Aroldo Cisneros

Growth Platform-Dependent and -Independent Phenotypic and Metabolic Responses of Arabidopsis and Its Halophytic Relative, Eutrema salsugineum , to Salt Stress

Eutrema salsugineum maintains its salt tolerance under very different growth conditions even though its development and metabolism show substantial growth condition-dependent differences. Comparative studies of the stress-tolerant Arabidopsis (Arabidopsis thaliana) halophytic relative, Eutrema salsugineum, have proven a fruitful approach to understanding natural stress tolerance. Here, we performed comparative phenotyping of Arabidopsis and E. salsugineum vegetative development under control and salt-stress conditions, and then compared the metabolic responses of the two species on different growth platforms in a defined leaf developmental stage. Our results reveal both growth platform-dependent and -independent phenotypes and metabolic responses. Leaf emergence was affected in a similar way in both species grown in vitro but the effects observed in Arabidopsis occurred at higher salt concentrations in E. salsugineum. No differences in leaf emergence were observed on soil. A new effect of a salt-mediated reduction in E. salsugineum leaf area was unmasked. On soil, leaf area reduction in E. salsugineum was mainly due to a fall in cell number, whereas both cell number and cell size contributed to the decrease in Arabidopsis leaf area. Common growth platform-independent leaf metabolic signatures such as high raffinose and malate, and low fumarate contents that could reflect core stress tolerance mechanisms, as well as growth platform-dependent metabolic responses were identified. In particular, the in vitro growth platform led to repression of accumulation of many metabolites including sugars, sugar phosphates, and amino acids in E. salsugineum compared with the soil system where these same metabolites accumulated to higher levels in E. salsugineum than in Arabidopsis. The observation that E. salsugineum maintains salt tolerance despite growth platform-specific phenotypes and metabolic responses suggests a considerable degree of phenotypic and metabolic adaptive plasticity in this extremophile.

Testing a Hypothesis of Intergeneric Allopolyploidy in Vine Cacti (Cactaceae: Hylocereeae)

Abstract Allopolyploidy is common in angiosperms, but only rarely involves different genera. One hypothesized case of intergeneric allopolyploidy is Hylocereus megalanthus, a member of Cactaceae tribe Hylocereeae, a group of vine cactus species, some of which are known for their edible fruits (“pitahaya” or “dragon fruit”). This polyploid species has been interpreted as morphologically intermediate between Hylocereus and Selenicereus. Plastid and nuclear ribosomal DNA ITS sequences from all H. megalanthus individuals sampled are either identical (plastid), or form a monophyletic clade despite considerable intraindividual polymorphism (ITS). Plastid and ITS phylogenies both show H. megalanthus nested within a well-supported Hylocereus, which in turn is nested within a paraphyletic Selenicereus. The absence of more than one lineage of ITS in H. megalanthus is consistent with autopolyploidy, but could be due to inter-homoeologue concerted evolution. Numerous low-copy nuclear genes were tested for utility in these vine cacti, and two (phyC and Vatp1) were sampled from H. megalanthus and a subset of Hylocereus and Selenicereus species. In both cases, H. megalanthus haplotypes were more closely related to each other than to other Hylocereus or Selenicereus haplotypes. Thus, we found no evidence for allopolyploidy, let alone intergeneric allopolyploidy, in H. megalanthus.

Anastatica hierochuntica, an Arabidopsis Desert Relative, Is Tolerant to Multiple Abiotic Stresses and Exhibits Species-Specific and Common Stress Tolerance Strategies with Its Halophytic Relative, Eutrema (Thellungiella) salsugineum

The search for novel stress tolerance determinants has led to increasing interest in plants native to extreme environments – so called “extremophytes.” One successful strategy has been comparative studies between Arabidopsis thaliana and extremophyte Brassicaceae relatives such as the halophyte Eutrema salsugineum located in areas including cold, salty coastal regions of China. Here, we investigate stress tolerance in the desert species, Anastatica hierochuntica (True Rose of Jericho), a member of the poorly investigated lineage III Brassicaceae. We show that A. hierochuntica has a genome approximately 4.5-fold larger than Arabidopsis, divided into 22 diploid chromosomes, and demonstrate that A. hierochuntica exhibits tolerance to heat, low N and salt stresses that are characteristic of its habitat. Taking salt tolerance as a case study, we show that A. hierochuntica shares common salt tolerance mechanisms with E. salsugineum such as tight control of shoot Na+ accumulation and resilient photochemistry features. Furthermore, metabolic profiling of E. salsugineum and A. hierochuntica shoots demonstrates that the extremophytes exhibit both species-specific and common metabolic strategies to cope with salt stress including constitutive up-regulation (under control and salt stress conditions) of ascorbate and dehydroascorbate, two metabolites involved in ROS scavenging. Accordingly, A. hierochuntica displays tolerance to methyl viologen-induced oxidative stress suggesting that a highly active antioxidant system is essential to cope with multiple abiotic stresses. We suggest that A. hierochuntica presents an excellent extremophyte Arabidopsis relative model system for understanding plant survival in harsh desert conditions.

Intergeneric hybridization within the tribe Hylocereeae, subfamily Cactoideae (Cactaceae)

Controlled hand-cross-pollinations were performed among the genera Hylocereus, Selenicereus, and Epiphyllum and synthetic intergeneric hybrids were obtained. Parental lines and the intergeneric hybrids were studied for their nuclear DNA (2C) content and for their morphological and phenological characteristics. Hybrids between Hylocereus and Selenicereus species produced viable seeds, a finding that showed the lack of post-zygote barriers. Partial to complete sterility was observed in the intergeneric Epiphyllum x Selenicereus hybrids. The dominant relationship between the alleles controlling morphological and phenological traits was elucidated. Among the traits studied, the presence of spines and/or hairy-spines on the fruit peel, the co-dominance of the ripening time, and the dominance of the high number of shoot ribs were the most significant characteristics proving hybridization. Further development of species-specific molecular markers will be valuable for a suitable taxonomic description of these spe...

Evaluation of Interspecific-Interploid Hybrids (F1) and Back Crosses (BC1) in Hylocereus Species (Cactaceae)

Vine cacti are night-blooming epiphytes plants, endemic to the Americas, and belong to Cactaceae, subfamily Cactoideae, tribe Hylocereeae (Br. and R.) Buxbaum (Barthlott & Hunt, 1993). According to the New Cactus Lexicon (Hunt, 2006), the genera Hylocereus (Berger) Br. and R., comprises 14 species, and they are widely distributed in tropical and subtropical regions of the Americas from Mexico to North Argentina (Mizrahi & Nerd, 1999; Merten, 2003). They inhabit a wide range of ecosystems, including coastal areas, high mountains and tropical rainforests (Ortiz, 1999). These species, known as pitahaya or Dragon fruit, are currently being marketed worldwide and have a high economic potential as exotic fruit crops in arid regions where water is scarce, since they use a Crassulacean acid metabolism (CAM) pathway and are exceptionally drought-tolerant (Raveh et al., 1998; Mizrahi & Nerd, 1999; Nobel & de la Barrera, 2004).