Characterization of the pheophorbide a oxygenase/phyllobilin pathway of chlorophyll breakdown in grasses
Characterization of the pheophorbide a oxygenase/phyllobilin pathway of chlorophyll breakdown in grasses
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DOI：
10.1007/s00425-018-2946-2
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发表时间：
2018-10-01
期刊：
影响因子：
4.3
通讯作者：
Hortensteiner, Stefan
中科院分区：
文献类型：
--
作者：
Das, Aditi;Christ, Bastien;Hortensteiner, Stefan
主要结论虽然叶绿素降解的PAO/叶绿蛋白途径在禾本科植物叶片衰老中是活跃的，但叶绿蛋白的丰度远低于叶绿素降解的量，完全发育的叶片变黄是植物衰老最显著的视觉症状。因此，叶绿素通过所谓的脱镁叶绿酸a加氧酶（PAO）/叶绿蛋白途径降解为一组物种特异性的叶绿蛋白，叶绿素分解的线性四吡咯产物。在这里，我们调查的多样性和丰富的叶绿蛋白在谷物和饲料作物，即大麦，水稻，黑麦草，高粱和小麦，使用液相色谱-质谱。共鉴定了13种叶白蛋白，其中4种是新的，尚未描述的，表明禾本科植物中存在相当高的叶白蛋白修饰活性多样性。沿着这些叶绿蛋白，已知拟南芥叶绿素分解代谢酶的大麦直向同源物被证明定位于叶绿体中，并且其中两个，即PAO和脱镁叶绿素脱镁叶绿酸水解酶，补充了各自的拟南芥突变体。这些数据证实了PAO/叶绿蛋白途径在禾本科植物中的功能。有趣的是，当比较叶绿蛋白丰度与衰老叶片中降解的叶绿素的量时，在大多数分析的草物种中，只有小部分叶绿素被回收为叶绿蛋白，与A. thaliana，其中叶绿蛋白量与降解的叶绿素相当匹配。这些数据表明，尽管在大麦（和其他谷物）中存在PAO/叶绿蛋白途径并具有活性，但叶绿蛋白不以化学计量方式积累，这意味着叶绿素可能降解超过叶绿蛋白水平。
Main conclusion Although the PAO/phyllobilin pathway of chlorophyll breakdown is active in grass leaf senescence,the abundance of phyllobilins is far below the amount of degraded chlorophyll.The yellowing of fully developed leaves is the most prominent visual symptom of plant senescence. Thereby, chlorophyll is degraded via the so-called pheophorbide a oxygenase (PAO)/phyllobilin pathway to a species-specific set of phyllobilins, linear tetrapyrrolic products of chlorophyll breakdown. Here, we investigated the diversity and abundance of phyllobilins in cereal and forage crops, i.e. barley, rice, ryegrass, sorghum and wheat, using liquid chromatography-mass spectrometry. A total of thirteen phyllobilins were identified, among them four novel, not yet described ones, pointing to a rather high diversity of phyllobilin-modifying activities present in the Gramineae. Along with these phyllobilins, barley orthologs of known Arabidopsis thaliana chlorophyll catabolic enzymes were demonstrated to localize in the chloroplast, and two of them, i.e. PAO and pheophytin pheophorbide hydrolase, complemented respective Arabidopsis mutants. These data confirm functionality of the PAO/phyllobilin pathway in grasses. Interestingly, when comparing phyllobilin abundance with amounts of degraded chlorophyll in senescent leaves, in most analyzed grass species only minor fractions of chlorophyll were recovered as phyllobilins, opposite to A. thaliana where phyllobilin quantities match degraded chlorophyll rather well. These data show that, despite the presence and activity of the PAO/phyllobilin pathway in barley (and other cereals), phyllobilins do not accumulate stoichiometrically, implying possible degradation of chlorophyll beyond the phyllobilin level.