Reactive oxygen species, as represented by their most stable form H2O2, play important roles as sig naling molecules in regulating plant growth and devel opment including cell proliferation, cell stress response, and signal transduction. H2O2 is known to be involved in biotic and abiotic stress responses. The observed drastic thereby increase in H2O2 levels in CSSL50 1 and the differential expression of several key regulatory genes involved on ROS production and scavenge collec tively suggest that ROS may play a critical role in regu lating rice endosperm chalkiness. Changes in H2O2 levels may affect multiple metabolism pathways in the rice endosperm, causing chalkiness phenotypic change. Further genetic and biochemical studies should further test such a possibility.

Conclusion Consistent with previous studies on the effect of adverse environmental conditions in causing chalky rice grain, our comparative transcriptome analysis of the caryopses of a near isogenic line CSSL50 1 and its low chalkiness parental line Asominori supports the notion that rice grain endosperm development is controlled by delicate, but complex genetic networks. Notably, several pathways related to signal transduction, cell rescue defense, transcription, protein degradation, carbohydrate metabolism and redox homeostasis were found to be predominant among the differentially expressed genes, suggesting that formation of rice endo sperm chalkiness may involve coordinated regulation of multiple pathways. Further refining of CSSL50 1 as a useful genetic material will help eventual cloning and engineering the major genes underlying the formation of rice grain chalkiness.

Methods Plant material and growth A japonica cultivar, Asominori, and its chromosome segment substitution line were used in this study. CSSL50 1 is a near isogenic line of Asominori with a substituted segment from the donor IR24. Seventy one F7 RILs were derived from a cross between Asominori and IR24 by single seed descent. To produce a series of CSSLs in a largely Asominori background, 19 selected RILs were crossed and then backcrossed with Asominori, without selection, until the BC3F1 generation. Sixty six individuals were then selected at BC3F1 on the basis of a whole genome survey and were denoted as CSSL1 CSSL66. CSSL50 was observed to have high grain chalkiness characteristics. To further reduce the intro gressed segment, CSSL50 was backcrossed with Asomi nori followed by two generations of self pollination, and the progeny were evaluated using marker assisted selec tion strategy. The homozygous line CSSL50 1 was found to have high chalky grains and contains a small segment of IR24 chromosome 8 in a largely Asominori genetic background. Four batches of seeds were sowed for both Asominori Cilengitide and CSSL50 1.