Khoshravesh Roxana, Stinson Corey R, Stata Matt, Busch Florian A, Sage Rowan F, Ludwig Martha, Sage Tammy L
Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks St., Ontario, ON M5S 3B2, Canada.
Research School of Biology, Australian National University, Canberra, ACT 2601, Australia.
J Exp Bot. 2016 May;67(10):3065-78. doi: 10.1093/jxb/erw150. Epub 2016 Apr 12.
Photorespiratory glycine shuttling and decarboxylation in bundle sheath (BS) cells exhibited by C2 species is proposed to be the evolutionary bridge to C4 photosynthesis in eudicots. To evaluate this in grasses, we compare anatomy, cellular localization of glycine decarboxylase (GDC), and photosynthetic physiology of a suspected C2 grass, Homolepis aturensis, with these traits in known C2 grasses, Neurachne minor and Steinchisma hians, and C3 S laxum that is sister to S hians We also use publicly available genome and RNA-sequencing data to examine the evolution of GDC subunits and enhance our understanding of the evolution of BS-specific GDC expression in C2 and C4 grasses. Our results confirm the identity of H aturensis as a C2 species; GDC is confined predominantly to the organelle-enriched BS cells in H aturensis and S hians and to mestome sheath cells of N minor Phylogenetic analyses and data obtained from immunodetection of the P-subunit of GDC are consistent with the hypothesis that the BS dominant levels of GDC in C2 and C4 species are due to changes in expression of a single GLDP gene in M and BS cells. All BS mitochondria and peroxisomes and most chloroplasts in H aturensis and S hians are situated centripetally in a pattern identical to C2 eudicots. In S laxum, which has C3-like gas exchange patterns, mitochondria and peroxisomes are positioned centripetally as they are in S hians This subcellular phenotype, also present in eudicots, is posited to initiate a facilitation cascade leading to C2 and C4 photosynthesis.
C2植物在维管束鞘（BS）细胞中表现出的光呼吸甘氨酸穿梭和脱羧作用被认为是真双子叶植物向C4光合作用进化的桥梁。为了在禾本科植物中评估这一点，我们将一种疑似C2禾本科植物阿图尔希阿单穗草的解剖结构、甘氨酸脱羧酶（GDC）的细胞定位和光合生理与已知的C2禾本科植物小类芦和纤毛鸭嘴草以及与纤毛鸭嘴草亲缘关系较近的C3植物疏花假虎尾草的这些特征进行了比较。我们还利用公开可用的基因组和RNA测序数据来研究GDC亚基的进化，并加深我们对C2和C4禾本科植物中BS特异性GDC表达进化的理解。我们的结果证实了阿图尔希阿单穗草是一种C2植物；GDC主要局限于阿图尔希阿单穗草和纤毛鸭嘴草中富含细胞器的BS细胞以及小类芦的维管束鞘细胞。系统发育分析以及从GDC的P亚基免疫检测获得的数据与以下假设一致，即C2和C4物种中GDC在BS中的优势水平是由于M和BS细胞中单个GLDP基因表达的变化。阿图尔希阿单穗草和纤毛鸭嘴草中所有的BS线粒体、过氧化物酶体以及大多数叶绿体都以与C2真双子叶植物相同的模式向心分布。在具有类似C3气体交换模式的疏花假虎尾草中，线粒体和过氧化物酶体的定位与纤毛鸭嘴草一样是向心的。这种亚细胞表型在真双子叶植物中也存在，被认为启动了一个促进级联反应，导致C2和C4光合作用。
