Trichomes control

Figure 3.3 Current models for the transcriptional control of anthocyanin biosynthetic genes in maize (A), petunia (B), and Arabidopsis (C). The Petunia model is based on Spelt et. al.,30 and the regulation of trichome formation was adapted from Payne et. al.3>...

The prenyltransferases that catalyse the s)mtheses of GPP, FPP and GGPP may be important regulatory enz)mies in plant terpenoid bios)mthesis since they are situated at the primary branch points of the pathway, directing flux among the various major classes of terpenoids. The level of prenyltransferase activity is, in fact, closely correlated with the rate of terpenoid formation in many experimental systems (Dudley et at, 1986 Hanley et at, 1992 Hugueney et at, 1996) consistent with the regulatory importance of these catalysts. The localization of specific prenyltransferases in particular types of tissue or subcellular compartments may control the flux and direction of terpenoid synthesis at these sites. For example, the GPP synthase in Salvia officinalis is restricted to the secretory cells of the glandular trichomes, which are the sole site of monoterpene bios)mthesis in this species (Croteau and Purkett, 1989). 

After several months in isolation without the Chaetoceros host, our cultured isolate, Calothrix SCOl, started to change its ceU character. For example, the trichome length extended, intercalary heterocysts were observed, and several trichomes appeared to branch. We interpreted such changes in the symbiont trichome and ceU integrity as due to loss of control by the diatom host over the symbiont since the symbiont is in a free-Uving state. These latter observations also suggest that the free-living trichome potentiaUy looks different than that which is observed when it lives symbioticaUy, and thus could be easily misidentified/overlooked in the field. 

Others (Foster et al., 2007 Kimor et al., 1978 ViUareal, 1989) have reported that vegetative ceUs degrade first, and often the heterocysts are the last part of the Richelia trichome to remain in a host diatom, which could also suggest host control or some sort of ceU signaUng between host and symbiont. Gomez et al. (2005) observed Calothrix symbionts associated with Chaetoceros diatoms which lacked chloroplasts suggesting that the host diatoms were senescent quite possibly though the symbiotic... 

Potatoes bearing glandular trichomes also defend against the Colorado potato beetle, a devastating pest capable of total crop destruction (4). Although resistance will not eliminate the need for other control tactics in management of this pest, our field experience indicates that present levels of resistance in hybrids can substitute for as much as 40% of the current insecticide usage on susceptible cultivars (14). 

Mechanisms of Plant Resistance to Insects, the second section, focuses on the role of trichomes in resistance to potatoes and tomatoes, enzymatic antinutritive defenses of tomatoes, and the roles of phytoalexins in insect control. 

The inheritance of glandular trichomes in S, berthaultii and in crosses with 5. tuberosum has been studied by several groups (11. 35-371. Type B trichome density, droplet size and presumably the presence of sucrose esters appear to be controlled by relatively few recessive genes. Heritability estimates ranged from 20-30% for density of Type A and B trichomes and 60% for Type B droplet size. The presence of polyphenoloxidase in type A trichomes is controlled by a single dominant gene (381. The inheritance of sesquiterpenes in type A trichomes has not been studied. 

Because 2-tridecanone-mediated resistance to sexta and L. decemlineata is genetically distinct from the lamellar-based resistance to zea, it would be possible to develop tomato cultivate resistant to sexta and decemlineata but susceptible to zea. On such cultivate, the 2-tridecanone-mediated induction of elevated tolerance to some insecticides might make control of zea more difficult (55, 58). The severity of H. zea as a pest problem on such cultivate would be further exacerbated if 2-tridecanone associated with the resistant foliage seriously interfered with insect parasitoids and predators important in suppressing natural populations of zea. Because of its occurrence in the foliar glandular trichomes, and because of its broad spectrum toxicity to insects, we considered it likely that parasitoids and predaceous insects searching the resistant foliage for prey would be exposed to potentially toxic levels of 2-tridecanone. 

Terpenoids can be found in various plant organs. Concentrations of terpenoids in plant tissues are controlled by the availability of substrate and the activity and type of biosynthesis enzymes. Emission rates of volatile terpenoids from plant leaves are controlled by their synthesis rates and compound-specific physicochemical characteristics, mainly their solubility, volatility and diffusivity. These are affected by physicochemical constraints caused by temperature, stomatal conductance and leaf structure. Storage of terpenoids could be isolated from other plant tissues with specialized structures such as secretory cavities, resin canals (Fig. 94.1), latex canals (lactifers), and glandular trichomes. 

Xie Z, Kapteyn J, Gang DR (2008) A systems biology investigation of the MEP/terpenoid and shikimate/phenylpropanoid pathways points to multiple levels of metabolic control in sweet basil glandular trichomes. Plant J 54 349-361. 

---