Assimilation of carbon by plants incurs water costs. In the many parts of the world where water is in short supply, plant transpiration efficiency, the ratio of carbon fixation to water loss, is critical to plant survival, crop yield and vegetation dynamics. When challenged by variations in their environment, plants often seem to coordinate photosynthesis and transpiration, but significant genetic variation in transpiration efficiency has been identified both between and within species. This has allowed plant breeders to develop effective selection programmes for the improved transpiration efficiency of crops, after it was demonstrated that carbon isotopic discrimination, Delta, of plant matter was a reliable and sensitive marker negatively related to variation in transpiration efficiency. However, little is known of the genetic controls of transpiration efficiency. Here we report the isolation of a gene that regulates transpiration efficiency, ERECTA. We show that ERECTA, a putative leucine-rich repeat receptor-like kinase (LRR-RLK) known for its effects on inflorescence development, is a major contributor to a locus for Delta on Arabidopsis chromosome 2. Mechanisms include, but are not limited to, effects on stomatal density, epidermal cell expansion, mesophyll cell proliferation and cell-cell contact.

Wheat seedlings were grown in soil of various strengths, obtained by changing the bulk density or the water content of the soil. Leaf expansion and transpiration rate were monitored from emergence until the main stem had 5-7 leaves. Leaf area, and shoot and root dry weights, were negatively correlated with soil strength as measured by penetrometer resistance. The growth of roots was less affected than that of shoots. Leaf expansion was reduced before the first leaf was fully expanded. Relative rates of leaf expansion thereafter were consistently lower at high soil strength, although not always significantly. High soil strength also produced substantially smaller stomatal conductances. All effects were the same whether variations of soil strength were brought about by changes in water content or in bulk density. Three possible causes of reduced shoot growth were examined: (1) a limiting supply of nutrients; or (2) of water, because of a restricted root system; or (3) a reduced carbon supply because of a higher carbon demand from the roots, or because of the low stomatal conductance. We conclude that these are all unlikely explanations for the onset of the effects of soil strength, which were independent of soil phosphorus content, of leaf water potential, and of the amount of carbon reserves in the seed. We suggest that growth of the shoot is primarily reduced in response to some hormonal message induced in the roots when they experience high soil strength.

The translationally controlled tumor protein (TCTP) is an important component of the TOR (target of rapamycin) signaling pathway, the major regulator of cell growth in animals and fungi. TCTP acts as the guanine nucleotide exchange factor of the Ras GTPase Rheb that controls TOR activity in Drosophila melanogaster. We therefore examined the role of Arabidopsis thaliana TCTP in planta. Plant TCTPs exhibit distinct sequence differences from nonplant homologs but share the key GTPase binding surface. Green fluorescent protein reporter lines show that Arabidopsis TCTP is expressed throughout plant tissues and developmental stages with increased expression in meristematic and expanding cells. Knockout of TCTP leads to a male gametophytic phenotype with normal pollen formation and germination but impaired pollen tube growth. Silencing of TCTP by RNA interference slows vegetative growth; leaf expansion is reduced because of smaller cell size, lateral root formation is reduced, and root hair development is impaired. Furthermore, these lines show decreased sensitivity to an exogenously applied auxin analog and have elevated levels of endogenous auxin. These results identify TCTP as an important regulator of growth in plants and imply a function of plant TCTP as a mediator of TOR activity similar to that known in nonplant systems.

SUMMARY Interactions between phytohormones play important roles in the regulation of plant growth and development, but knowledge of the networks controlling hormonal relationships, such as between oxylipins and auxins, is just emerging. Here, we report the transcriptional regulation of two Arabidopsis YUCCA genes, YUC8 and YUC9, by oxylipins. Similarly to previously characterized YUCCA family members, we show that both YUC8 and YUC9 are involved in auxin biosynthesis, as demonstrated by the increased auxin contents and auxin-dependent phenotypes displayed by gain-of-function mutants as well as the significantly decreased IAA levels in yuc8 and yuc8/9 knockout lines. Gene expression data obtained by qPCR analysis and microscopic examination of promoter-reporter lines reveal an oxylipin-mediated regulation of YUC9 expression that is dependent on the COI1 signal transduction pathway. In support of these findings, the roots of the analyzed yuc knockout mutants displayed a reduced response to methyl jasmonate (MeJA). The similar response of the yuc8 and yuc9 mutants to MeJA in cotyledons and hypocotyls suggests functional overlap of YUC8 and YUC9 in aerial tissues, while their function in roots show some specificity, likely in part related to different spatio-temporal expression patterns of the two genes. These results provide evidence for an intimate functional relationship between oxylipin signaling and auxin homeostasis.

The relationship between transpiration ratio (R, ratio of water transpired to carbon fixed) or cabon isotopic discrimination (Δ) and leaf dry-matter mineral content (m) was examined within several C3 species including monocot and dicotyledonous species (Triticum aestivum, Hordeum spontaneum, Helianthus annuus and Nicotiana tabacum) and the C4 species, Sorghum bicolor. In all species, whether grown in the glasshouse or in the field, m was positively correlated to R; consistently, m was positively correlated to Δ in the C3 species, and negatively in sorghum. These genetic correlations between transpiration ratio and mineral content contrast with the absence of such a relationship when variations of R were environmentally induced by changes of atmospheric humidity or carbon dioxide concentration. In those circumstances, and consistent with earlier reports, ash content or its major constituents were remarkably stable and insensitive to a doubling or more of R. The genetic relationships shown in the present study are to some extent brought about by variations in assimilation and transpiration rates per se (i.e. passive mineral uptake), but the data indicate that they involve other quantitatively more important mechanisms, some of which may be related to the control of leaf water content. With the exception of Hordeum spontaneum, potassium was the element contributing the most to the relationship between R, or Δ, and mineral content. Depending on the species other elements showed a significant correlation with R or Δ. Further experiments are needed to unravel the nature of the genetic association between R and m. Meanwhile plant mineral content may be a useful criterion in selection programs to improve transpiration efficiency since its correlation with R was not much lower than that of carbon isotope discrimination.