Improvement in the grain filling of barley, together with the elimination of seed shattering, were perhaps the first breeding activities of mankind, presumably made by the preferential collection of plump grains remaining on the spike at maturity. The difference in protein and starch quantity between Hordeum spontaneum, the wild ancestor of cultivated barley, and all extant landraces and cultivars is great, and is due to human intervention. These changes have been critically dependent on the components of the grain, the majority of which is starch and protein. A
fuller understanding of grain filling in barley and biosynthesis of storage reserves will not only benefit current applications, but make many more possible.
Two directions may be taken in the future: improvement of barley yield; improvement of barley starch or protein properties. Yield improvement per se, particularly tinder the agro-economic conditions of over-production such as obtained in Europe, is not being strongly emphasized at present. Nevertheless, yield stability, maintenance of yield under unfavorable growing conditions, remains a breeding target. Yield stability itself may have many components. One aspect is the accumulation of sufficient photosynthate and nitrogen early in the growing season to sustain grain filling should conditions later deteriorate. Disease resistance, particularly as it affects the photosynthetic performance of the flag leaf, is extremely important. Drought resistance, important both for northern European conditions early in the growing season, as well as for other regions under dryland agricultural regimes, is valuable for maintaining yield. At least in wheat, water deficit has a direct effect on starch biosynthesis, reducing both the number of B-granules and the size of A-granules (116).
Cold and heat tolerance play a role in yield. Regarding barley starch biosynthesis and starch yield itself, there is considerable evidence that the soluble starch synthases in particular are heat sensitive (117-119). The temperature of the spike during grain filling is particularly critical. The long awns of most barley varieties are effective heat sinks, although they require sufficient transpirational water flow for this function (117). Hence, protein engineering of the starch synthases to increase their heat tolerance would be one goal, but would require transgenic barley in order to be implemented. Barley transformation is not at present compatible with stability of yield under "biodynamic" or "organic" agricultural regimes, a major growth sector in the market driven by consumer preferences particularly in Western Europe and North America.
Improvement of barley starch and protein quality, in particular, has much potential. Starch functional qualities are derived from the degree of starch crystallinity, granule size, lipid content, and amylose content. These in principal are all under genetic control. The degree and pattern of branching in amylopectin is critical to starch properties. Modification of starch through alteration of SBE, GBSS, and AGP has been undertaken in potato (79, 120-122). The presence of multiple SBE forms which, though having different substrate affinities and chain-transfer preferences, may partially substitute for one another in knock-outs or knock-downs, considerably complicates the venture (79). Improvements through modulation of enzyme levels or activities will here, too, require transgenic modification of barley. Although barley can be transformed (123-125), it is a labor-intensive process compared with the transformation of rice or starch-producing dicotyledonous plants such as potato. Nevertheless, the irreplaceable application niches that barley has, such as malt production, and its major importance as a grain crop in many parts of the world, argue in favor of making this effort in the future.
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