High Temperature-Induced Expression of Rice α-Amylases in Developing Endosperm Produces Chalky Grains

Abstract

Global warming impairs grain filling in rice and reduces starch accumulation in the endosperm, leading to chalky-appearing grains, which damages their market value. We found previously that high temperature-induced expression of starch-lytic α-amylases during ripening is crucial for grain chalkiness. Because the rice genome carries at least eight functional α-amylase genes, identification of the α-amylase(s) that contribute most strongly to the production of chalky grains could accelerate efficient breeding. To identify α-amylase genes responsible for the production of chalky grains, we characterized the histological expression pattern of eight α-amylase genes and the influences of their overexpression on grain appearance and carbohydrate components through a series of experiments with transgenic rice plants. The promoter activity of most α-amylase genes was elevated to various extents at high temperature. Among them, the expression of Amy1A and Amy3C was induced in the internal, especially basal to dorsal, region of developing endosperm, whereas that of Amy3D was confined near the ventral aleurone. These regions coincided with the site of occurrence of chalkiness, which was in clear contrast to conventionally known expression patterns of the enzyme in the scutellum and aleurone during seed germination. Furthermore, overexpression of α-amylase genes, except for Amy3E, in developing endosperm produced various degrees of chalky grains without heat exposure, whereas that of Amy3E yielded normal translucent grains, as was the case in the vector control, even though Amy3E-overexpressing grains contained enhanced α-amylase activities. The weight of the chalky grains was decreased due to reduced amounts of starch, and microscopic observation of the chalky part of these grains revealed that their endosperm consisted of loosely packed round starch granules that had numerous pits on their surface, confirming the hydrolysis of the starch reserve by α-amylases. Moreover, the chalky grains contained increased amounts of soluble sugars including maltooligosaccharides at the expense of starch. The integrated analyses proposed that expression of Amy1A, Amy3C, and Amy3D at the specific regions of the developing endosperm could generate the chalkiness. This finding provides the fundamental knowledge to narrow down the targets for the development of high temperature-tolerant premium rice.

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

“…4 and 5 ). To date, most investigators have been conducting microscopic observations, mostly focusing on the surface structure of starch granules using scanning electron microscopy in dried chalky kernels (Zakaria et al 2002 ; Yamakawa et al 2007 ; Wada et al 2014 ; Xi et al 2014 ; Kaneko et al 2016 ; Dou et al 2017 ; Nakata et al 2017 ). Because the tissues need to be dehydrated prior to scanning electron microscopy observations under a vacuum, this approach does not provide any information on what exactly occurred in the cytosol in the chalky zone.…”

Effects of OsCDPK1 on the Structure and Physicochemical Properties of Starch in Developing Rice Seeds

“…These effects decrease the market value because the rice will have poor milling qualities (being easily broken), poor cooking and eating qualities, and a poor appearance [41,42]. It has recently been found that a high temperature (33 °C for 12 h light, 28 °C for 12 h dark) also induced the expression of three -amylase genes, Amy1A, Amy3C, and Amy3D, in developing endosperm, causing the grains to be chalky, probably because of starch degradation and the accumulation of soluble sugars in the endosperm [43]. In contrast, we found that the Ri-1 endosperm was more transparent at both a low temperature (22 °C for 16 h light, 20 °C for 8 h dark) and a high temperature (31°C for 16 h light, 28 °C for 8 h dark) than that of the WT during the grain-filling process (Figure 2 and Table 1).…”

Occurrence of rice blast on and grain quality of irrigated rice fertilized with nitrogen and silicates

“…It should be noted that works on the effects of the association between N, Si, and fungicide applications on the industrial quality of irrigated rice grains are still scarce. Some studies have related grain quality to factors such as different humidity levels and fungicide use (Teló et al, 2011), irrigation suppression (Londero et al, 2015), high temperature (Nakata et al, 2017), and shading (Deng et al, 2018).…”