Sage Tammy L, Sage Rowan F
Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, ON M5S3B2, Canada.
Plant Cell Physiol. 2009 Apr;50(4):756-72. doi: 10.1093/pcp/pcp033. Epub 2009 Feb 25.
One mechanism to enhance global food stocks radically is to introduce C4 photosynthesis into C3 crops from warm climates, notably rice. To accomplish this, an understanding of leaf structure and function is essential. The chlorenchyma structure of rice and related warm-climate C3 grasses is distinct from that of cool temperate C3 grasses. In temperate C3 grasses, vacuoles occupy the majority of the cell, while chloroplasts, peroxisomes and mitochondria are pressed against the cell periphery. In rice, 66% of protoplast volume is occupied by chloroplasts, and chloroplasts/stromules cover >95% of the cell periphery. Mitochondria and peroxisomes occur in the cell interior and are intimately associated with chloroplasts/stromules. We hypothesize that the chlorenchyma architecture of rice enhances diffusive CO(2) conductance and maximizes scavenging of photorespired CO2. The extensive chloroplast/stromule sheath forces photorespired CO(2) to exit cells via the stroma, where it can be refixed by Rubisco. Deep cell lobing and small cell size, coupled with chloroplast sheaths, creates high surface area exposure of stroma to intercellular spaces, thereby enhancing mesophyll transfer conductance. In support of this, rice exhibits higher mesophyll transfer conductance, greater stromal CO2 content, lower CO2 compensation points at warm temperature and less oxygen sensitivity of photosynthesis than cool temperate grasses. Rice vein length per leaf, mesophyll thickness and intercellular space volume are intermediate between those of most C3 and C4 grasses, indicating that the introduction of Kranz anatomy into rice may not require radical changes in leaf anatomy; however, deep lobing of chlorenchyma cells may constrain efforts to engineer C4 photosynthesis into rice.
从根本上提高全球粮食储备的一种机制是将C4光合作用引入来自温暖气候地区的C3作物，尤其是水稻。要实现这一点，了解叶片结构和功能至关重要。水稻和相关温暖气候下的C3禾本科植物的叶肉结构与凉爽温带地区的C3禾本科植物不同。在温带C3禾本科植物中，液泡占据细胞的大部分空间，而叶绿体、过氧化物酶体和线粒体则被挤压在细胞周边。在水稻中，原生质体体积的66%被叶绿体占据，叶绿体/基质小管覆盖细胞周边的比例超过95%。线粒体和过氧化物酶体出现在细胞内部，并与叶绿体/基质小管密切相关。我们推测，水稻的叶肉结构增强了CO2扩散传导，并使光呼吸产生的CO2清除最大化。广泛的叶绿体/基质小管鞘迫使光呼吸产生的CO2通过基质排出细胞，在那里它可以被核酮糖-1,5-二磷酸羧化酶/加氧酶重新固定。细胞深裂和小细胞尺寸，再加上叶绿体鞘，使得基质与细胞间隙的表面积暴露增加，从而提高了叶肉传递传导。支持这一观点的是，与凉爽温带禾本科植物相比，水稻表现出更高的叶肉传递传导、更高的基质CO2含量、在温暖温度下更低的CO2补偿点以及对光合作用更低的氧气敏感性。水稻每片叶的叶脉长度、叶肉厚度和细胞间隙体积介于大多数C3和C4禾本科植物之间，这表明将花环结构引入水稻可能不需要对叶片解剖结构进行彻底改变；然而，叶肉细胞的深裂可能会限制将C4光合作用引入水稻的努力。
