Flooding of soils results in acute oxygen deprivation (anoxia) of plant roots during winter in temperate latitudes, or after irrigation, and is a major problem for agriculture. One early response of plants to anoxia and other environmental stresses is downregulation of water uptake due to inhibition of the water permeability (hydraulic conductivity) of roots (Lp(r)). Root water uptake is mediated largely by water channel proteins (aquaporins) of the plasma membrane intrinsic protein (PIP) subgroup. These aquaporins may mediate stress-induced inhibition of Lp(r) but the mechanisms involved are unknown. Here we delineate the whole-root and cell bases for inhibition of water uptake by anoxia and link them to cytosol acidosis. We also uncover a molecular mechanism for aquaporin gating by cytosolic pH. Because it is conserved in all PIPs, this mechanism provides a basis for explaining the inhibition of Lp(r) by anoxia and possibly other stresses. More generally, our work opens new routes to explore pH-dependent cell signalling processes leading to regulation of water transport in plant tissues or in animal epithelia.

The arbuscular mycorrhizal (AM) symbiosis is a mutualistic endosymbiosis formed by plant roots and AM fungi. Most vascular flowering plants have the ability to form these associations, which have a significant impact on plant health and consequently on ecosystem function. Nutrient exchange is a central feature of the AM symbiosis, and AM fungi obtain carbon from their plant host while assisting the plant with the acquisition of phosphorus (as phosphate) from the soil. In the AM symbiosis, the fungus delivers P i to the root through specialized hyphae called arbuscules. The molecular mechanisms of Pi and carbon transfer in the symbiosis are largely unknown, as are the mechanisms by which the plant regulates the symbiosis in response to its nutrient status. Plants possess many classes of P i transport proteins, including a unique clade (Pht1, subfamily I), members of which are expressed only in the AM symbiosis. Here, we show that MtPT4, a Medicago truncatula member of subfamily I, is essential for the acquisition of P i delivered by the AM fungus. However, more significantly, MtPT4 function is critical for AM symbiosis. Loss of MtPT4 function leads to premature death of the arbuscules; the fungus is unable to proliferate within the root, and symbiosis is terminated. Thus, Pi transport is not only a benefit for the plant but is also a requirement for the AM symbiosis.biotrophic ͉ membrane ͉ mineral nutrition ͉ mutualism ͉ root P lants and their arbuscular mycorrhizal (AM) fungal symbionts have coexisted for Ͼ400 million years (1), and the association is widespread in terrestrial ecosystems. AM symbiosis provides multiple benefits for the plant, not only enhanced phosphorus and nitrogen nutrition but also tolerance to pathogens and abiotic stresses (2, 3). For AM fungi, formation of a symbiosis is an obligate requirement. Their large spores contain reserves sufficient to support growth of a hyphal germ tube, but they must form an association with a plant to acquire additional carbon to complete their life cycle (4). Development of symbiosis, in particular intraradical development of the fungus in the root cortex, is reduced if P i availability is high (5-7). This suggests that the plant has a mechanism of regulating fungal growth, presumably to avoid the unnecessary allocation of carbon resources. The molecular basis is unknown but could occur by the control of carbon allocation (8).In the AM symbiosis, the fungal hyphae grow through the intercellular spaces of the root and subsequently invade the inner cortical cells, developing branched hyphae, called arbuscules, within the cells (9, 10). As each arbuscule forms, the plant cell envelops it in a membrane, the periarbuscular membrane, and the result is an extensive plant-fungal interface specialized for nutrient exchange (11)(12)(13)(14). It is predicted that P i and carbon transfer occur at the arbuscule/cortical cell interface, although direct evidence for carbon transfer at this location is lacking (14). Current data suggest that P i is translocated through the ...

Compartmentation of the eukaryotic cell requires a complex set of subcellular messages, including multiple retrograde signals from the chloroplast and mitochondria to the nucleus, to regulate gene expression. Here, we propose that one such signal is a phosphonucleotide (39-phosphoadenosine 59-phosphate [PAP]), which accumulates in Arabidopsis thaliana in response to drought and high light (HL) stress and that the enzyme SAL1 regulates its levels by dephosphorylating PAP to AMP. SAL1 accumulates in chloroplasts and mitochondria but not in the cytosol. sal1 mutants accumulate 20-fold more PAP without a marked change in inositol phosphate levels, demonstrating that PAP is a primary in vivo substrate. Significantly, transgenic targeting of SAL1 to either the nucleus or chloroplast of sal1 mutants lowers the total PAP levels and expression of the HL-inducible ASCORBATE PEROXIDASE2 gene. This indicates that PAP must be able to move between cellular compartments. The mode of action for PAP could be inhibition of 59 to 39 exoribonucleases (XRNs), as SAL1 and the nuclear XRNs modulate the expression of a similar subset of HL and drought-inducible genes, sal1 mutants accumulate XRN substrates, and PAP can inhibit yeast (Saccharomyces cerevisiae) XRNs. We propose a SAL1-PAP retrograde pathway that can alter nuclear gene expression during HL and drought stress.