Graduate School of Biological Sciences, Tokyo Institute of Technology Yokohama, Japan.
Center for Biological Resources and Informatics, Tokyo Institute of Technology Yokohama, Japan.
Front Plant Sci. 2014 Jun 23;5:280. doi: 10.3389/fpls.2014.00280. eCollection 2014.
Plant galactolipid synthesis on the outer envelope membranes of chloroplasts is an important biosynthetic pathway for sustained growth under conditions of phosphate (Pi) depletion. During Pi starvation, the amount of digalactosyldiacylglycerol (DGDG) is increased to substitute for the phospholipids that are degraded for supplying Pi. An increase in DGDG concentration depends on an adequate supply of monogalactosyldiacylglycerol (MGDG), which is a substrate for DGDG synthesis and is synthesized by a type-B MGDG synthase, MGD3. Recently, sucrose was suggested to be a global regulator of plant responses to Pi starvation. Thus, we analyzed expression levels of several genes involved in lipid remodeling during Pi starvation in Arabidopsis thaliana and found that the abundance of MGD3 mRNA increased when sucrose was exogenously supplied to the growth medium. Sucrose supplementation retarded the growth of the Arabidopsis MGD3 knockout mutant mgd3 but enhanced the growth of transgenic Arabidopsis plants overexpressing MGD3 compared with wild type, indicating the involvement of MGD3 in plant growth under sucrose-replete conditions. Although most features such as chlorophyll content, photosynthetic activity, and Pi content were comparable between wild-type and the transgenic plants overexpressing MGD3, sucrose content in shoot tissues decreased and incorporation of exogenously supplied carbon to DGDG was enhanced in the MGD3-overexpressing plants compared with wild type. Our results suggest that MGD3 plays an important role in supplying DGDG as a component of extraplastidial membranes to support enhanced plant growth under conditions of carbon excess.
在质体的外被膜上合成植物半乳糖脂是在磷（Pi）耗尽条件下持续生长的重要生物合成途径。在 Pi 饥饿时，二半乳糖基二酰基甘油（DGDG）的量增加以替代为提供 Pi 而降解的磷脂。DGDG 浓度的增加取决于单半乳糖基二酰基甘油（MGDG）的充足供应，MGDG 是 DGDG 合成的底物，由 B 型 MGDG 合酶 MGD3 合成。最近，蔗糖被认为是植物对 Pi 饥饿反应的全局调节剂。因此，我们分析了拟南芥在 Pi 饥饿期间参与脂质重塑的几个基因的表达水平，发现当蔗糖外源性供应到生长培养基中时，MGD3 mRNA 的丰度增加。蔗糖补充延缓了拟南芥 MGD3 敲除突变体 mgd3 的生长，但与野生型相比，增强了过表达 MGD3 的转基因拟南芥植物的生长，表明 MGD3 参与了蔗糖充足条件下的植物生长。尽管野生型和过表达 MGD3 的转基因植物的大多数特征（如叶绿素含量、光合作用活性和 Pi 含量）相似，但与野生型相比，茎组织中的蔗糖含量降低，并且外源碳掺入 DGDG 增强。我们的结果表明，MGD3 在提供 DGDG 作为质体外膜的组成部分以支持在碳过剩条件下增强的植物生长方面发挥重要作用。
