The diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae) is a very destructive crucifer-specialized pest that has resulted in significant crop losses worldwide. DBM is well attracted to glucosinolates (which act as fingerprints and essential for herbivores in host plant recognition) containing crucifers such as wintercress, Barbarea vulgaris (Brassicaceae) despite poor larval survival on it due to high-to-low concentration of saponins and generally to other plants in the genus Barbarea. B. vulgaris build up resistance against DBM and other herbivorous insects using glucosinulates which are used in plant defense. Aside glucosinolates, Barbarea genus also contains triterpenoid saponins, which are toxic to insects and act as feeding deterrents for plant specialist herbivores (such as DBM). Previous studies have found interesting relationship between the host plant and secondary metabolite contents, which indicate that attraction or resistance to specialist herbivore DBM, is due to higher concentrations of glucosinolates and saponins in younger leaves in contrast to the older leaves of Barbarea genus. As a response to this phenomenon, herbivores as DBM has developed a strategy of defense against these plant biochemicals. Because there is a lack of full knowledge in understanding bioactive molecules (such as saponins) role in plant defense against plant herbivores. Thus, in this review, we discuss the role of secondary plant metabolites in plant defense mechanisms against the specialist herbivores. In the future, trials by plant breeders could aim at transferring these bioactive molecules against herbivore to cash crops.

ABSTRACT:The leaf ultrastructure of mangrove Kandelia candel (L.) Druce planted in pots under different salinity conditions was compared under a transmission electron microscope (TEM). The results showed that the plasmalemma in plants grown in salinity conditions of 0‰ treatment (control) and 25‰ treatment was tightly combined, while in plants with salinity of 50‰ treatment, the plasmalemma crimpled remarkably and plasmolysis occurred. The nucleus and its two-layer membranes were obvious in control plants. In the case of 25‰ treatment, the membrane breakdown was observed, nucleoplasm dispersed in cytoplasm, and the electron density of cells was lower than that in control plants. In plants treated with 50‰ salinity the nucleus collapsed and no structure of the nucleus could be observed. As far as chloroplasts in control plants were concerned, they were oblong with a typical arrangement of grana and stroma thylakoids and one or two grains of starch. However, the chloroplasts in plants treated with 25‰ salinity were swelling and usually contained more grains of starch and few plastoglobuli. Most chloroplasts had a reduced number of grana, particularly of thylakoids in grana as compared with control plants. The chloroplasts of plants treated with 50‰ salinity had a considerably reduced system of grana and stroma thylakoids, and sometimes they were even deformed morphologically. They were mixed-up and contained more grains of starch and plastoglobuli. The indistinct structure of mitochondrial cristae was observed only in plants treated with 50‰ salinity. These showed that mitochondria are cell organs less sensitive to hypersaline conditions than chloroplasts and nucleus, and it was deduced that respiration was more conservative to an environment change than photosynthesis.

Effects of salinities on leaf characteristics and CO 2 /H 2 O exchange of mangrove species Kandelia candel seedlings were studied in a pot experiment. The seedlings grown in salinity of 50‰ caused a strong reduction in the rate of growth, but their leaves were black-green, smaller and less expanded or distorted than those of plants in the control and treatment of 25‰. As compared with control plants, leaves of plants treated with salinity of 25‰ were shiny and smooth. Stomatal number and density under the epidermis in leaves were reduced with the increase of salinity. Mesophyll cells in plants grown in salinity of 50‰ were smallest, 25‰ ranked the second and 0‰ were largest. Their arrangement was compact in 50‰, while in the control it was loose. Cells of the upper epidermis in leaves of control plants were loose and the cell wall was thin while that of 50‰ was more compact and the cell wall was thicker than that of 25‰. Chlorophyll (Chl) (a + b) content (μmol/cm 2 ) in plants grown in 50‰ salinity increased significantly compared with that in 0‰, and Chl a/b was also reduced. Carotenoid pigments (Car) increased significantly in different treatments. Photosynthesis (Pn) was significantly inhibited by higher salinity, and the light compensation point of higher salinity leaves increased. Net photosynthetic rate (Pn), stomatal conductance (Cs), and transpiration rate (Tr) were reduced with the increase of salinity while dark respiration (Rd) increased.

