Sugarcane (Saccharum Spp.) physiological profile and phosphate transporters expression during an arbuscular mycorrhizal symbiosis

Author(s)

Raul Santin Almeida

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Centro de Energia Nuclear na Agricultura (CENA/STB)
• Defense date: 2007-06-22

Examining board members

Antonio Vargas de Oliveira Figueira; Marcos Silveira Buckeridge; Arnoldo Rocha Façanha; Adriana Pinheiro Martinelli Rodriguez; Alexandre Magno Sebbenn

• Advisor: Antonio Vargas de Oliveira Figueira

Abstract

Plants display a wide array of physiological adaptations to low soil phosphorus (Pi) availability. This work discussed physiological and energetic costs associated with these strategies, focusing on sugarcane responses to Pi availability during the development of arbuscular mycorrhizal (AM) symbiosis. Such costs are important components of adaptation to low phosphorus soils affecting phosphorus acquisition and leve, growth and soluble sugars concentration in plant tissues. Sugarcane plants were grown in pots, with or without AM (Glomus clarum), and with low (20 mg kg-1) or high (200 mg kg-1) phosphorus supply. Roots and shoots were harvest for analysis after 14, 30, 44 and 58 days post-inoculation (dpi) with the fungus Glomus clarum,. The low Pi supply caused Pi deficiency in mycorrhizedl or non-mycorrhizedl plants. The efficiency of Pi absorption, indicated by shoot Pi accumulation in correlation to root biomass, suggested that root and mycorrhizal Pi absorption were similar, regardless of the Pi doses. Phosphorus availability did not affect the whole-plant biomass, and plants under low Pi supply, used more efficiently this nutrient. On the other hand, mycorrhized plants supplemented with low Pi presented the highest root growth and shoot reduction, resulting in high root:shoot ratio. At 58 dpi, glucose, fructose and sucrose concentrations in leaves of mycorrhized plants were 3.8, 2.3 and 2.4-fold higher respectively, than in non-mycorrhizedl plants. These results suggested that, under these experimental conditions, mycorrhizal symbiosis establishment was not a typical mutualistic association affecting sugarcane growth profile and allometry, when cultivated under low Pi. The photosynthate levels of leaves from mycorrhizd plants indicated an increase in photossynthetic rate but withut resulting in higher macrobiont growth. The quantitative amplification of reversed transcripts (RT-PCR) technology has become a method of choice for functional gene validation, with sensitiveness, accurate quantification and high-throughput. The relative quantification is easier determined by relative expression in comparison to a constitutively expressed refernce gene. Here, the expression variability of a sugarcane actin gene (Actin) glyceraldehydo phosphate dehydrogenase (GAPDH), tubulin (Tubulina), and ubiquitins (UbiQ1 and UbiQ2) from various tissues were analysed and compared based on the ranking list from the Genorm and NormFinder softwares. Expression analysis based on the REST software gave the proper statistic validation. UbiQ1 was the most stable gene among the candidate gen-references along the various tissues or organs tested: meristem, inflorescence, leaf, stem and roots treated with high and low phosphorus. Considering UbiQ1 as the reference gene, the relative expression of the sugarcane high-affinity phosphate transporters genes PT7 and PT8 were assessed from roots fertilized with low Pi or high Pi , inoculated or not with the mycorrhizal fungus, harvest 58 dpi. Both genes belong to the Pht1 Pi transporter and share similarity with the rice orthologs ORYsat;Pht1;7 and ORYsat;Pht1;8. Under Pi deficiency, the phosphate transporter PT7 was induced in non-colonized roots, but less expressed in mycorrhizedl ones, with high root colonization rate suppressing PT7 expression. PT8 showed low variability in expression, slightly more expressed in mycorrhized plants than in non-mycorhized plants under low Pi supply. These results indicated that PT7 was induced in Pi stressed roots, and possibly associated with the root Pi uptake, while PT8 had limited modulation in expression and probably involved on Pi fluxes or homeostasis, likely associated with both root and mycorrhizal phosphate uptake pathways. PT7 and PT8 were induced during medium/long-term treatments, showing induction or constant expression in both acquisition and mobilization of Pi in response to Pi deprivation, which is consistent with the proposed role of this tranporter family. Mycorrhizedl sugarcane showed a highly plastic response to low Pi (AU)