In situ localization and changes in the expression level of transcripts related to intercellular transport and phloem loading in leaves of maize (Zea mays L.) treated with low temperature

Intercellular transport in C4 plant, including maize, is particularly important due to the necessity of efficient exchange of photoassimilates between different types of cells, where specific stages of C4 photosynthesis occur. The transport on the trail: mesophyll—bundle sheath—vascular parenchyma occurs by symplast, through plasmodesmata. The next stage, phloem loading (the transport of photosynthetic products to the companion cell/sieve element complex) belongs to the apoplasmic type, without the participation of plasmodesmata. This work concerns verification of the hypothesis about the impact of low temperature on changes in expression of genes, strictly related to phloem loading process as well as intercellular (symplasmic) transport. For that purpose, in situ hybridization assay for several mRNAs and the determination of changes in the expression level of these transcripts using RT-qPCR technique were conducted. Transcripts related to phloem loading, e.g., sucrose transporters, interactor of proton pump, aquaporins and plasmodesmata-associated genes, coding: TMV-MP30 binding protein and, remorins were selected. Studies were performed on two lines of maize differing in chilling-sensitivity level. Plants were treated with low temperature (day/night): 14/12 and 8/6 °C in two periods: 4 and 28 h. Cold-treatment resulted in a diverse response of tested maize plants, mainly changes in the gene expression under exposition at shock temperature level (6–8 °C) and after prolonged chilling-treatment (28 h). The localization of sucrose transporter and aquaporin transcripts in the thick-walled sieve tubes indicates the potential role of these cells in the phloem loading process. The pronounced changes in expression of plasmodesmata-related transcripts indicate their evident participation in response to cold stress.