (Medicago sativa L.) production is negatively affected by drought stress. This is particularly true for alfalfa grown on non-irrigated rangelands. Thus, the development of drought-tolerant alfalfa cultivars is of great significance. A greenhouse study was conducted to evaluate 11 alfalfa accessions including several that are adapted to rangeland conditions and two commercial accessions, for their performance under drought condition. Water supply was adjusted based on the transpiration rate of individual plants to compensate for 100, 75, 50 or 25 % of transpirational water loss. We found that RS, a naturalized alfalfa collected from the Grand River National Grassland in South Dakota, showed the best resistance to drought condition. It showed the smallest reduction in stem elongation (36 %), relative growth rate (14 %), and shoot dry mass (40 %) production under the severest drought tested in this study relative to the non-drought treatment. While RS showed less biomass production under well-watered conditions, it produced similar or more shoot biomass under drought conditions compared to other accessions. Associated with the drought resistance or less sensitivity to drought, RS showed greater capability to maintain root growth, shoot relative water content, and leaf chlorophyll content compared to other accessions. Different from other accessions, RS showed increasing water use efficiency (WUE) as water deficit became severe, reaching the greatest WUE among 11 accessions. Our results suggest that RS is a valuable genetic resource that can be used to elucidate physiological and molecular mechanisms that determine drought resistance in alfalfa and to develop alfalfa with improved WUE.

14-3-3s are highly conserved, multigene family proteins that have been implicated in modulating various biological processes. The presence of inherent polyploidy and genome complexity has limited the identification and characterization of 14-3-3 proteins from globally important Brassica crops. Through data mining of Brassica rapa, the model Brassica genome, we identified 21 members encoding 14-3-3 proteins namely, BraA.GRF14.a to BraA.GRF14.u. Phylogenetic analysis indicated that B. rapa contains both ε (epsilon) and non-ε 14-3-3 isoforms, having distinct intron-exon structural organization patterns. The non-ε isoforms showed lower divergence rate (Ks < 0.45) compared to ε protein isoforms (Ks > 0.48), suggesting class-specific divergence pattern. Synteny analysis revealed that mesohexaploid B. rapa genome has retained 1–5 orthologs of each Arabidopsis 14-3-3 gene, interspersed across its three fragmented sub-genomes. qRT-PCR analysis showed that 14 of the 21 BraA.GRF14 were expressed, wherein a higher abundance of non-ε transcripts was observed compared to the ε genes, indicating class-specific transcriptional bias. The BraA.GRF14 genes showed distinct expression pattern during plant developmental stages and in response to abiotic stress, phytohormone treatments, and nutrient deprivation conditions. Together, the distinct expression pattern and differential regulation of BraA.GRF14 genes indicated the occurrence of functional divergence of B. rapa 14-3-3 proteins during plant development and stress responses.

2051 RESEARCHT emperature is one of the major environmental cues regulating growth and development of plants. Chilling or subzero freezing temperature results in cold stress, which is one of the major factors limiting the production and the overall yield of plants. Cold-induced damage is evident at both the physiological and molecular levels. The formation of ice in vegetative tissues and the shutting down of important metabolic machineries are but a few of the adverse effects of freezing. However, these effects are greatly reduced when plants are exposed to low nonfreezing temperatures prior to being subjected to freezing. Such acquired tolerance to subzero temperatures by many temperate plants is termed cold acclimation (Guy, 1990;Thomashow, 1999;Chinnusamy et al., 2007). Cold acclimation is achieved through biochemical and physiological reprograming at the tissue and cellular levels. Cell plasma membranes are the first sensors and responders to low-temperature stress. Early responses include the rigidification of cellular membranes, followed by cytoskeletal rearrangement, Ca 2+ influx, and ABSTRACT We recently identified the alfalfa (Medicago sativa L.) germplasm River Side (RS) and Foster Ranch (FR), both naturally adapted to the Grand River National Grassland environment in South Dakota and showing superior freezing tolerance. To understand the molecular basis of freezing tolerance in RS and FR, we examined expression of the C-repeat binding factor-like (CBFl) genes in alfalfa. Eighteen CBFl genes were identified after examining the genome of Medicago truncatula Gaertn., a close relative of alfalfa. Phylogenetic analysis clustered Medicago CBFs into four subgroups. Expression profiling of these genes in alfalfa seedlings revealed diverse cold-induction patterns. Four of the genes that showed an early induction as CBF3 in Arabidopsis under cold stress were selected for detailed expression analyses. These genes varied in expression patterns, in different tissues and at different developmental stages, and exhibited different diurnal regulation without cold treatment. Two of the genes, MsCBFl-17 and MsCBFl-18, showed an early and high induction under cold stress in RS and Apica, a cold-tolerant cultivar, when compared with a nonfreezing tolerant germplasm, suggesting that these two genes are potentially the functional homologs of CBF3. On the other hand, MsCBFl-11 was the only gene that was induced in all three cold-tolerant germplasm, including FR, but the induction was relatively late compared with MsCBFl-17 and MsCBFl-18. Together, these findings suggest that the CBFs may play an important role in the regulation of freezing tolerance in alfalfa, and additional mechanisms exist to explain the superior freezing tolerance in RS and FR.

Recent studies mainly in Arabidopsis have renewed interest and discussion in some of the key issues in hydrotropism of roots, such as the site of water sensing and the involvement of auxin. We examined hydrotropism in maize (Zea mays) primary roots.We determined the site of water sensing along the root using a nonintrusive method. Kinematic analysis was conducted to investigate spatial root elongation during hydrotropic response. Indole-3-acetic acid (IAA) and other hormones were quantified using LC-MS/MS. The transcriptome was analyzed using RNA sequencing.Main results: The very tip of the root is the most sensitive to the hydrostimulant. Hydrotropic bending involves coordinated adjustment of spatial cell elongation and cell flux. IAA redistribution occurred in maize roots, preceding hydrotropic bending. The redistribution is caused by a reduction of IAA content on the side facing a hydrostimulant, resulting in a higher IAA content on the dry side. Transcriptomic analysis of the elongation zone prior to bending identified IAA response and lignin synthesis/wall cross-linking as some of the key processes occurring during the early stages of hydrotropic response.We conclude that maize roots differ from Arabidopsis in the location of hydrostimulant sensing and the involvement of IAA redistribution.