Environmentally Friendly Selectable Marker Genes

The production of transgenic plants involves a suitable transformation vector carrying a selectable marker gene driven by a promoter to select the transformed tissue/ regenerants. Hygromycin is an excellent selectable agent for rice transformation when hph is driven by a constitutive promoter using all three transformation systems.17-19 However, though there has been no report of any toxic effect of HPH on the human system and it is not used in preparing any antibiotic drug for human use, public concerns still remain about having HPH in the final product. Alternatively, a new selection strategy involves the use of mannose, which cannot be metabolized by many plant species. A schematic presentation shows the conversion of mannose to man-nose- 6-phosphate (M-6-P) by a hexokinase,

Fig. 22.2. a) Symptoms of "whiteheads" showing panicles without grain caused by stem borer (left), and rice plants with stem borer resistance (right). b) Control plants showing the symptoms and damage due to sheath blight (left) versus transgenic plants showing enhanced resistance (right; from Datta et al, 1999a). c) Rice plants cv. IR72 with Xa-21 showing resistance to bacterial blight (race 6) versus control plants showing disease symptoms (right, from Tu et al, 1998b). T=transgenic; C=control.

Fig. 22.2. a) Symptoms of "whiteheads" showing panicles without grain caused by stem borer (left), and rice plants with stem borer resistance (right). b) Control plants showing the symptoms and damage due to sheath blight (left) versus transgenic plants showing enhanced resistance (right; from Datta et al, 1999a). c) Rice plants cv. IR72 with Xa-21 showing resistance to bacterial blight (race 6) versus control plants showing disease symptoms (right, from Tu et al, 1998b). T=transgenic; C=control.

Table 22.6. Pathogenesis-relatedprotein families in plants (recognized and proposed)

Family

Representative

Molecular

Biochemical

plants

weight (kDa)

properties

PR-1

Tobacco

14, 15, 17

Antifungal, unknown

PR-2

Tobacco

31, 33, 35

b-1, 3-glucanase

PR-3

Tobacco

27, 28, 32, 34

Chitinase

PR-4

Tobacco

13, 15, 20

Similar to potato win proteins

PR-5 Tobacco 24, 26 Thaumatin-like

Rice 23

PR-6

Tomato

8, 13

Proteinase inhibitor

PR-7

Tomato

69

Endo-proteinase

PR-8

Cucumber

28

Chitinase

PR-9

Tobacco

39, 40

Lignin-forming peroxidase

PR-10

Parsley

17-19

Ribonuclease-like

PR-11

Tobacco

41, 43

Class V chitinase

Others

a. Thionins

Barley

5

Antimicrobial

b. Plant defensins

Radish

5

Antifungal

resulting in severe growth inhibition (Fig. 22.3). The gene product phosphomannose isomerase converts M-6-P to fructose 6-phos-phate, which is readily metabolized, giving the transformed cells a metabolic advantage. Mannose-6-phosphate is toxic to non-transgenic cells/tissue, which supports the mechanism of mannose selection. We are now working on this positive selection, aiming at an environmentally friendly and publicly acceptable marker gene in rice. Similar work has been reported in the selection of transgenic sugar beet.20

Was this article helpful?

0 0
Organic Gardeners Composting

Organic Gardeners Composting

Have you always wanted to grow your own vegetables but didn't know what to do? Here are the best tips on how to become a true and envied organic gardner.

Get My Free Ebook


Post a comment