充實期高溫影響稻米品質形成的生理途徑

The Physiological Pathway of High Temperature Effects on Rice Quality Formation during Grain-Filling Stage

林芹如(Chin-Ju Lin)

指導教授 ： 盧虎生

國立臺灣大學/生物資源暨農學院/農藝學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02601

摘要

稻米品質的形成牽涉到複雜的生理及生化反應過程，也與環境之間具有密切的關係，其中溫度是影響稻米品質形成主要的環境因子之一。對於台灣稻米栽培環境而言，高溫造成之品質下降尤其是白堊質的產生，一直是提升台灣稻米品質需要解決的問題。過去對於高溫與稻米品質間的研究多是針對高溫對品質相關之澱粉與蛋白質等大分子在合成途徑酵素表現與含量組成上的影響變化，鮮少針對水稻穀粒充實期間高溫影響水稻穎果發育與米質形成過程中可能之生理途徑與分子機制，因此本研究的目的為探討高溫下種子內部由於生理或是能量狀態的變化所可能導致稻米品質下降的生理途徑，並建立一水稻充實期高溫對稻米外觀品質影響的生理途徑；進而探討調節根部溫度對降低或減緩高溫對米質不良影響的可能性，及其與稻米品質形成間的關係。
結果顯示，高溫降低了結實率、成熟穗重以及糙米粒重。外觀方面高溫造成嚴重白堊質的產生，也明顯降低完整米率及提高未熟粒與死米比率，顯示充實期高溫處理確實造成了稻米品質的下降。
高溫可能之生理途徑方面，結果顯示，高溫雖然加速了穎果的充實，但也加速其主要source來源葉片(劍葉)及穎果果(種)皮葉綠素的降解，減少充實期間利用果(種)皮光合作用獲得之氧氣與能量的補充，降低穎果內的能量狀態(ATP濃度的下降)。由基因表現之分析顯示，為了調節高溫下穎果內之能量的狀態，糖解作用與酒精發酵作用被提升，此兩作用的提升也增加了各類反應基質糖類的需求，因而可能導致澱粉的降解。而高溫下穀粒主要選擇酒精發酵作用而非蘋果酸發酵途徑，可避免穎果的過度酸化。此外，也發現高溫抑制了檸檬酸循環的進行，無氧發酵作用與檸檬酸循環一升一降的結果也暗示了高溫穎果內部是處於低氧狀態。另一方面，高溫促進了穎果內活化氧族中H2O2的大量生成，SOD與catalase等與抗氧化相關之酵素基因也在高溫下被提升，顯示穎果內部氧化逆境的加劇，可能導致細胞膜脂質過氧化的發生及加速水稻穎果DNA分解及細胞死亡。上述這些生理反應過程干擾了充實過程中澱粉及儲藏性蛋白質等大分子的合成與累積，最終導致穎果發育與充實不良，因而產生白堊質現象。而根溫可在某些方面可降低或緩和此現象。此外，高溫也調控了與逆境反應相關之基因的表現，如高溫促進了GLOI、RAB24、OsSAMDC等基因的表現，抑制了PDI、cyPPDKB基因的表現，顯示與這些基因相關的分子機制也參與在穎果對高溫的反應中。
綜合本研究的各項結果顯示，穎果對高溫的生理反應並不單純，本研究提供了一個基本的推論架構，未來仍需進一步確認各相關途徑間的互控機制及重要性。

關鍵字

水稻 ； 高溫 ； 白蝁質

並列摘要

Rice plants frequently expose to high temperature during grain-filling stage in Taiwan. It renders chalky grain appearance and results in lower rice quality. Moreover, with the influence of global warming in recent years, high temperature has become a major challenge for rice production in Taiwan. Previous studies usually focus on the high temperature effects of enzyme activities involved in starch or protein biosynthetic pathway and the content/composition alteration of these storage products. Few documents, however, report the possible physiological pathways or molecular mechanisms related to energy/physiological status which lead to poor grain quality formation under high temperature. The present study focused on the physiological pathway of high temperature effects on rice quality during grain-filling stage, and expected to reveal the relation between high temperature and rice quality formation. The current study also hoped to confirm the possibility of ameliorating high temperature negative effects on rice quality via root temperature regulation.
The results showed that under high temperature oxygen depletion inside rice caryopsis may limit ATP synthesis. Moreover, high temperature induced rapid growth of caryopsis but also speeded up its chlorophyll degradation, and so decreased the extra supply for oxygen and energy by photosynthesis during grain-filling state. High temperature also accelerated the aging of flag leaf, the major source organ for grain filling, thus may cause the shortage of photosynthetic assimilate. According to gene expression analysis for energy metabolism related genes, glycolysis and fermentation pathways were elevated by high temperature, and therefore meight accelerate the starch degradation to provide more sugar substrates for both pathways. Alcohol fermentation pathway rather than organic acids fermentation was elevated by high temperature which might avoid further acidification in caryopsis. In addition high temperature enhanced the H2O2 production, one of reactive oxygen species, and enhanced expressions of genes encoding antioxidants, such as SOD and CATB, indicating the existence of oxidative stress and homeostasis in caryopsis under high temperature. Lipid peroxidation, nuclear DNA fragmentation and endosperm cell death were also found to be promoted under high temperature. These phenomenons seemed to be caused by oxidative stress. Above physiological reactions might in turn interfere the synthesis and accumulation of starch or storage protein during grain-filling stage, and finally lead to rice chalky appearance formation. Moreover, lowering root temperature could ameliorate these effects exerted by high temperature with respect of analyzed physiological traits.
High temperature also regulated the expression of some stress-related gene, including up-regulated GLOI, RAB24, OsSAMDC, but down-regulated PDI, cyPPDKB expressions. It reveals that these gene-related molecular mechanisms may also participate in physiological responses to high temperature.
The results in this study revealed that the physiological pathway between high temperature and rice quality formation seems to be complicate. The present study provided a basic frame; however, it still needs more works to elucidate the relationship among different pathways.

並列關鍵字

Oryza sativa ； high temperature ； chalky

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