Breeding strategy for P-efficient plants

通过提高对土壤磷的吸收能力，培育出比普通作物生长得更好的高效磷植物，将是植物育种的重要目标。从根部增加有机酸排泄可能参与缺磷反应机制，是实现这一育种目标的一个有希望的途径。此外，要培育出在自然环境中具有优异生长性能的“真正的高效磷植物”，还应考虑植物与微生物的相互作用和植物与土壤的相互作用，因为这些相互作用会影响植物排泄的OA对磷的溶解效率。根据这些概念，我们完成了研究，得到了以下发现：1)胡萝卜IPG细胞系和白色羽扇豆的OA排泄机制：IPG（不溶性磷酸盐生长者）比亲本细胞系释放更多的柠檬酸盐。利用RNAi技术，我们发现质子泵浦参与了这种能力。此外，我们还发现OA代谢的改变在白豆OA排泄中起关键作用。2)拟南芥苹果酸转运蛋白的基因组学研究我们确定了控制拟南芥AL耐受性的QTL，并确定了该位点上的一个关键基因AtALMT1，该基因编码了拟南芥AL响应型苹果酸转运蛋白。利用微阵列和遗传学方法，我们发现了基因表达水平与苹果酸释放能力之间的相关性。我们还表征了AtALMTI的表达和调控，并发现可逆磷酸化参与调节这些机制。3)植物-微生物和土壤相互作用研究发现，从碳经济的角度来看，OA释放和微根菌系统对小麦磷素吸收有共同的促进作用，因此OA释放应由微根菌系统平衡。此外，我们还利用ICP-MASS建立了灵敏、高通量的拟南芥粕含量定量系统。利用这个系统，我们发现转基因植物对养分的吸收得到了改善。

Development of P-efficient plants, which can grow better than regular crop plants by enhanced ability of P-uptake from soil, would be important targets in plant breeding. Enhancement of organic acid excretion from the root, which seemed to be involved in P-deficiency responsive mechanisms, is one promising approach to realize this breeding target. Furthermore, it should be considered plant-microbial interaction and plant-soil interaction to develop "real P-efficient plants" showing superior growth performance in natural environment because these interactions should affect efficiency of P solublization by excreted OA. From these concepts, we completed research and obtained following findings.1) OA excretion mechanism in Carrot IPG cell line and white lupinThe IPG (Insoluble phosphate grower) can release large amounts of citrate than parental cell line. Using RNAi techniques, we showed proton pumping is involved in this capacity. Also, we found that altered OA metabolism play a key role in OA excretion in white lupin.2) Genomics approach for malate transporter in ArabidopsisWe identified QTL controlling AL tolerance in Arabidopsis and identified a key gene in this locus, namely AtALMT1 encoding a Al-responsive malate transporter in Arabidopsis. Using micro-array and genetic approaches, we found correlation between expression level of the gene and malate release capacity. We also have characterized the AtALMTI expression and regulation and found that involvement of a reversible phosphorylation in regulating these mechanisms.3) Plant-Microbe and Soil interactionsWe found that OA release should be balanced by micoryiza system in terms of carbon economy because OA release and micoryiza system additively contribute on P-uptake in wheat. In addition, we established sensitive and high throughput system for quantifying system for meal contents in Arabidopsis using ICP-MASS. Using this system, we identified improved uptake of nutrients by transgenic plants.