Rice grain yield is predicted to decrease in the future because of an increase in tropospheric ozone concentration. However, the underlying mechanisms are unclear. Here, we investigated the responses to ozone of two rice (Oryza Sativa L.) cultivars, Sasanishiki and Habataki. Sasanishiki showed ozone-induced leaf injury, but no grain yield loss. By contrast, Habataki showed grain yield loss with minimal leaf injury. A QTL associated with grain yield loss caused by ozone was identified in Sasanishiki/Habataki chromosome segment substitution lines and included the ABERRANT PANICLE ORGANIZATION 1 (APO1) gene. The Habataki allele of the APO1 locus in a near-isogenic line also resulted in grain yield loss upon ozone exposure, suggesting APO1 involvement in ozone-induced yield loss. Only a few differences in the APO1 amino acid sequences were detected between the cultivars, but the APO1 transcript level was oppositely regulated by ozone exposure: i.e., it increased in Sasanishiki and decreased in Habataki. Interestingly, the levels of some phytohormones (jasmonic acid, jasmonoyl-L-isoleucine, and abscisic acid) known to be involved in attenuation of ozone-induced leaf injury tended to decrease in Sasanishiki but to increase in Habataki upon ozone exposure. These data indicate that ozone-induced grain yield loss in Habataki is caused by a reduction in the APO1 transcript level through an increase in the levels of phytohormones that reduce leaf damage.
BackgroundIt is becoming clear that ozone affects not only grain yield but also grain quality in rice. However, the biochemical mechanisms responsible for ozone-induced changes in appearance quality or components are poorly understood. We analyzed appearance quality and starch composition in the rice cultivars “Koshihikari” (japonica) and “Kasalath” (indica) grown under elevated ozone conditions.ResultsElevated ozone significantly increased the proportion of immature (mainly chalky) kernels in “Koshihikari” but not in “Kasalath”. Scanning electron microscopy of transverse sections of kernels showed that endosperm starch granules of “Koshihikari” ripened under elevated ozone were loosely packed with large spaces and contained irregular rounded granules. Amylose content was increased in “Koshihikari” kernels with ozone exposure, but was unchanged in “Kasalath” kernels. Distribution analysis of amylopectin chain length showed that ozone induces a decrease of long-side chains and alterations of short side-chains in “Koshihikari” kernels. Furthermore, Starch Synthase (SS) IIIa transcript levels in “Koshihikari” caryopses were decreased by elevated ozone.ConclusionsThe japonica cultivar “Koshihikari” showed significant deterioration in appearance quality of kernels caused by abnormal starch accumulation due to exposure to ozone. The alteration patterns of amylose and amylopectin in ozone-exposed rice kernels are similar to those in rice kernels harvested from SSIIIa-deficient mutants. These findings suggest that the increase of chalky kernels in ozone-treated “Koshihikari” is partly attributable to the repressed expression of SSIIIa involved in amylopectin side-chain elongation with ozone exposure. Elevated ozone reduced appearance quality in “Koshihikari” although it did not impair starch properties contributing to the eating quality of cooked rice.Electronic supplementary materialThe online version of this article (doi:10.1186/s12284-016-0079-4) contains supplementary material, which is available to authorized users.
After fruit development has been triggered by pollination, the abscission zone (AZ) in the pedicel strengthens its adhesion to retain the fruit. In tobacco, unpollinated flowers are shed from the AZ, while enlargement of the same tissue occurs in pollinated flowers. The abscission rate depends on effective pollination. In this study, we focused on pectin and boron (B) distribution and their functions in the AZ. B is an essential micronutrient that forms cross-links in pectin, which are important for organ differentiation and maturation. B content and the formation rate of the borate-ester cross-linked rhamnogalacturonan II dimer (dRG-II-B) were measured in tobacco. Expression of NtGUT1, which is important for the formation of dRG-II-B, and the B transporter NtNIP3;1 were analyzed in the peduncle and AZ. To determine B content and dRG-II-B formation rate, the following samples were collected: bud peduncle, bud AZ, fruit peduncle, fruit AZ, and flower AZs, which were treated to prevent pollination or were artificially pollinated. Samples treated in HNO 3 were assayed for total B content with ICP-MS. Endo-polygalacturonase (EPG)-treated alcohol-insoluble residues were assayed for formation rates of dRG-II-B with HPLC. Although there was more total B in fruit AZs than in branches, no differences in total B, watersoluble B, and B relevant to cross-linking with RG-II were found between buds and fruits. The formation rates of dRG-II-B were also the same in peduncles and AZs in both buds and fruits. These have the same characteristics of variation with or without pollination. Expression of both NtGUT1 and NtNIP3;1 was highest in AZs of flowers before opening, however, expression of NtGUT1 in flower AZs without pollination was not significantly different from after artificial pollination. These results suggest that the presence of relatively large amounts of B and pectin is important in structural enhancement of AZs after anthesis, but is not related to abscission with or without pollination.
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