Summary Desert plants are hypothesized to survive the environmental stress inherent to these regions in part thanks to symbioses with microorganisms, and yet these microbial species, the communities they form, and the forces that influence them are poorly understood.Here we report the first comprehensive investigation of the microbial communities associated with species of Agave, which are native to semiarid and arid regions of Central and North America and are emerging as biofuel feedstocks. We examined prokaryotic and fungal communities in the rhizosphere, phyllosphere, leaf and root endosphere, as well as proximal and distal soil samples from cultivated and native agaves, through Illumina amplicon sequencing.Phylogenetic profiling revealed that the composition of prokaryotic communities was primarily determined by the plant compartment, whereas the composition of fungal communities was mainly influenced by the biogeography of the host species. Cultivated A. tequilana exhibited lower levels of prokaryotic diversity compared with native agaves, although no differences in microbial diversity were found in the endosphere.Agaves shared core prokaryotic and fungal taxa known to promote plant growth and confer tolerance to abiotic stress, which suggests common principles underpinning Agave–microbe interactions.

The role of plasma membrane aquaporins (PIPs) in water relations of Arabidopsis was studied by examining plants with reduced expression of PIP1 and PIP2 aquaporins, produced by crossing two different antisense lines. Compared with controls, the double antisense (dAS) plants had reduced amounts of PIP1 and PIP2 aquaporins, and the osmotic hydraulic conductivity of isolated root and leaf protoplasts was reduced 5-to 30-fold. The dAS plants had a 3-fold decrease in the root hydraulic conductivity expressed on a root dry mass basis, but a compensating 2.5-fold increase in the root to leaf dry mass ratio. The leaf hydraulic conductance expressed on a leaf area basis was similar for the dAS compared with the control plants. As a result, the hydraulic conductance of the whole plant was unchanged. Under sufficient and under water-deficient conditions, stomatal conductance, transpiration rate, plant hydraulic conductance, leaf water potential, osmotic pressure, and turgor pressure were similar for the dAS compared with the control plants. However, after 4 d of rewatering following 8 d of drying, the control plants recovered their hydraulic conductance and their transpiration rates faster than the dAS plants. Moreover, after rewatering, the leaf water potential was significantly higher for the control than for the dAS plants.From these results, we conclude that the PIPs play an important role in the recovery of Arabidopsis from the water-deficient condition.Water transport through cellular membranes is facilitated by aquaporins, proteins that form waterselective channels. The presence of aquaporins in a membrane can increase the osmotic hydraulic conductivity of the membrane (L P , meters per second per megapascal) by 10-to 20-fold (Preston et al., 1992). In plants, the physiological importance of aquaporins is currently mainly inferred from their widespread occurrence (Johansson et al., 2000) and the use of HgCl 2 , a nonspecific inhibitor (Tyerman et al., 2002). Aquaporins, which are found in almost all types of tissues (Maurel, 1997), have changed the way we think about plant water relations (Maurel and Chrispeels, 2001).Water movement through a living organ such as a root or a leaf can take an apoplastic route, which has a low resistance to flow, or a transcellular route, which has a higher resistance because water has to move through lipid bilayer membranes . Bulk water flow associated with the transpiration stream is mostly apoplastic, except in the root exo-and endodermis (Zimmermann et al., 2000) and in the leaf bundle sheath (Koroleva et al., 2002), where apoplastic barriers (Casparian band, suberin lamellae, and secondary cell wall thickening) restrict the apoplastic path. Other important processes such as cell enlargement, refilling of embolized vessels, and movement of guard cells and pulvini may require rapid transport of water across membranes. Furthermore, the considerable growth-associated water potential difference (0.1-0.3 MPa) found in most growing organs of herbaceous plants (e.g. Nonami and Boyer, 1...

