Studies On Photosynthetic Characteristics And Leaves Proteomics Of Maize Inbred Lines Under Low Phosphorus Stress

As one of essential macronutrients for plant growth and development, phosphorus(P) is involved in almost all of plant life activities.Maize(Zea mays L.) is an important food and forage crop,which occupies a pivotal position in the agricultural production of the world.In the main cultivation areas for maize, insufficient of available Pi in soil has been an important constraint for yield and increases its production cost.Therefore,better knowing mechanisms of maize absorption,utilization and tolerance for Pi,exploiting biological characteristics of crops and developing P-efficient cultivars may be of great theoretical significance and practical value.In this study,we take P nutrient mutants 99038,99095 coming from cell engineering technology and the original line Qi-319 which is a skeleton maize inbred line as the materials.The root morphological,physiological and biochemical responses to P-starvation were investigated under solution culture conditions in order to constracting P efficiency of the above three lines.Some photosynthetic parameters of the three hydroponic lines were studied under two P level conditions,which is for knowing the difference among their photosynthetic characteristics.The proteome changes of Qi-319 leaves responsive to P-stress were analyzed,which can provide a basis for understanding low-P tolerance mechanism of maize photosynthetic system.There were significant various P nutrient characteristics among 99038,Qi319 and 99095.When subjected to low P treatment for 25 days,the biomass and phosphorus content of 99038 were higher than that of Qi319 and 99095,which indicated that the sensibility of 99038 was the lowest among the three lines.99038 had advantages in root-shoot ratio,total root length,the number and length of later root,while average root diameter was the thinnest,compared with Qi319 and 99095. The root development of 99095 was much weaker than 99038 and Qi319.The amount of secreted organic acid and protons increased under P starvation,which was helpful for the activation and absorption of insoluble phosphate in the soil.Compared to Qi319 and 99095,99038 had the strongest ability of rhizosphere acidification.Low P treatment obviously increased the activities of APase in roots and on root surfaces. APase activities of 99095 were lower than 99038 and Qi319,suggested its weaker ability to release and recycle of organic P.Furthermore,99038 was advantageous in total uptake area,active uptake area and P-uptake kinetics(the higherâ… _max value and the lower K_m,C_min values).Our data demonstrated that root vigour and phosphorous affinity of 99095 were weaker than 99038 and Qi319.Those results above showed that the difference of root morphology,physiology and the capability of P-absorption might result in various low-P tolerance of 99038,Qi319 and 99095.Maize inbred lines 99038,Qi319 and 99095 have different photosynthetic characteristics under P deficit.The results were shown that low phosphorus stress decreased net photosynthetic rate,stomatal conductance and transpiration rate,but increased intercellular CO₂ concentration.The decrease of photosynthetic rate in three lines may be caused by no-stomatal limitation.Compared to Qi319 and 99095,99038 could keep higher photosynthetic rate under low P condition.The efficiency of light energy conversion and utilization of 99038 was higher than Qi319 and 99095,which suggested the superior tolerance to low P stress.Low P supply decreased the content of chlorophyll and carotenoid,as well as the rate of Chla/Chlb.The extent of decline of 99095 was much than 99038 and Qi319.P starvation changed the amount of photosynthetic carbon partitioning into sucrose and starch.After 25 days of low P stress,the rate of sucrose/starch in leaves of 99038 was higher than that of Qi319 and 99095.Under the same P level conditions,inorganic phosphate content in leaves of 99038 was significantly higher than that of Qi319 and 99095,whereas 99095 showed the worst tolerance to P deficiency.The different inorganic phosphorus content in leaves may be contrubuted to the variation of photosynthesis characteristics in maize inbred lines with different P-efficiency.The expressions of proteins in Qi-319 leaves were changed under low P stress. The method of PEG fractional precipitation was used to extract maize leaf proteins. Approximate 1296 protein spots were detected by 2-DE,200(15.43%) of which changed significantly in amount(P＜0.05;≥2 folds).144 differentially expressed proteins were identified by MALDI-TOF MS and classified according to Arabidopsis MATDB.The 144 identified proteins represented a large range of functional categories,including 17 unknown or unclassified proteins,12 proteins in protein fate, 9 proteins in protein synthesis,12 proteins in cell rescue,defense and virulence,7 proteins in secondary metabolism,17 proteins in energy,47 proteins in metabolism, 15 proteins in transcription/cellular communication/signal transduction mechanism and 8 proteins in cell cycle/transport.A large number of proteins involved in photosynthesis,such as rubisco lager subunit,rubisco activase(RCA),chloroplast ATP synthase subunit,NADP-dependent malate dehydrogenase,light-harvesting complex(LHC),sucrose-phosphatase,UDP-sulfoquinovose synthase and so on, which indicated that photosynthetic system of maize leaves was strongly affected by P deficiency.Several other proteins,such as S-adenosylmethionine synthase,14-3-3 protein,RNA binding protein,G proteinβsubunit,protein kinases and PP2A were involved in the secondary metabolism,signal transduction and transcriptional regulation.Their inducement suggested that they might play important roles in linking the change of external P concentration and the metabolic adaptation of plants,to regulate gene expression facilitating P-homeostasis,growth and development for plant. Those results above pointed out that the low-P tolerance of maize leaves is involved in a complicated process related with multiple metabolic and signal pathways at proteome level and also offered the valuable information for better understanding P starvation responsive mechanism of maize plants.