The importance of starch in plant growth and human and animal nutrition has led to a long and rich history of research into the biosynthesis and catabolism of the storage material. It does seem that there was a period in the late 1960's and early 1970's when there was a waning interest among plant scientists in studying starch metabolism. That pattern has been dramatically reversed now with the advent of new tools for studying plant molecular biology, cell biology and biochemistry. The combination of these new technologies, along with the ability to produce transgenic plants, has generated an explosion of information on starch metabolism in recent years.
It is easy to see from the above descriptions that the synthesis of starch during grain filling of wheat is a very dynamic and complicated process which can be influenced by a number of controls. Perhaps most intriguing of these is the relationship of final seed size, and hence starch content, to the pre-pollination dimension of the reproductive structures which ultimately produce the caryopsis. As part of this relationship, endosperm cells seem to follow a course of formation that is tied to floral structure magnitude in a way that subsequently leads to a predetermined number of sites (i.e., cells) where starch can be stored. Although this strong allometric connection exits, there is also a degree of pliability in the relation. Expectedly, post-anthesis stress, such as restricted assimilate availability or high temperature, can reduce grain filling relative to the potential which prevailed at fertilization. More surprisingly, though, is that final seed mass can be modulated upwards, to a degree, by situations which provide a greater-than-normal source/sink balance. Understanding these changes as the unfolding of intracellular development and operation of compartmentalized carbon metabolism is very intriguing and only just beginning to be fathomed.
The formation of amyloplasts and starch granule development therein is at the heart of grain filling in wheat, as well as cereals in general. Our understanding of amyloplast development in wheat takes on new meaning with the confirmation that B-type starch granules take form in organelles originating from amyloplasts containing A-type granules. It now appears that B-type amyloplasts do not exist and there is a close physical relationship between the formation of A- and B-granules. This raises the question of how metabolism of the amyloplast is capable of producing at least two different types of granules that possess different properties, all contained within organelles having so much relatedness. Answers to this question are likely to come from obtaining a better understanding of the biosynthesis of the linear and branched polymers that are formed and aggregate into starch granules.
There is still much confusion about which catalysts and/or transporters are involved in controlling the amount of starch produced during grain filling. Clear examples from other crops demonstrate the importance of AGP, SS, and BE enzymes in regulating starch deposition. The picture is not as clear, however, with regard to the other enzymes of starch biosynthesis and enzymes related via sugar metabolism. Studies of many sorts can be found which demonstrate one over another enzyme as being a significant control point in regulating starch production. Determination of the actual importance of these enzymes will probably have to await the production of genetic and/or transgenic mutants which selectively knock out the protein activity. When these tools are available, specific steps of metabolism can be assessed for their sole contribution to the control of starch deposition.
As described above, the expanding pace of isolating and characterizing wheat genes related to starch and sugar metabolism offers hope that answers to the importance of particular steps in metabolism will be available in the not too distant future. Then, sure to ensue would be development of superior cultivars of wheat that possess tailored characteristics of seed growth and, in all likelihood, grain starches with new and useful functionalities.
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