Presentation Information
[3GteX-10]Is resistant starch also resistant to rice itself? Physiological analysis of early seedling growth in be2b rice mutants
○Ryutaro Morita1 (1. Graduate School of Agricultural and Life Sciences, The University of Tokyo (Japan))
Keywords:
Resistant starch,Rice (Oryza sativa L.),Branching enzyme IIb,Early seedling growth,Starch degradation
[Purpose] Starch is a polysaccharide biosynthesized through a series of photosynthetic reactions in plants. Humanity has used starch not only as a primary energy source but also in various industrial applications, ranging from traditional papermaking to modern biodegradable plastics. Recently, cereals with high levels of resistant starch (RS), which is less digestible and suppresses postprandial blood glucose surges, have been developed for health benefits. However, elevated RS levels in the endosperm of some crops suppress post-germination growth, which is disadvantageous for agricultural use. In rice, a deficiency in branching enzyme IIb (BEIIb), which synthesizes short branches of amylopectin, dramatically increases RS content. In this study, we analyzed the physiological effects of RS accumulation on the post-germination growth of rice.
[Method] We used a be2b mutant (EM10) of the Kinmaze background with normal seed size and a be2b mutant (A10) of the Akita63 background with large seed size. To evaluate the physiological effects of RS accumulation on seedling growth, we analyzed carbon metabolism in the germinating endosperm and embryo of the be2b mutants grown on MS agar medium. Additionally, we grew the be2bmutants in soil and analyzed seedling and root growth at 10-20 days after sowing under various light conditions.
[Results] The starch in the germinating endosperm of be2b mutants was minimally degraded, resulting in lower sugar levels within the germinating endosperm. This sugar starvation signal dramatically up-regulates the expression of gibberellin biosynthesis-related genes such as GA20ox2 and GA3ox2 in the embryo, which subsequently boosts the transcriptional levels of α-amylases in the aleurone layer of the germinating endosperm. Despite the exceptionally strong induction of starch-degradation signals, the RS-rich endosperm starch remains undegraded due to its high resistance to α-amylase. Consequently, post-germination growth was poorer in be2b mutants compared to wild-type rice. However, under high- and medium-light conditions, A10 exhibited significantly more vigorous growth than EM10. Conversely, under low-light conditions, no distinct growth differences were observed between the two lines.
[Consideration] This profound resistance of endosperm accumulated starch to degradation against high metabolic demand induces a vicious cycle in RS-rich grains, leading to the reduction of seedling growth in be2b mutants. We hypothesized that increasing seed size could compensate for the early seedling growth inhibition caused by the be2b mutation by providing a larger pool of non-starch storage materials, such as soluble sugars and proteins. However, the alleviating effect of larger seed size was light-intensity dependent, diminishing as photosynthetic activity decreased. This suggests that mitigating the growth inhibition depends not only on the amount of storage materials but also on the cultivar’s ability to efficiently utilize both storage materials in the seed and photoassimilates during the early heterotrophic-to-autotrophic transition.
[Conclusion] Our findings demonstrate that to seamlessly integrate RS-rich cereals into agricultural systems, it is essential to consider not only the be2b mutation itself, but also the physiological characteristics of the background cultivar, including seed size and seedling vigor after germination, and the cultivation environment.
[Method] We used a be2b mutant (EM10) of the Kinmaze background with normal seed size and a be2b mutant (A10) of the Akita63 background with large seed size. To evaluate the physiological effects of RS accumulation on seedling growth, we analyzed carbon metabolism in the germinating endosperm and embryo of the be2b mutants grown on MS agar medium. Additionally, we grew the be2bmutants in soil and analyzed seedling and root growth at 10-20 days after sowing under various light conditions.
[Results] The starch in the germinating endosperm of be2b mutants was minimally degraded, resulting in lower sugar levels within the germinating endosperm. This sugar starvation signal dramatically up-regulates the expression of gibberellin biosynthesis-related genes such as GA20ox2 and GA3ox2 in the embryo, which subsequently boosts the transcriptional levels of α-amylases in the aleurone layer of the germinating endosperm. Despite the exceptionally strong induction of starch-degradation signals, the RS-rich endosperm starch remains undegraded due to its high resistance to α-amylase. Consequently, post-germination growth was poorer in be2b mutants compared to wild-type rice. However, under high- and medium-light conditions, A10 exhibited significantly more vigorous growth than EM10. Conversely, under low-light conditions, no distinct growth differences were observed between the two lines.
[Consideration] This profound resistance of endosperm accumulated starch to degradation against high metabolic demand induces a vicious cycle in RS-rich grains, leading to the reduction of seedling growth in be2b mutants. We hypothesized that increasing seed size could compensate for the early seedling growth inhibition caused by the be2b mutation by providing a larger pool of non-starch storage materials, such as soluble sugars and proteins. However, the alleviating effect of larger seed size was light-intensity dependent, diminishing as photosynthetic activity decreased. This suggests that mitigating the growth inhibition depends not only on the amount of storage materials but also on the cultivar’s ability to efficiently utilize both storage materials in the seed and photoassimilates during the early heterotrophic-to-autotrophic transition.
[Conclusion] Our findings demonstrate that to seamlessly integrate RS-rich cereals into agricultural systems, it is essential to consider not only the be2b mutation itself, but also the physiological characteristics of the background cultivar, including seed size and seedling vigor after germination, and the cultivation environment.
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