The ER luminal binding protein, BiP, has been linked to prolamine protein body formation in rice. To obtain further information on the possible role of this chaperone in protein body formation we have cloned and sequenced a BiP cDNA homolog from rice endosperm. The rice sequence is very similar to the maize BiP exhibiting 92% nucleotide identity and 96% deduced amino acid sequence identity in the coding region. Substantial amino acid sequence homology exists between rice BiP and BiP homologs from several other plant and animal species including long stretches of conservation through the amino-terminal ATPase domain. Considerable variation, however, is observed within the putative carboxy-terminal peptide-binding domain between the plant and nonplant BiP sequences. A single hand of approximately 2.4 kb was visible when RNA gel blots of total RNA purified from seed tissue were probed with radiolabeled rice BiP cDNA. This band increased in intensity during seed development up to 10 days after flowering, and then decreased gradually until seed maturity. Protein gel blots indicated that BiP polypeptide accumulation parallels that of the prolamine polypeptides throughout seed development. Immunocytochemical analysis demonstrated that BiP is localized in a non-stochastic fashion in the endoplasmic reticulum membrane complex of developing endosperm cells. It is abundant on the periphery of the protein inclusion body but not in the central portion of the protein body or in the cisternal ER membranes connecting the protein bodies. These data support a model which proposes that BiP associates with the newly synthesized prolamine polypeptide to facilitate its folding and assembly into a protein inclusion body, and is then recycled.

The small-grain cereals, wheat and rice, are two of the major crops grown in the world and are used mainly as food. As the world population is projected to increase by 40% by the year 2020, these cereals can be expected to assume a much larger role in providing the basic daily dietary requirements required for human growth and development. This is especially true for rice where this cereal provides many of the dietary calories for about 50% of the world’s population, most of whom live in Asia. In view of constraints caused by the amount of available arable land and the limitations in chemical inputs in the environment imposed by the increasing use of sustainable agricultural practices, new approaches to increase the genetic yield potential of crop plants must be developed and implemented. Although dramatic improvements in the genetic yield potentials of wheat and rice were achieved during the so-called green revolution, only relatively small annual increases (1-2%) in the genetic yield potential have been attained in recent years. This trend is even true for maize (Duvick, 1992), despite the employment of the most modern biotechnological tools and resources available to the maize plant breeder. If we are to meet the challenge of feeding 8 billion people in the year 2020, it is clear that a major increase in genetic yield potential of cereal crops must be achieved. In very general terms, the genetic yield potential is dependent on source-sink relationships (Ho, 1988; Turgeon, 1989). Source leaves capture light energy and fix carbon dioxide to produce sugars and other metabolites. These organic compounds are exported from the source leaves and transported to developing sink tissues, for example, young developing leaves and new root tissue, which utilize these basic precursors for growth and development. Because of the importance of the primary processes of photosynthesis in controlling plant productivity, considerable research effort has been directed to increasing the efficiency of the source leaves. Plant productivity is also influenced by the capacity of sink tissues to uptake and assimilate photosynthate produced by source leaves or reconverted from storage reserves (Ho, 1988).