Identification of a gene involved in the juvenile-to-adult transition (JAT) in cultivated olive trees

The juvenile-to-adult transition is a complex and poorly understood process in plant development required to reach reproductive competence. For woody plants, knowledge of this transition is even scantier and no genes have been definitively identified as involved in this transition. To search for genes involved in the juvenile-to-adult transition in olive, we constructed juvenile and adult subtractive cDNA gene libraries and identified genes that were differentially expressed in the juvenile and adult phases. In the analysis of theses libraries, we found 13 differentially expressed genes. One of these genes designated as juvenile to adult transition (JAT) was of special interest because it was highly expressed at the mRNA level in the early developmental phases but repressed in the adult phase. The analysis of mutant trees altered in the juvenile-to-adult transition, as well as a segregating progeny of 31 trees from a “Picual” x “Jabaluna” cross, support the contention that its activity might be required for a non-delayed transition. The study of an Arabidopsis thaliana JAT mutant strain confirmed this hypothesis as it showed a delayed flowering phenotype. JAT is expressed in different parts of the plant, showing an unexpectedly high level of mRNA in the roots. However, the JAT expression level is not determined by the distance to the roots, but rather depends on the developmental stage of the branch meristems. JAT is a widely represented gene in plants that appears to be involved in the control of the juvenile-to-adult transition in olive.     

Comparative analysis of expressed sequence tags (ESTs) from Triticum monococcum shoot apical meristem at vegetative and reproductive stages

Triticum monococcum has recently drawn the attention of biologists to discover and utilize novel genes and alleles. To explore the molecular features of the genetic network governing floral transition in shoot apical meristem (SAM) of spring growth habit T. monococcum, two expressed sequence tag (EST) libraries containing 3,031 ESTs from vegetative SAM (VS) and 2,647 ESTs from early reproductive SAM (RS) were analyzed. Assembly of ESTs resulted in 2,303 unigenes for VS library (368 contigs and 1,935 singletons) and 1,890 unigenes (337 contigs and 1,553 singletons) for RS library. The 67.05 % of VS unigenes and 66.30 % of RS unigenes showed significant similarity with genes of known, putative and or unknown function, whereas the remaining 32.95 % of the VS unigenes and 33.7 % of RS unigenes displayed no significant match with the public protein database. The 1,064 and 866 unigenes of VS and RS libraries were assigned to functional categories using Pageman ontology tool. Further analysis revealed that the switch from VS to RS caused significant changes in the abundance of unigenes assigned to some functional categories. A total of 37 genes were identified which were significantly differentially expressed between vegetative and reproductive stages of T. monococcum SAM. Investigation of the differentially expressed genes revealed the importance of the genes involved in energy metabolism, ubiquitin/26S proteasome system, polyamines biosynthesis and signaling of reactive oxygen species in SAM differentiation towards floral transition in T. monococcum.     

Transcript profiling and gene expression analysis under drought stress in <Emphasis Type="Italic">Ziziphus nummularia</Emphasis> (Burm.f.) Wright &amp; Arn.

Drought is one of the prime abiotic stresses responsible for limiting agricultural productivity. A number of drought responsive genes have been isolated and functionally characterized but these studies have been restricted to a few model plant systems. Very few drought responsive genes have been reported till date from non model drought tolerant plants. The present study aimed at identifying differentially expressed genes from a drought tolerant, non-model plant, Ziziphus nummularia (Burm.f.) Wight & Arn. One month old seedlings of Z. nummularia were subjected to drought stress by 30% Polyethylene glycol (PEG 6000) treatment for 6, 12, 24, 48 and 72 h. A significant reduction in RWC and increase in proline was observed at 24 h and 48 h of treatment. Suppression subtractive hybridization (SSH) library was constructed with drought stressed seedlings after 24 h and 48 h of PEG 6000 treatment. A total of 142 and 530 unigenes from 24 h and 48 h library were identified respectively. Gene ontology studies revealed that about 9.78% and 15.07% unigenes from 24 h and 48 h SSH libraries were expressed in “response to stress”. Fifteen putative drought responsive genes identified in SSH library were validated for drought responsive differential expression by RT-qPCR. Significant changes in fold expressions were observed with time in the treated samples compared to the control. A heat map revealing the expression profile of genes was constructed by hierarchical clustering. Various genes identified in SSH libraries can serve as a resource for marker discovery and selection of candidate genes to improve drought tolerance in other susceptible crops.     

Suppression Subtractive Hybridization Analysis of Genes Regulated by Application of Exogenous Abscisic Acid in Pepper Plant (Capsicum annuum L.) Leaves under Chilling Stress

Low temperature is one of the major factors limiting pepper (Capsicum annuum L.) production during winter and early spring in non-tropical regions. Application of exogenous abscisic acid (ABA) effectively alleviates the symptoms of chilling injury, such as wilting and formation of necrotic lesions on pepper leaves; however, the underlying molecular mechanism is not understood. The aim of this study was to identify genes that are differentially up- or downregulated in ABA-pretreated hot pepper seedlings incubated at 6°C for 48 h, using a suppression subtractive hybridization (SSH) method. A total of 235 high-quality ESTs were isolated, clustered and assembled into a collection of 73 unigenes including 18 contigs and 55 singletons. A total of 37 unigenes (50.68%) showed similarities to genes with known functions in the non-redundant database; the other 36 unigenes (49.32%) showed low similarities or unknown functions. Gene ontology analysis revealed that the 37 unigenes could be classified into nine functional categories. The expression profiles of 18 selected genes were analyzed using quantitative RT-PCR; the expression levels of 10 of these genes were at least two-fold higher in the ABA-pretreated seedlings under chilling stress than water-pretreated (control) plants under chilling stress. In contrast, the other eight genes were downregulated in ABA-pretreated seedlings under chilling stress, with expression levels that were one-third or less of the levels observed in control seedlings under chilling stress. These results suggest that ABA can positively and negatively regulate genes in pepper plants under chilling stress.     

miRNA control of vegetative phase change in trees

After germination, plants enter juvenile vegetative phase and then transition to an adult vegetative phase before producing reproductive structures. The character and timing of the juvenile-to-adult transition vary widely between species. In annual plants, this transition occurs soon after germination and usually involves relatively minor morphological changes, whereas in trees and other perennial woody plants it occurs after months or years and can involve major changes in shoot architecture. Whether this transition is controlled by the same mechanism in annual and perennial plants is unknown. In the annual forb Arabidopsis thaliana and in maize (Zea mays), vegetative phase change is controlled by the sequential activity of microRNAs miR156 and miR172. miR156 is highly abundant in seedlings and decreases during the juvenile-to-adult transition, while miR172 has an opposite expression pattern. We observed similar changes in the expression of these genes in woody species with highly differentiated, well-characterized juvenile and adult phases (Acacia confusa, Acacia colei, Eucalyptus globulus, Hedera helix, Quercus acutissima), as well as in the tree Populus x canadensis, where vegetative phase change is marked by relatively minor changes in leaf morphology and internode length. Overexpression of miR156 in transgenic P. x canadensis reduced the expression of miR156-targeted SPL genes and miR172, and it drastically prolonged the juvenile phase. Our results indicate that miR156 is an evolutionarily conserved regulator of vegetative phase change in both annual herbaceous plants and perennial trees.