Evidence for preservation of vacuolar compartments during foehn-induced chalky ring formation of Oryza sativa L.

Abstract

Main conclusionVacuolar compartments being sustained among the amyloplasts inadequately accumulated in rice endosperm cells are the main cause of chalky ring formation under dry wind conditions.Foehn-induced dry wind during the grain-filling stage induces shoot water deficit in rice (Oryza sativa L.) plants, which form a ring-shaped chalkiness in their endosperm that degrades milling quality and rice appearance. Air spaces formed in several inner cells cause significant transparency loss due to irregular light reflection. Although starch synthesis was suggested to be retarded by osmotic adjustment at foehn-induced moderately low water potential, the source of these air spaces remains unknown. We hypothesised that the preservation of vacuoles accompanied by a temporary reduction in starch biosynthesis in the inner cells leads to the chalky ring formation. Panicle water status measurement, light and transmission electron microscopic (TEM) observations, and an absolute qPCR analysis were conducted. Most starch synthesis-related genes exhibited temporarily reduced expression in the inner zone in accordance with the decrease in panicle water status. TEM observations provided evidence that vacuolar compartments remained among the loosely packed starch granules in the inner endosperm cells, where a chalky ring appeared after kernel dehydration. Taken together, we propose that vacuolar compartments sustained among the amyloplasts inadequately accumulated in rice endosperm cells and caused air space formation that leads to ring-shaped chalkiness under dry wind conditions.Electronic supplementary materialThe online version of this article (10.1007/s00425-018-2975-x) contains supplementary material, which is available to authorized users.

Multiple strategies for heat adaptation to prevent chalkiness in the rice endosperm

“…6). It has recently been demonstrated that the formation of vacuole-like structures among inadequately accumulated amyloplasts in the endosperm cells of rice are the main cause of chalky ring formation in plants exposed to the hot, dry winds (Hatakeyama et al , 2018). The loss of transparency similarly observed in the dorsal side of kernel in this study (Fig.…”

“…As reported previously (Wada et al , 2011, 2014), osmotic adjustment also occurs in the endosperm cells of rice growing under dry wind conditions prior to chalky ring formation. Hatakeyama et al (2018) have proposed that the appearance of vacuole-like structures occurs as a consequence of osmotic adjustment in the inner endosperm under such wind-induced low water potential. In our current work, on-site cell metabolomics and cell turgor assays in the outer endosperm at the early ripening stage indicated notable treatment differences, including in redox metabolites, such as Cys, ascorbic acid, and glutathione, as well as differences in cell turgor as heat adaption responses (Fig.…”

“…Kernels for microscopic observation were sampled in the growth chambers, fixed, and embedded according to the methodology described previously by Saito et al (2010, 2012) with slight modifications (Hatakeyama et al , 2018). Transverse segments (1−2 mm thick) from the middle of the kernels at 12, 20, and 40 DAH were fixed with 4% (w/v) paraformaldehyde in 100 mM sodium phosphate (pH 7.2) for 3 h at room temperature and then washed in 100 mM phosphate buffer (pH 7.2).…”

“…Sections were cut with an ultramicrotome (Sorvall MT-5000, DuPont, Newtown, CT, USA) using a diamond knife. For the image analysis of the arrangement of organelles, the outline of all amyloplasts, PBs, and other areas (referred to as ‘gaps’) in the cells, as well as cells from light-microscope images taken from three plants, were traced using the ImageJ software (), as described previously (Hatakeyama et al , 2018). The areas of PSVs and both types of PBs on TEM images were similarly traced.…”

“…Recently, however, Masumura’s group has established a new fixation technique suitable for TEM observation in hard kernels (Saito et al , 2010, 2012). More recently, Hatakeyama et al (2018) have used this fixation technique to identify the source of the air spaces that are formed along the chalky ring in dry wind-treated kernels. In this current study, we used our newly developed on-site cell-specific analysis combined with time-course TEM observations to test our hypothesis regarding the heat-induced formation of rice chalkiness, in order to identify the exact source of the air spaces formed in the cells.…”