The results suggest that SA responses in tomato play a less important role in defense against Phytophthora capsici than to Pst. The impact of SA and plant activators on ABA accumulation was measured in tomato roots and shoots.However, ABA accumulation in non-stressed TDL and BTH treatments trended higher than those observed in salt-stressed plants that did not receive a plant activator treatment . Protection by TDL against Pst is likely the result of a triggered SAR response and not the result of an antagonistic effect on ABA levels. The efficacy of plant activators depends on the specific diseases targeted and the environmental context, which may present additional stressors to confound defense network signaling in the plant. A challenge for successful deployment of plant activators in the field is to manage the allocation, ecological and fitness costs that are associated with induced defenses . These costs can be manifested by reduced growth and reproduction, vulnerability to other forms of attack, and potential interference with beneficial associations . It would seem that the severity of these costs is conditioned in part by the milieu of abiotic stressors operative at any given time. Reactive oxygen species contribute to the initiation of SAR , are induced by SA and BTH , and are essential co-substrates for induced defense responses such as lignin synthesis . ROS also are important in modulating abiotic stress networks, for example in ABA signaling and response . The potential compounding effect of ROS generated from multiple stressors presents a dilemma in that the plant must reconcile these to adapt or else suffer the negative consequences of oxidative damage for failure to do so . Paradoxically, SA and BTH also are reported to protect plants against paraquat toxicity, blueberry grow pot which involves ROS generation for its herbicidal action . How plants balance ROS’s signaling roles and destructive effects within multiple stress contexts is unresolved and a critically important area of plant biology with relevance for optimizing induced resistance strategies in crop protection .
Although our experiments were conducted under highly controlled conditions, the results with TDL are encouraging and show that chemically induced resistance to bacterial speck disease occurs in both salt-stressed and non-stressed plants and in plants severely compromised in SA accumulation. Future research with plant activators should consider their use within different abiotic stress contexts to fully assess outcomes in disease and pest protection.These syntenies of wheat and rye chromosomes permit the formation of compensating translocations of wheat and rye chromosomes. A compensating translocation is genetically equivalent to either of the two parental chromosomes; that is, it carries all relevant genes, but not necessarily in the same order. On the other hand, homoeology between wheat group 1S and rye 1S arms permitted induction of homoeologous genetic recombination, thus the development of recombinants of much smaller segments of rye 1RS to wheat than the entire arm. Many of the present wheat cultivars developed by breeding for disease resistance carry a spontaneous centric rye-wheat translocation 1RS.1BL that has been very popular in wheat breeding programs . This translocation contains a short arm of rye chromosome 1, and the long arm of wheat chromosome 1BL . It must have occurred by misdivision of centromeres of the two group 1 chromosomes, and fusion of released arms and first appeared in two cultivars from the former Soviet Union, Aurora and Kavkaz. Rye chromosome arm 1RS in the translocation contains genes for resistance to insect pest and fungal disease but as it spread throughout wheat breeding programs it became apparent that the translocation was also responsible for a yield boost in the absence of pests and disease . Besides the presence of genes for resistance and yield advantage on 1RS, there is a disadvantage of 1RS in wheat due to the presence of the rye seed storage protein secalin, controlled by the Sec-1 locus on 1RS, and the absence of the wheat loci, Gli-B1 and Glu-B3, on the 1RS arm. Lukaszewski modified the 1RS.1BL translocation by removing the Sec-1 locus and adding Gli-B1 and Glu-B3 on the 1RS arm. Lukaszewski developed a set of wheat−rye translocations, derived from ‘Kavkaz’ winter wheat that added 1RS to wheat arms 1AL, 1BL, and 1DL in spring bread wheat ‘Pavon 76’, a high yielding spring wheat from CIMMYT.
Studies showed that the chromosomal position of 1RS in the wheat genome affected agronomic performance as well as bread-making quality . Using the 1RS translocation, Lukaszewski developed a total of 183 wheatrye short arm recombinant lines for group 1 chromosomes in a near-isogenic background of cv. Pavon 76 bread wheat. Out of 183 recombinant chromosomes, 110 were from 1RS- 1BS combinations, 26 from 1RS-1AS and 47 from1RS-1DS combinations. Mago et al. used some of these lines to link molecular markers with rust resistance genes on 1RS. These recombinant brea kpoint populations provide a powerful platform to locate region specific genes. Wheat roots have two main classes, seminal roots and nodal roots . Seminal roots originate from the scutellar and epiblast nodes of the germinating embryonic hypocotyls, and nodal roots, emerge from the coleoptiler nodes at the base of the apical culm . The subsequent tillers produce their own nodal roots, two to four per node and thus contribute towards correlation of root and shoot development . The seminal roots constitute from 1-14% of the entire root system and the nodal roots constitute the rest . Genetic variation for root characteristics was reported in wheat and other crop species . Genetic variability for seedling root number was studied among different Triticum species at diploid, tetraploid, and hexaploid level and it was found to be positively correlated with seed weight . In a hydroponic culture study in winter wheat, Mian et al. found significant genotypic differences in root and shoot fresh weights, number of roots longer than 40 cm, longest root length and total root length. Wheat genotypes with larger root systems in hydroponic culture were higher yielding in field conditions than those with smaller root systems . Also, wheat yield stability across variable moisture regimes was associated with greater root biomass production under drought stress . Studies in other cereal crops associated quantitative trait loci for root traits with the QTL for grain yield under field conditions. Champoux et al. provided the first report of specific chromosomal regions in any cereal likely to contain genes affecting root morphology. They reported that QTL associated with root traits such as root thickness, root dry weight per tiller, root dry weight per tiller below 30 cm,hydroponic bucket and root to shoot ratio shared common chromosomal regions with putative QTL associated with field drought avoidance/tolerance in rice. Price and Tomos also mapped QTL for root growth using a different population than that used by Champoux et al. in rice.
