高溫下水稻穀粒粒型對米質形成之生理影響

Abstract

前人研究指出種子在發育時間其內部是處於低氧之環境，越靠近種子中心氧分子濃度逐漸下降，能量生成越少。而種子內部氧氣濃度亦會隨溫度之提升而下降。水稻在高溫下容易使產量及品質下降，尤其在開花後20天以內的影響最甚。一般認為，在稉稻與秈稻兩個亞種中，秈稻的耐熱性相對較佳;因此，本研究期望探討不同之水稻粒型在高溫下對米質形成之生理過程之影響差異，期望作為耐熱育種改良之參考。本試驗以農業試驗所提供之臺農67號 (TNG67，短粒型)及經由疊氮化鈉誘變之品系SA1739 (長粒型)為材料，於開花後進行15天之日夜溫25/20 oC及35/30 oC處理，分別取開花後6、9、15、20、25 DAF (Days after flowering)及成熟之種子進行各項品質及生理試驗分析。 經米粒外觀分析結果顯示SA1739較TNG67心腹白比例較少，且其成熟穀粒重量受高溫處理影響少，屬較耐熱之品系。 在常溫環境下，SA1739的米質外觀無TNG67常見之腹白現象，且在能量代謝之基因表現上與穎果內部ATP濃度果可知。常溫下SA1739在6~15DAF尚可由有氧呼吸途徑提供ATP，約自15DAF才開始走向無氧呼吸。然而TNG67穎果在常溫下其有氧呼吸相關途徑隨發育減少其表現，取而代之逐漸提升無氧呼吸之表現。由此顯示SA1739粒型改變後，已對穎果內能量合成造成影響。 在高溫環境下，TNG67之穎果澱粉合成基因(SSIIa、GBSS)抑制較多，為高溫下穀粒重量下降之原因。SA1739之穎果在高溫下其Glycolysis、TCA cycle相關基因(IDH、SUSB、MDH)及ATPSB皆受抑制，但PPDKA大量表現，推測SA1739穎果於高溫下其PPDKA可扮演PK(pyruvate kinase)之角色，將PEP轉pyruvate，提供無氧呼吸之原料，以生產ATP供應其內部之代謝反應。反觀TNG67高溫下參與有氧呼吸途徑之基因亦受到抑制但ADH提升之幅度較少，可能無法供應生理代謝所需之足夠能量。能量方面，高溫下SA1739在早期其ATP仍可維持常溫之狀態，且常溫與高溫下ATP最大濃度差異小，可能有利於減少白堊質粒產生。另SA1739榖粒充實早期有較高之SOD表現，推測可將superoxide轉換為毒性較弱之H2O2，降低脂肪過氧化程度(MDA)，此外其多元胺合成基因(OsSAMDCI、SPDSYI)表現提升，可能有較多含量多元胺而使得其在高溫下細胞死亡指標(PBZ)基因表現相較TNG67低。反觀TNG67高溫下ACO1早期表現提升幅度大，可能有較多乙烯之產生，另多元胺合成基因提升幅度相對較少，加上有多元胺降解基因(PAO)大量表現之現象，而導致其細胞死亡指標(PBZ)相較嚴重。 依本試驗TNG67與SA1739初步之結果推斷，外觀具細長粒型之SA1739具有較好之能量利用效率，可能減緩高溫對穀粒充實之傷害，但是否代表其穎果內部具有較高之含氧含量還需待實測該穎果內部氧含量才能驗證。本試驗之水稻長粒型與短粒型品系對高溫之品質與生理反應之差異性的研究成果，將可提供因應氣候暖化之耐熱性育種的理論基礎。

It is known that the seed usually develops under hypoxia state with the oxygen concentration decreased from outer to inner layers of the seed. Reports also found that increasing temperature would further decrease oxygen concentration in the seed. Many researches reveal that high temperature stress may lead to a poor yield and grain quality, especially before 20 days after flowering. Furthermore, indica type rice, which usually has long grain shape, usually had better heat tolerance than does the japonica type. In the present study, we want to understand the effects of grain shape on grain quality formation under high temperature stress. After flowering, TNG67 and SA1739 (a mutant line fromTNG67, with slender/long grain shape) were treated with 25/20oC (day/night temperature) and 35/30oC for 15 days . The current results revealed that grains of mutant line had a lower chalkiness than that of TNG67 under high temperature. From the results of energy metabolism related gene expression and ATP concentration under normal temperature (25/20oC, day/night), we found that SA1739 could go through oxidative phosphorylation during 6~15 DAF to supply ATP. However, TNG67 only could produce ATP from fermentation even under 25/20oC. Mutation of the grain shape of SA1739 from TNG67 might changed their energy metabolism during grain filling. In gene expression, starch synthesis genes (SSIIa, GBSS) decreased seriously in TNG67 under high temperature. The grains of SA1739 had higher ATP concentration at early developmental stage. SA1739 showed up-regulation in expressions of PPDKA, ADH and down regulation of TCA cycle genes under high temperature, suggesting a role of PPDKA to change PEP to pyruvate, so that alcohol fermentation in the long grain mutant line could supply enough ATP under high temperature. In addition, SA1739 had higher expression levels on SOD expression to decrease lipid peroxidation and high expression level of OsSAMCDI, SPDSYI , but less expression on PAO, that might implied a higher content of polyamines under high temperature. SA1739 also showed lower expression of cell death related gene PBZ under high temperature. On the other hand, the short grain cultivar TNG67 had higher expression rate of ACO1 at early stage and less expression on OsSAMCDI and SPDSYI under high temperature, suggesting that it might release higher amount of ethylene and lower amount of polyamines under high temperature, and in turn enhanced a higher expression of cell death related gene PBZ. From the present results, it is suggested that SA1739 may have higher energy metabolizing efficiecly under high temperature, so that it may have better dry matter accumulation and less chalk grains. However, the relationship between the internal oxygen status in caryopsis with grain shape needs to be clarified in the future .