Agricultural College, Yangzhou University, Yangzhou 225009, China.
Innovation Center of Rice Cultivation Technology in Yangtze River Valley of Ministry of Agriculture, Yangzhou 225009, China.
Plant Cell Physiol. 2022 Jul 14;63(7):991-1007. doi: 10.1093/pcp/pcac066.
The grain filling of rice depends on photoassimilates from leaves and stems. Phloem loading is the first crucial step for the transportation of sucrose to grains. However, phloem loading mechanisms in rice leaves and stems and their response to nitrogen (N) remain unclear. Here, using a combination of electron microscopy, transportation of a phloem tracer and 13C labeling, phloem loading was studied in rice leaves and stems. The results showed that the sieve element-companion cell complex lacked a symplastic connection with surrounding parenchyma cells in leaves and stems. The genes expression and protein levels of sucrose transporter (SUTs) and sugars will eventually be exported transporters (SWEETs) were detected in the vascular bundle of leaves and stems. A decrease in the 13C isotope remobilization from leaves to stems and panicles following treatment with p-chloromercuribenzenesulfonic acid indicated that rice leaves and stems actively transport sucrose into the phloem. Under low-N (LN) treatment, the activities of α-amylase, β-amylase and sucrose phosphate synthase (SPS) in stems and activity of SPS in leaves increased; genes expression and protein levels of SUTs and SWEETs in leaves and stems increased; the 13C isotope reallocation in panicles increased. These indicated that LN enhanced apoplastic phloem loading in stems and leaves. This improved the translocation of photoassimilates and consequently increased grain filling percentage, grain weight and harvest index. This study provides evidence that rice leaves and stems utilize an apoplastic loading strategy and respond to N stimuli by regulating the genes expression and protein levels of SUTs and SWEETs.
水稻的灌浆依赖于叶片和茎中的光合产物。韧皮部装载是蔗糖向籽粒运输的第一步关键步骤。然而，水稻叶片和茎中的韧皮部装载机制及其对氮（N）的响应仍不清楚。在这里，使用电子显微镜、韧皮部示踪剂运输和 13C 标记的组合，研究了水稻叶片和茎中的韧皮部装载。结果表明，筛管-伴胞复合体与叶片和茎中的周围薄壁细胞缺乏质外体连接。在叶片和茎的维管束中检测到蔗糖转运蛋白（SUTs）和糖最终输出转运蛋白（SWEETs）的基因表达和蛋白水平。用对氯汞苯甲酸处理后，从叶片到茎和穗的 13C 同位素再分配减少，表明水稻叶片和茎主动将蔗糖运入韧皮部。在低氮（LN）处理下，茎和叶片中α-淀粉酶、β-淀粉酶和蔗糖磷酸合成酶（SPS）的活性增加；叶片和茎中 SUTs 和 SWEETs 的基因表达和蛋白水平增加；穗中 13C 同位素再分配增加。这表明 LN 增强了茎和叶的质外体韧皮部装载。这改善了光合同化物的转运，从而增加了灌浆百分率、粒重和收获指数。本研究提供的证据表明，水稻叶片和茎利用质外体装载策略，并通过调节 SUTs 和 SWEETs 的基因表达和蛋白水平来响应 N 刺激。