In a field study of maize recombinant lines, QTL for root architecture and above ground biomass production shared the same location . Tuberosa et al. reported the overlap of QTL for root characteristics in maize grown in hydroponic culture with QTL for grain yield in the field under well-watered and droughted regimes occurred in 8 different regions. They observed that QTL for weight of nodal and seminal roots were most frequently and consistently overlapped with QTL for grain yield in drought and well watered field conditions. Also, at four QTL regions, increase in weight of the nodal and seminal roots was positively associated with grain yield under both irrigation regimes in the field. There are a few reports on QTL studies for root traits in durum wheat but none has been reported in bread wheat. Kubo et al. studied root penetration ability in durum wheat. They used discs of paraffin and Vaseline mixtures as substitute for compact soil. Later, a QTL analysis was done for the number of roots penetrating the poly vinyl disc, total number of seminal and crown roots, root penetration index and root dry weight . The QTL for number of roots penetrating the poly vinyl disc and root penetration index was located on chromosome 6A and a QTL for root dry weight was located on 1B. Wang et al. demonstrated significant positive heterosis for root traits among wheat F1 hybrids. They showed that 27% of the genes were differentially expressed between hybrids and their parents. They suggested the possible role of differential gene expression in root heterosis of wheat, and possible other cereal crops. In a recent molecular study of heterosis, Yao et al. speculated that up-regulation of TaARF, an open reading frame encoding a putative wheat ARF protein, might contribute to heterosis observed in wheat root and leaf growth. Rye, wheat and barley develop 4-6 seminal roots which show a high degree of vascular segmentation . Feldman traced files of metaxylem to their levels of origin in maize root apex and showed their differentiation behind the root apex in three-dimensional model. In drier environments, Richards and Passioura demonstrated that genotypes, when selected for narrow root xylem vessels as against unselected controls, yielded up to 3%-11% more than the unselected controls depending upon their genetic background. This yield increase in the selections with narrow root vessel was correlated with a significantly higher harvest index, also higher biomass at maturity and kernel number. Huang et al. indicated the decrease in diameter of metaxylem vessel and stele with increase in temperature which resulted in decreased axial water flow in wheat roots. The decrease in axial water flow is very critical in conserving water during vegetative growth and making it available during reproductive phase of the plant. In a recent study on root anatomy, QTL for metaxylem were identified on the distal end of the long arm of chromosome 10 of rice . In another comparative study of rye DNA sequences with rice genome, the distal end of the long arm of chromosome 10 of rice showed synteny to the 1RS chromosome arm . The 1RS.1BL chromosome is now being used in many wheat breeding programs. Rye has the most highly developed root system among the temperate cereals and it is more tolerant to abiotic stresses such as drought, heat, and cold than bread wheat .
Introgression of rye chromatin into wheat may enlarge the wheat root system. Manske and Vlek reported thinner roots and higher relative root density for 1RS.1BL translocations compared with their non-translocated bread wheat checks in an acid soil, but not under better soil conditions. Repeated studies with the 1RS translocation lines of Pavon 76 have demonstrated a consistent and reproducible association between root biomass and the presence and position, of the rye 1RS arm . The increased grain yield of 1RS translocations under field conditions observed and reported earlier may be due to the consistent tendency of 1RS to produce more root biomass and also to the higher transpiration rate measured .Those authors have shown a significant increase of root biomass in wheat lines with 1RS translocations, and a positive correlation between root biomass and grain yield. All translocations of 1RS: with 1A, 1B, and 1D chromosomes have shown increased root biomass and branching as compared to Pavon 76 and there was differential expression for root biomass among these translocation lines with ranking 1RS.1AL > 1RS.1DL > 1RS.1BL > Pavon 76. In Colorado, the 1RS.1AL translocation with 1RS from Amigo showed 23% yield increase under field conditions over its winter wheat check, Karl 92 . Many present day bread wheat cultivars carry a centric rye-wheat translocation 1RS.1BL in place of chromosome 1B . Originally the translocation was thought to have been fixed because the 1RS arm of rye carries genes for resistance to various leaf and stem fungal diseases and insects . However, the translocation increased grain yield even in the absence of pathogens . It has been shown recently that this yield increase may be a direct consequence of a substantially increased root biomass . Studies by Ehdaie et al. 2003 showed a significant increase of root biomass in wheat lines with 1RS translocations and a positive correlation between root biomass and grain yield. In sand cultures, all three 1RS translocations on 1AL, 1BL, and 1DL in ‘Pavon 76’ genetic background showed clear position effects with more root biomass and root branching over Pavon 76 .