Maize, like other cereals, accumulate starch in the seed endosperm as an energy reserve.

Moreover, its starch is one of the most important plant products and has various direct and indirect applications in food, feed, and industries.

Attempts to increase starch accumulation have received a great deal of attention by plant breeders and plant scientists.

Starch biosynthesis is a central function in plant metabolism that is accomplished by a multiplicity of conserved enzymatic activities.

Roughly three-quarters of the total starch is amylopectin, which consists of branched glucose chains that form insoluble, semi-crystalline granules.The remainder of the starch is amylose, which is composed of linear chains of glucose that adopt a helical configurationwithin the granule

Starch biosynthesis in seeds is dependent upon several environmental, physiological, and genetic factors.

Maize kernel is a suitable system for studying the genetic control of starch biosynthesis.

A large number of mutations that cause defects in various steps in the pathway of starch biosynthesis in the kernel have been reported. Their analysis has contributed greatly to the understanding of starch synthesis.

Many biochemical and molecular studies on starch synthesis have been also focused on identifying the rate limiting enzymes to control metabolism.

ADP-glucose pyrophosphorylase (AGPase) plays a key role in regulating starch biosynthesis in cereal seeds.

The AGPase in the maize endosperm is a heterotetramer of two small subunits encoded by Brittle2 (Bt2) gene, and two large subunits, encoded by the Shrunken2 (Sh2) gene.

Bt2 and Sh2 genes from maize was over expressed with an endosperm-specific promoter from 27-kDa zein into elite inbred lines by Agrobacterium tumefaciens mediated transformation.

Developing transgenic maize kernels exhibited higher Bt2 and Sh2 gene expression, higher AGPase activity, higher seed weight, and the kernels accumulated more starch compared with non-transgenic plants.

In many heterotrophic organs, sucrose synthase (SuSy) is a major determinant highly controls the channeling of incoming sucrose into starch and cell wall polysaccharides.

SuSy is a highly regulated enzyme that catalyzes the reversible conversion of sucrose and a nucleoside diphosphate into the corresponding nucleoside diphosphate glucose and fructose.

Enhancing SuSy activity resulted in increased levels of starch in transgenic potato tubers.

Endosperms of transgenic maize plants ectopically expressing StSUS4 from potato exhibited a significant increase in SuSy activity, and in starch.

Starch metabolism in maize seed endosperm cells

To increase the root starch production, modified form of the bacterial gene (glgC) encoding AGPase under the control of the root-specific (in cassava) patatin promoter was expressed. The transgenic plants had a 260% increase in total root starch biomass.

This increase in starch production was associated with an increase both in root number and root size but not an increase in root starch density. Increases in root biomass in transgenic plants were shown to be correlated with the relative increase in root AGPase activity.

Expression glgC16 of the bacterial AGPase in potato tubers resulted in a 36% increase in AGPase activity and a corresponding 35% increase in tuber starch content.

Modification of wheat starch is presently targeted in various laboratories to improve its potential utility.

The ratio of amylose to amylopectin in starch determines its physico-chemical characteristics.

Wheat flour, low in amylose content is desirable for noodlemaking as it improves noodle texture.

In wheat, scientists are thus working towards increasing the amylopectin content of starch thereby reducing amylose content leading to the formation of a value added, low-amylose flour.

The starch branching enzymes catalyse formation of 1, 6-linkages in the glucan polymer and control the amount of amylopectin produced.The expression of genes coding for enzymes involved in starch biosynthesis ( amylose and amylopectins can be modulated to enhance the production of amylopectins .

In potato to enhance the starch level and protein levels, the genes involved are expressed under Class-I patatin promoter from potato, which is regulated promoter specific to tubers, encoding the major storage protein in tubers.

Patatin is a family of glycoproteins that have lipid acylhydrolase and transferase activities.

Patatins are abundant glycoprotein in the tubers of potato plants

Transgenic plants with enhanced Vitamins

Vitamin A deficiency is prevalent in the developing world and is probably responsible for the deaths of two million children annually.

Humans can synthesize vitamin A if provided with the precursor molecule b-carotene (also known as provitamin A), a pigment found in many plants but not in cereal grains.

Therefore, a strategy was devised to introduce the correct metabolic steps into rice endosperm to facilitate b-carotene synthesis.

An initial breakthrough was the development of a rice line expressing a daffodil (Narcissus pseudonarcissus) phytoene synthase, enabling the accumulation of the vitamin A precursor phytoene in the endosperm.

Followed shortly thereafter by the original ‘Golden Rice’ variety, expressing two daffodil enzymes and one from Erwinia uredovora, which reconstituted the entire pathway and enabled the rice endosperm to accumulate b-carotene, resulting in itseponymous golden color.

In the best lines, the grain contained >1.5 mg of b-carotene per gram of dry weight.

Recently, the ‘Golden Rice II’ variety in which the daffodil phytoene synthase gene is replaced with its more efficient maize homolog, resulting in a 23-fold improvement in b-carotene content (up to 37 mg g1).This has led to similar progress in other crops, including, most recently, ‘yellow potato’ , ‘orange cauliflower’ , carrots withenhanced b-carotene in the taproot and tomatoes withthe b-carotene metabolic pathway transferred to the plastids

A recently developed potato variety containing the phytoene synthase, phytoene desaturase and lycopene b-cyclase from Erwinia herbicola contained 114 mg carotenoids per gram of dry weight and 47 mg b-carotene per gram of dry weight.

Vitamin E is a group of eight hydrophobic compounds (known as vitamers), the most potent of which is a-tocopherol.

Dietary vitamin E is obtained mainly from seeds, and its functionin the body is to prevent the oxidation and polymerizationof unsaturated fatty acids.

Vitamin E deficiency leads to general wasting, kidney degeneration and infertility.

In plants, tocopherol synthesis requires input from two metabolicPathways.

Work in Arabidopsis has shown that the levels of vitamin E activity can be increased either by increasing the total amount of vitamin E or by shifting the metabolic flux towards a-tocopherol.

Arabidopsis genes encoding g-tocopherol methyltransferase (ʏ-TMT) in Arabidopsis seeds, resulting in a fundamental shift fromʏ/δ- to α/β-tocopherol; this showed that nutritionalenhancement in plants was possible without altering totalvitamin E levels.

The expression of Arabidopsis homogentisic acid prenyltransferase (HPT) produced twice the level of vitamin E found in normal seeds.

whereas expression of the Escherichia coli tyrA gene, whichencodes a dual-function enzyme (chorismate mutase andprephenate dehydrogenase) resulted in up to three timesthe normal level of vitamin E