水稻幼苗耐鹽相關數量性狀之基因座定位與分析

Abstract

鹽害是目前世界上主要的非生物逆境之一，鹽逆境會抑制作物生長並影響產量。水稻，作為餵養世界上大多數人口的重要糧食作物，是禾本科作物中最感鹽的，因此提升水稻的耐鹽性顯得非常重要。鹽害逆境可以分為兩個時期：第一為滲透壓的逆境 (osmotic stress)，由於土壤中的鹽離子濃度升高，使得土壤的滲透勢 (osmotic potential) 低於植株體內的滲透勢造成植株無法順利汲取水分，並延遲植株的生長。第二為離子逆境 (ionic stress)，是指鹽害後期當鈉離子累積到一定濃度後，造成老葉枯萎黃化。本研究中，我們使用兩組水稻自然種原，觀察其株高、葉面積、氣孔密度與鹽害指數在正常環境及鹽處理後的變化，並針對這些性狀進行關聯性座定位 (association mapping)。另外我們使用Nipponbare與IR64的重組自交系針對氣孔密度進行數量基因座定位，基因型資料經滑動視窗法處理定序錯誤所造成的緊密二次互換 (tight double crossing over)，定位到染色體2號與6號有兩個明顯的尖峰，帶有Nipponbare對偶基因的品系通常具有較低的氣孔密度。我們預期氣孔密度較低的種原，其蒸散速率較慢，藉此減少鈉離子由蒸散流進入地上部，達到耐鹽的效果。結果顯示，氣孔密度與鹽害指數呈現低度的相關，但仍可以觀察到鹽害指數最高 (較感鹽) 者確實具有較大的氣孔密度，反之亦然。此外，株高與葉面積在鹽環境下生長受抑制的程度與氣孔密度呈中度相關，顯示氣孔密度對於滲透勢逆境可能有較大的影響。本研究結果使我們瞭解氣孔密度對水稻幼苗期耐鹽的貢獻，以及不同種原在鹽逆境下的耐鹽性程度，這些可以為耐鹽水稻育種提供新的方向。

Salinity is one of the major abiotic stresses that limited the growth of crops. Salinity stress can be divided into two phases: the osmotic stress happened rapidly when plant senses high level of salt in soil; the ionic stress usually occurs later when too much sodium ion is accumulated in the shoot. Rice is the most salt sensitive crop among cereals, we hope to identify QTLs that significantly contribute to salt tolerance in rice. In this study we used two sets of rice diversity accessions for association mapping and IR64/Nipponbare recombinanat inbred lines for linkage mapping. For phenotypic traits, we measured stomata density, seedling height, leaf area and injury score under normal and salt conditions, we expect the accessions with lower stomata density could help reducing the transpiration rate, therefore sodium uptake in leaves is further minimized. Our result indicates that stomata density has weak positive relationship with injury score, and has moderate correlation with relative seedling height change and the reduction of leaf area. This suggests that stomata density might be involved in osmotic tolerance rather than ionic tolerance. For linkage mapping study, we mapped the significant loci on chromosome 2 and 6 explained 6 and 12% of the phenotypic variation of stomata density, respectively; and in these regions, the accessions with Nipponbare allele tend to have lower stomata density. These results demonstrate the contribution of stomata density to salinity tolerance at rice seedling stage and identify the accessions with multiple tolerance mechanisms, which can be further used in genetic breeding of salt tolerant rice.