SUMMARYCellular exchange of carbon dioxide (CO 2 ) is of extraordinary importance for life. Despite this significance, its molecular mechanisms are still unclear and a matter of controversy. In contrast to other living organisms, plants are physiologically limited by the availability of CO 2 . In most plants, net photosynthesis is directly dependent on CO 2 diffusion from the atmosphere to the chloroplast. Thus, it is important to analyze CO 2 transport with regards to its effect on photosynthesis. A mutation of the Arabidopsis thaliana AtPIP1;2 gene, which was characterized as a non-water transporting but CO 2 transport-facilitating aquaporin in heterologous expression systems, correlated with a reduction in photosynthesis under a wide range of atmospheric CO 2 concentrations. Here, we could demonstrate that the effect was caused by reduced CO 2 conductivity in leaf tissue. It is concluded that the AtPIP1;2 gene product limits CO 2 diffusion and photosynthesis in leaves.

The impact of aquaporin function on plant water balance is discussed. The significance of these proteins for root water uptake, water conductance in the xylem, including embolism refilling and the role of plant aquaporins in leaf physiology, is described. Emphasis is placed on certain aspects of water stress reactions and the correlation of aquaporins to abscisic acid as well as on the relation of water and CO2 permeability in leaves.

BackgroundMaize stover is an important source of crop residues and a promising sustainable energy source in the United States. Stalk is the main component of stover, representing about half of stover dry weight. Characterization of genetic determinants of stalk traits provide a foundation to optimize maize stover as a biofuel feedstock. We investigated maize natural genetic variation in genome-wide association studies (GWAS) to detect candidate genes associated with traits related to stalk biomass (stalk diameter and plant height) and stalk anatomy (rind thickness, vascular bundle density and area).ResultsUsing a panel of 942 diverse inbred lines, 899,784 RNA-Seq derived single nucleotide polymorphism (SNP) markers were identified. Stalk traits were measured on 800 members of the panel in replicated field trials across years. GWAS revealed 16 candidate genes associated with four stalk traits. Most of the detected candidate genes were involved in fundamental cellular functions, such as regulation of gene expression and cell cycle progression. Two of the regulatory genes (Zmm22 and an ortholog of Fpa) that were associated with plant height were previously shown to be involved in regulating the vegetative to floral transition. The association of Zmm22 with plant height was confirmed using a transgenic approach. Transgenic lines with increased expression of Zmm22 showed a significant decrease in plant height as well as tassel branch number, indicating a pleiotropic effect of Zmm22.ConclusionSubstantial heritable variation was observed in the association panel for stalk traits, indicating a large potential for improving useful stalk traits in breeding programs. Genome-wide association analyses detected several candidate genes associated with multiple traits, suggesting common regulatory elements underlie various stalk traits. Results of this study provide insights into the genetic control of maize stalk anatomy and biomass.Electronic supplementary materialThe online version of this article (10.1186/s12870-019-1653-x) contains supplementary material, which is available to authorized users.

Heterologous expression in yeast cells revealed that NtAQP1, a member of the so-called PIP1 aquaporin subfamily, did not display increased water transport activity in comparison with controls. Instead, an increased CO 2 -triggered intracellular acidification was observed. NtPIP2;1, which belongs to the PIP2 subfamily of plant aquaporins, behaved as a true aquaporin but lacked a CO 2 -related function. Results from split YFP experiments, protein chromatography, and gel electrophoresis indicated that the proteins form heterotetramers when coexpressed in yeast. Tetramer composition had effects on transport activity as demonstrated by analysis of artificial heterotetramers with a defined proportion of NtAQP1 to NtPIP2;1. A single NtPIP2;1 aquaporin in a tetramer was sufficient to significantly increase the water permeability of the respective yeast cells. With regard to CO 2 -triggered intracellular acidification, a cooperative effect was observed, where maximum rates were measured when the tetramer consisted of NtAQP1 aquaporins only. The results confirm the model of an aquaporin monomer as a functional unit for water transport and suggest that, for CO 2 -related transport processes, a structure built up by the tetramer is the basis of this function.Water moves across biological membranes by diffusion. In most living organisms, the rate of water diffusion can be increased via pore-forming transmembrane proteins, the socalled aquaporins. These consist of six-membrane-spanning helices; N and C termini of the proteins face the cytosol. The helix-connecting loops B and E are themselves short helices that dip into the membrane from opposite sides and form the water-conducting channel (1, 2). Aquaporin monomers can assemble into tetramers (3, 4). Evidence for their function as water transport facilitators was observed by results from experiments using heterologous expression systems, such as Xenopus laevis oocytes (5). Despite this initially detected function, a facilitated membrane transport for glycerol or volatile substances like CO 2 or NH 3 was postulated (6 -9). Plant aquaporins were subdivided into protein groups according to the cellular location in which they have been initially detected and on the basis of sequence similarities (10). Accordingly, the PIPs (plasma membrane-intrinsic proteins) were split into two major groups, the PIP1 and PIP2 aquaporins. Compared with PIP1 proteins, PIP2 proteins have a shorter N-terminal extension and a longer C terminus (11)(12)(13)(14). In general, plant PIP2 proteins have been shown to facilitate membrane water transport in heterologous expression systems; however, PIP1 proteins display low or no activity in this respect (8,11,(15)(16)(17)(18). For the human AQP1 as well as for the tobacco PIP1 aquaporin NtAQP1, an increased cellular acidification rate under CO 2 -enriched buffer was obtained in oocytes expressing the respective aquaporin in addition to a carbonic anhydrase (9,19,20). The observations on NtAQP1, together with the fact that plants deficient in NtAQP1 express...