The Transcriptome Analyses Of Mazie (Zea Mays L.) Roots In Response To Low Phosphorus Stress

Phosphorus (P) is one of essential macronutrients for plant growth and development. However, the concentration of available Pi by roots in soil is usually about 1μM, which cannot satisfy the demand of plants to acquire the sufficient Pi. In the main area of corn production, lack of available phosphorus in soil has become one of the important factors for restricting yield of corn and increasing its production costs. Therefore, the in-depth study of low-P tolerance mechanism of maize is of important theoretical significance and practical value. As the major organs for absorption of mineral elements in higher plants, root is of high plasticity. Due to the uneven distribution of phosphorus in soil and its relatively low mobility, root development and morphology is particularly important for the absorption of phosphorus. Changing the morphology of root system to improve the capacity of assimilating phosphorus from soil is an effective strategy of plant for coping with low phosphorus environment.Root morphological adaptive changes of maize inbred lines Qi-319 seedlings in response to phosphorus deficiencyFirst of all, seedlings of maize inbred line Qi-319, endosperms of which had been removed, were used to observe and analyze the impact of low phosphorus treatment (5μM KH₂PO₄) on their root morphology, and confirm the adaptive changes of maize roots in response to low phosphorus environment The results showed that with the extension of the time for low phosphorus treatment, phosphorus concentration in vivo significantly reduced, while phosphorus use efficiency obviously increased. However, compared with the shoots, the plant distributed the greater proportion of phosphorus to the roots under phosphorus stress, which might be an important reason for maintaining root growth and development under low-phosphorus stress. The growth of shoots was significantly inhibited under low phosphorus condition, while the growth of roots was better maintained during the treatment of low phosphorus. Root-shoot ratio of plant under low phosphorus treatment was significantly higher than the control, which means that under low-phosphorus stress the plant will assign more carbohydrates to the roots and give priority to ensure the normal growth and development of root system. Under low phosphorus stress the growth rate of Qi-319 axile roots was accelerated and the root length was significantly increased, compared with the control. Through the PI staining, the length of mature root cells was not found to be changed significantly under low phosphorus stress, and it was speculated that the increased elongation of axile roots under low phosphorus stress is likely to the results of increased activity of apical cell proliferation and accelerated cell division. Feulgen staining showed that under low-phosphorus stress the number of lateral root primordia was significantly reduced; and statistical results showed that the treatment of low phosphorus decreased the number of lateral roots. The results above showed that phosphorus deficiency significantly impacts root morphology of maize, and the responses of different root parts to low phosphorus stress are not the same: under low phosphorus stress axile root length of Qi-319 was significantly increased, while lateral root formation was obviously inhibited, which means that Qi-319 was more inclined to form root system with long axile roots and fewer branches. These root morphological changes mediated by phosphorus deficiency are obviously different from Arabidopsis. These results revealed the species-specificity of root morphological changes mediated by low phosphorus environment, and root system of different plants may have different ways in response to low phosphorus environment.Changes of Qi-319 root gene expression in response to low phosphorus stress detected by maize whole-genome oligonucleotide microarrayThe adaption of plant to low phosphorus environment is essentially caused by the changes of gene expression regulation. In the study seedlings of Qi-319, endosperms of which had been removed, were used as plant materials. The gene expression profile of root tip treated by low phosphorus for 1 day, 8 days and the section of the lateral root primordium formation for 2, 8 days were detected by maize genome-wide oligonucleotide microarray. The significantly changed probe of biological valve must be in accordance with the following conditions: p < 0.05, the average change folds≥1.5 and the change folds≥1.5 in all three independent repeated experiments. The results showed that 130 and 1221 genes were differentially expressed in root tip after 1 day and 8 day of low phosphorus stress, while 202 and 717 differentially expressed genes in section of lateral root primordium formation after 2 days, 8 days of low phosphorus treatment. These differentially expressed genes involved in metabolism, cell signal transduction, transcription, cell proliferation and growth regulation, energy, transport, protein synthesis and fate, cell defense and response etc. Functional classification also showed that the response of maize roots to phosphorus deficiency is closely related to the degree of low phosphorus stress, and differs obviously in various parts of root, indicating that the response of plant roots to phosphorus starvation is a complex response process. Expression of a variety of phosphorus transport, mobilization and metabolism-related gene, such as phosphate transporter, acid phosphatase, ribonuclease, SPX domain genes as well as phospholipids, glycolipids and sulfolipid metabolism-related genes, is impacted by low phosphorus stress. The changes in expression level of these genes are of great importance for enhancing phosphorus absorption capacity and improving the phosphorus use efficiency.In the transcriptomic analysis of root tips under low phosphorus treatment, we found a large number of differentially expressed genes related to signal transduction, transcription regulation, cell proliferation and growth, such as a variety of protein kinases and phosphatases, multiple families of transcription factors, hormone metabolism and signal transduction related genes, cell cycle regulation protein etc, may play important roles in connecting changes of external phosphorus concentration and changes of roots adaptations. This reveals that the change in activity of apical cell division is are closely related to variety in expression level of differentially expressed genes regulating cell proliferation and growth.In the transcripteomic analysis of section of lateral root primordium formation under phosphorus deficiency, the expression levels of a number of lateral root growth-related gene change, such as LOB domain gene, protein phosphatase PP2A, Response Regulator, ACC oxidation enzymes. These genes involved in signal transduction, hormones regulation etc, which may be of great significance in mediating the low phosphorus signal-regulated lateral root primordia formation. This indicates that the multiple lateral development related genes are likely involved in restraining the formation of lateral root primordial under low phosphorus stress, and play important roles in this process.In this work, we observed root morphology changes of Qi-319 and conducted the transcriptome analyses of different parts of axile roots (root tip and section of lateral root primordium formation) under low phosphorus treatment. The results made clear the detailed response of root morphology to low phosphorus stress and revealed the effect of phosphorus starvation on maize root growth and transcriptome. Our study provided valuable information for searching genes related to maize root morphology and phosphorus efficiency in phosphorus deficient environment and shedding light on exploring molecular mechanisms of plant roots remodeling under phosphorus starvation, which was of great importance for the excavation and use of phosphorus efficiency key genes and proposing strategies for crop molecular breeding of phosphorus efficiency.