The regeneration frequency of okra (Abelmoschus esculentus) is greatly influenced by its genetic makeup and recalcitrant nature. Phenolic secretion, in particular, is a major problem in okra tissue culture. This study describes a reproducible, rapid, and more efficient in vitro regeneration method using cotyledonary node explants of okra. Explants were incubated on Murashige and Skoog (MS) medium containing different concentrations and combinations of various plant growth regulators (PGRs) [benzyladenine (BA), thidiazuron (TDZ), and α-naphthylacetic acid (NAA)], and regeneration enhancers [silver nitrate (AgNO3) and Pluronic F-68]. Cut ends of cotyledonary node segments rapidly turned brown and cultures failed to establish. Antibrowning additives, such as activated charcoal (AC), ascorbic acid (AA), and AgNO3 at various concentrations in PGR-free MS basal medium were tested for their ability to control phenolic secretion from explants. Among these additives, 15 mg·L−1 AA was found to be optimal for controlling phenolic secretion, resulting in healthy explants and culture establishment. The highest number of shoots (a mean of 9.3 ± 0.9 shoots per cotyledonary node explant) was obtained on MS media containing 0.5 mg·L−1 NAA + 1 mg·L−1 TDZ + 0.1% Pluronic F-68. Individual shoots were elongated on MS medium + 1 mg·L−1 BA + 0.1 mg·L−1 gibberellic acid (GA3) (shoot length 5.3 ± 0.2 cm) and rooted on ½ MS medium + 1 mg·L−1 indole-3-butyric acid (IBA) and 200 mg·L−1 AC (5.3 ± 0.2 roots per shoot). Rooted plantlets were acclimatized in plastic pots inside a plant growth chamber at 25 ± 2 °C and 70% relative humidity, with an 80% survival rate. This optimized protocol can be used for producing transgenic plants of commercial okra cultivars through genetic transformation.

Longan is one of the most important subtropical fruit trees and a famous special product in south China. Increased fruit demand brings longan cultivation to Pb-affected regions. Seedlings of longan (cv. Wulongling) in pots with sands were irrigated daily for 30 d with a freshly prepared nutrient solution containing different concentrations of Pb(NO 3 ) 2 (0, 100 , 200 , 400 , 600 , 800 and 1000 mg L -1 ) to determine physiological and biochemical responses of longan seedlings to various levels of lead (Pb). The results indicated that Pb stress substantially inhibited the growth of longan plants and markedly declined in their dry biomass. However, when the plants were grown at 100 mg L -1 Pb, the growth and dry biomass of the plants showed no significant difference from control. In addition, the chlorophyll a fluorescence and gas exchange parameters were correlated with the growth and yield response. Pb treatments increased the minimum fluorescence (Fo) and caused a decrease in maximum fluorescence (Fm), variable fluorescence (Fv), the maximum quantum efficiency of PSII photochemistry (Fv/Fm), trapped energy flux per cross section (CS) at t=0 (TRo/CSo), electron transport flux per CS at t=0 (ETo/CSo), dissipated energy flux per CS at t=0(DIo/CSo), and the amount of active PSII reaction centers (RCs) per CS at t = 0 (RC/CSo). Furthermore, Pb stress led to decreases in the protein contents, the activity of peroxidase (POD, EC 1.11.1.7) and the accumulation of proline and malondialdehyde (MDA), and enhanced superoxide dismutase activity (SOD, EC 1.15.1.1), whereas catalase (CAT, EC 1.11.1.6) and ascorbate peroxidase (APX,EC 1.11.1.11) were enhanced at low Pb levels and decreased under high Pb stress. Nonetheless, these changes were closely related to the severity of the Pb stress.