Forage Seed Laboratory, Key Laboratory of Pratacultural Science, Beijing Municipality, China Agricultural University, Beijing 100193, China.
College of Qinghai-Tibetan Plateau, Southwest Minzu University, Chengdu 610041, China.
Int J Mol Sci. 2020 Jun 23;21(12):4457. doi: 10.3390/ijms21124457.
Increasing photosynthetic ability as a whole is essential for acquiring higher crop yields. Nonleaf green organs (NLGOs) make important contributions to photosynthate formation, especially under stress conditions. However, there is little information on the pod wall in legume forage related to seed development and yield. This experiment is designed for alfalfa () under drought stress to explore the photosynthetic responses of pod walls after 5, 10, 15, and 20 days of pollination (DAP5, DAP10, DAP15, and DAP20) based on ultrastructural, physiological and proteomic analyses. Stomata were evidently observed on the outer epidermis of the pod wall. Chloroplasts had intact structures arranged alongside the cell wall, which on DAP5 were already capable of producing photosynthate. The pod wall at the late stage (DAP20) still had photosynthetic ability under well-watered (WW) treatments, while under water-stress (WS), the structure of the chloroplast membrane was damaged and the grana lamella of thylakoids were blurry. The chlorophyll a and chlorophyll b concentrations both decreased with the development of pod walls, and drought stress impeded the synthesis of photosynthetic pigments. Although the activity of ribulose-1,5-bisphosphate carboxylase (RuBisCo) decreased in the pod wall under drought stress, the activity of phosphoenolpyruvate carboxylase (PEPC) increased higher than that of RuBisCo. The proteomic analysis showed that the absorption of light is limited due to the suppression of the synthesis of chlorophyll a/b binding proteins by drought stress. Moreover, proteins involved in photosystem I and photosystem II were downregulated under WW compared with WS. Although the expression of some proteins participating in the regeneration period of RuBisCo was suppressed in the pod wall subjected to drought stress, the synthesis of PEPC was induced. In addition, some proteins, which were involved in the reduction period of RuBisCo, carbohydrate metabolism, and energy metabolism, and related to resistance, including chitinase, heat shock protein 81-2 (Hsp81-2), and lipoxygenases (LOXs), were highly expressed for the protective response to drought stress. It could be suggested that the pod wall in alfalfa is capable of operating photosynthesis and reducing the photosynthetic loss from drought stress through the promotion of the C4 pathway, ATP synthesis, and resistance ability.
提高整体光合作用能力是获得更高作物产量的关键。非叶绿色器官（NLGO）对光合产物的形成做出了重要贡献，尤其是在胁迫条件下。然而，关于与种子发育和产量相关的豆科饲料荚壁的信息很少。本实验以紫花苜蓿（）为研究对象，在干旱胁迫下，对授粉后 5、10、15 和 20 天（DAP5、DAP10、DAP15 和 DAP20）的豆荚壁进行超微结构、生理和蛋白质组学分析，以探讨其光合作用响应。在豆荚壁的外表皮上可以明显观察到气孔。叶绿体结构完整，沿细胞壁排列，DAP5 时已经能够产生光合产物。在水分充足（WW）处理下，后期（DAP20）的荚壁仍具有光合作用能力，而在水分胁迫（WS）下，叶绿体膜结构受损，类囊体的基粒片层模糊。随着荚壁的发育，叶绿素 a 和叶绿素 b 的浓度都降低，干旱胁迫阻碍了光合色素的合成。尽管干旱胁迫下荚壁中的核酮糖-1,5-二磷酸羧化酶（RuBisCo）活性降低，但磷酸烯醇丙酮酸羧化酶（PEPC）的活性升高，高于 RuBisCo。蛋白质组学分析表明，干旱胁迫抑制叶绿素 a/b 结合蛋白的合成，从而限制了对光的吸收。此外，与 WW 相比，PS I 和 PS II 相关蛋白在 WW 下表达下调。尽管干旱胁迫下参与 RuBisCo 再生期的一些蛋白的表达受到抑制，但 PEPC 的合成被诱导。此外，一些参与 RuBisCo 还原期、碳水化合物代谢和能量代谢以及与抗性相关的蛋白，包括几丁质酶、热休克蛋白 81-2（Hsp81-2）和脂氧合酶（LOXs），在干旱胁迫下高度表达，以产生对干旱胁迫的保护响应。可以认为，紫花苜蓿的荚壁能够通过促进 C4 途径、ATP 合成和抗性能力来进行光合作用，并减轻干旱胁迫造成的光合作用损失。
