Linking Auxin with Photosynthetic Rate via Leaf Venation.

Land plants lose vast quantities of water to the atmosphere during photosynthetic gas exchange. In angiosperms, a complex network of veins irrigates the leaf, and it is widely held that the density and placement of these veins determines maximum leaf hydraulic capacity and thus maximum photosynthetic rate. This theory is largely based on interspecific comparisons and has never been tested using vein mutants to examine the specific impact of leaf vein morphology on plant water relations. Here we characterize mutants at the Crispoid (Crd) locus in pea (Pisum sativum), which have altered auxin homeostasis and activity in developing leaves, as well as reduced leaf vein density and aberrant placement of free-ending veinlets. This altered vein phenotype in crd mutant plants results in a significant reduction in leaf hydraulic conductance and leaf gas exchange. We find Crispoid to be a member of the YUCCA family of auxin biosynthetic genes. Our results link auxin biosynthesis with maximum photosynthetic rate through leaf venation and substantiate the theory that an increase in the density of leaf veins coupled with their efficient placement can drive increases in leaf photosynthetic capacity.

Auxin-Induced Resistance to Common Scab Disease of Potato Linked to Inhibition of Thaxtomin A Toxicity.

Production of the phytotoxin thaxtomin A by pathogenic Streptomyces spp. is essential for induction of common scab disease in potato. Prior studies have shown that foliar application of sublethal concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) and other auxin or auxin-like compounds significantly reduced severity and occurrence of common scab in subsequently produced tubers. However, the means of disease suppression by these compounds was not known. We confirm the disease suppressive activity of 2,4-D. Detailed tuber physiological examination showed that lenticel numbers, lenticel external dimensions, and periderm thickness and structure, physiological features believed to be critical to Streptomyces scabiei infection, were not substantially changed by 2,4-D treatments, negating a possible mechanism for disease suppression through alteration of these structures. In contrast, our studies show accumulation of 2,4-D in tubers of treated plants occurs and is associated with an enhanced tolerance to thaxtomin A. Applying 2,4-D to cultures of S. scabiei did not significantly alter in vitro growth of the pathogen. Thaxtomin A production by the pathogen was inhibited by 2,4-D, but only at the highest rate tested (1.0 mM), which is at least 200-fold more than is found in 2,4-D treated tubers. These data suggest 2,4-D has no direct effect on the pathogen or its virulence. Confirmatory evidence from studies with Arabidopsis thaliana seedlings demonstrated that the auxins 2,4-D and IAA ameliorate thaxtomin A toxicity. The evidence presented whereby auxin treatment inhibits toxicity of thaxtomin A secreted by the pathogen suggests a novel indirect means of disease suppression.

The role of strigolactones and ethylene in disease caused by Pythium irregulare.

Plant hormones play key roles in defence against pathogen attack. Recent work has begun to extend this role to encompass not just the traditional disease/stress hormones, such as ethylene, but also growth-promoting hormones. Strigolactones (SLs) are the most recently defined group of plant hormones with important roles in plant-microbe interactions, as well as aspects of plant growth and development, although the knowledge of their role in plant-pathogen interactions is extremely limited. The oomycete Pythium irregulare is a poorly controlled pathogen of many crops. Previous work has indicated an important role for ethylene in defence against this oomycete. We examined the role of ethylene and SLs in response to this pathogen in pea (Pisum sativum L.) at the molecular and whole-plant levels using a set of well-characterized hormone mutants, including an ethylene-insensitive ein2 mutant and SL-deficient and insensitive mutants. We identified a key role for ethylene signalling in specific cell types that reduces pathogen invasion, extending the work carried out in other species. However, we found no evidence that SL biosynthesis or response influences the interaction of pea with P. irregulare or that synthetic SL influences the growth or hyphal branching of the oomycete in vitro. Future work should seek to extend our understanding of the role of SLs in other plant interactions, including with other fungal, bacterial and viral pathogens, nematodes and insect pests.

Using excised leaves to screen lucerne for salt tolerance: physiological and cytological evidence.

Salinity affects many physiological processes at all levels of plant structural organization. Being a physiologically and genetically complex trait, salinity tolerance implies a coordinated contribution of multiple mechanisms, making plant screening for salt tolerance extremely difficult. In this work, we show how the use of excised leaves can fulfill that task. We argue that, by adding NaCl directly to the transpiration stream, the protective effects of several mechanisms regulating Na(+) delivery to the shoot are eliminated, enhancing PSII exposure to salinity treatment and resulting in a significant decline in leaf photochemistry (Fv/Fm characteristics). We suggest that measuring Fv/Fm characteristics on excised salt-treated leaves provides an opportunity to evaluate the efficiency of vacuolar Na(+) compartmentation, arguably the most important feature for salt tolerance. We also explain the observed decline in Fv/Fm values as salt-induced structural damage to chloroplasts caused by oxidative stress.