Effective regulation of water balance in plants requires localized extracellular barriers that control water and solute movement. We describe a clade of five Arabidopsis thaliana ABCG half-transporters that are required for synthesis of an effective suberin barrier in roots and seed coats (ABCG2, ABCG6, and ABCG20) and for synthesis of an intact pollen wall (ABCG1 and ABCG16). Seed coats of abcg2 abcg6 abcg20 triple mutant plants had increased permeability to tetrazolium red and decreased suberin content. The root system of triple mutant plants was more permeable to water and salts in a zone complementary to that affected by the Casparian strip. Suberin of mutant roots and seed coats had distorted lamellar structure and reduced proportions of aliphatic components. Root wax from the mutant was deficient in alkylhydroxycinnamate esters. These mutant plants also had few lateral roots and precocious secondary growth in primary roots. abcg1 abcg16 double mutants defective in the other two members of the clade had pollen with defects in the nexine layer of the tapetum-derived exine pollen wall and in the pollen-derived intine layer. Mutant pollen collapsed at the time of anther desiccation. These mutants reveal transport requirements for barrier synthesis as well as physiological and developmental consequences of barrier deficiency.

Water and nitrogen availability limit crop productivity globally more than most other environmental factors. Plant availability of macronutrients such as nitrate is, to a large extent, regulated by the amount of water available in the soil, and, during drought episodes, crops can become simultaneously water and nitrogen limited. In this review, we explore the intricate relationship between water and nitrogen transport in plants, from transpiration-driven mass flow in the soil to uptake by roots via membrane transporters and channels and transport to aerial organs. We discuss the roles of root architecture and of suberized hydrophobic root barriers governing apoplastic water and nitrogen movement into the vascular system. We also highlight the need to identify the signalling cascades regulating water and nitrogen transport, as well as the need for targeted physiological analyses of plant traits influencing water and nitrogen uptake. We further advocate for incorporation of new phenotyping technologies, breeding strategies, and agronomic practices to improve crop yield in water- and nitrogen-limited production systems.

Aliphatic alcohols naturally exist in many organisms as important cellular components; however, their roles in extracellular polymer biosynthesis are poorly defined. We report here the isolation and characterization of a rice (Oryza sativa) male-sterile mutant, defective pollen wall (dpw), which displays defective anther development and degenerated pollen grains with an irregular exine. Chemical analysis revealed that dpw anthers had a dramatic reduction in cutin monomers and an altered composition of cuticular wax, as well as soluble fatty acids and alcohols. Using map-based cloning, we identified the DPW gene, which is expressed in both tapetal cells and microspores during anther development. Biochemical analysis of the recombinant DPW enzyme shows that it is a novel fatty acid reductase that produces 1-hexadecanol and exhibits >270-fold higher specificity for palmiltoyl-acyl carrier protein than for C16:0 CoA substrates. DPW was predominantly targeted to plastids mediated by its N-terminal transit peptide. Moreover, we demonstrate that the monocot DPW from rice complements the dicot Arabidopsis thaliana male sterile2 (ms2) mutant and is the probable ortholog of MS2. These data suggest that DPWs participate in a conserved step in primary fatty alcohol synthesis for anther cuticle and pollen sporopollenin biosynthesis in monocots and dicots.

Temperatures higher than the optimum negatively affects plant growth and development. Tolerance to high temperature is a complex process that involves several pathways. Understanding this process, especially in crops such as rice, is essential to prepare for predicted climate changes due to global warming. Here, we show that OsMYB55 is induced by high temperature and overexpression of OsMYB55 resulted in improved plant growth under high temperature and decreased the negative effect of high temperature on grain yield. Transcriptome analysis revealed an increase in expression of several genes involved in amino acids metabolism. We demonstrate that OsMYB55 binds to the promoter regions of target genes and directly activates expression of some of those genes including glutamine synthetase (OsGS1;2) glutamine amidotransferase (GAT1) and glutamate decarboxylase 3 (GAD3). OsMYB55 overexpression resulted in an increase in total amino acid content and of the individual amino acids produced by the activation of the above mentioned genes and known for their roles in stress tolerance, namely L-glutamic acid, GABA and arginine especially under high temperature condition. In conclusion, overexpression of OsMYB55 improves rice plant tolerance to high temperature, and this high tolerance is associated with enhanced amino acid metabolism through transcription activation.

Rice is an important crop that is very sensitive to salinity. However, some varieties differ greatly in this feature, making investigations of salinity tolerance mechanisms possible. The cultivar Pokkali is salinity tolerant and is known to have more extensive hydrophobic barriers in its roots than does IR20, a more sensitive cultivar. These barriers located in the root endodermis and exodermis prevent the direct entry of external fluid into the stele. However, it is known that in the case of rice, these barriers are bypassed by most of the Na⁺ that enters the shoot. Exposing plants to a moderate stress of 100 mM NaCl resulted in deposition of additional hydrophobic aliphatic suberin in both cultivars. The present study demonstrated that Pokkali roots have a lower permeability to water (measured using a pressure chamber) than those of IR20. Conditioning plants with 100 mM NaCl effectively reduced Na⁺ accumulation in the shoot and improved survival of the plants when they were subsequently subjected to a lethal stress of 200 mM NaCl. The Na⁺ accumulated during the conditioning period was rapidly released when the plants were returned to the control medium. It has been suggested that the location of the bypass flow is around young lateral roots, the early development of which disrupts the continuity of the endodermal and exodermal Casparian bands. However, in the present study, the observed increase in lateral root densities during stress in both cultivars did not correlate with bypass flow. Overall the data suggest that in rice roots Na⁺ bypass flow is reduced by the deposition of apoplastic barriers, leading to improved plant survival under salt stress.