Concurrent determinations of changes in hydraulic conductivity and tissue anatomy were made for roots of Agave deserti excised during drying and following rewetting in soil. At 30 d of drought, hydraulic conductivity had declined less than twofold for older nodal roots, tenfold for young nodal roots, and more than 20-fold for lateral roots ("rain roots" occurring as branches on the nodal roots). These decreases were consistent with increases in cortical lacunae caused by cell shrinkage and collapse. Similarly, reduction of lacunae in response to rewetting after 7 d of drought corresponded to levels of recovery in hydraulic conductivity, with young nodal roots showing full recovery, lateral roots returning to only 21% of initial conductivity, and older nodal roots changing only slightly. Increases in suberization in the exodermis, endodermis, and cortex adjacent to the endodermis in response to drying coincided with decreases in hydraulic conductivity. Measurements of axial hydraulic conductance per unit length before and after pressurization indicated that embolism caused reductions in axial conductance of98% for lateral roots, 35% for young nodal roots, and 20% for older nodal roots at 7 d of drought. Embolism, cortical lacunae, and increasing suberization caused hydraulic conductivity to decline during drought in the three root types, thereby helping limit water loss to dry soil; the recovery in hydraulic conductivity for young nodal roots after rewetting would allow them to take up water readily once soil moisture is replenished.

Recent advances in the retrieval of Chl fluorescence from space using passive methods (solar-induced Chl fluorescence, SIF) promise improved mapping of plant photosynthesis globally. However, unresolved issues related to the spatial, spectral, and temporal dynamics of vegetation fluorescence complicate our ability to interpret SIF measurements. We developed an instrument to measure leaf-level gas exchange simultaneously with pulse-amplitude modulation (PAM) and spectrally resolved fluorescence over the same field of view - allowing us to investigate the relationships between active and passive fluorescence with photosynthesis. Strongly correlated, slope-dependent relationships were observed between measured spectra across all wavelengths (F , 670-850 nm) and PAM fluorescence parameters under a range of actinic light intensities (steady-state fluorescence yields, F ) and saturation pulses (maximal fluorescence yields, F ). Our results suggest that this method can accurately reproduce the full Chl emission spectra - capturing the spectral dynamics associated with changes in the yields of fluorescence, photochemical (ΦPSII), and nonphotochemical quenching (NPQ). We discuss how this method may establish a link between photosynthetic capacity and the mechanistic drivers of wavelength-specific fluorescence emission during changes in environmental conditions (light, temperature, humidity). Our emphasis is on future research directions linking spectral fluorescence to photosynthesis, ΦPSII, and NPQ.

Drought-induced changes in root hydraulic conductance (L P ) and mercury-sensitive water transport were examined for distal (immature) and mid-root (mature) regions of Opuntia acanthocarpa. During 45 d of soil drying, L P decreased by about 67% for distal and mid-root regions. After 8 d in rewetted soil, L P recovered to 60% of its initial value for both regions. Axial xylem hydraulic conductivity was only a minor limiter of L P . Under wet conditions, HgCl 2 (50 m), which is known to block membrane water-transport channels (aquaporins), decreased L P and the radial hydraulic conductance for the stele (L R, S ) of the distal root region by 32% and 41%, respectively; both L P and L R, S recovered fully after transfer to 2-mercaptoethanol (10 mm). In contrast, HgCl 2 did not inhibit L P of the mid-root region under wet conditions, although it reduced L R, S by 41%. Under dry conditions, neither L P nor L R, S of the two root regions was inhibited by HgCl 2 . After 8 d of rewetting, HgCl 2 decreased L P and L R, S of the distal region by 23% and 32%, respectively, but L P and L R, S of the mid-root region were unaltered. Changes in putative aquaporin activity accounted for about 38% of the reduction in L P in drying soil and for 61% of its recovery for the distal region 8 d after rewetting. In the stele, changes in aquaporin activity accounted for about 74% of the variable L R, S during drought and after rewetting. Thus, aquaporins are important for regulating water movement for roots of O. acanthocarpa.