Background: Most vascular flowering plants have the capacity to form symbiotic associations with arbuscular mycorrhizal (AM) fungi. The symbiosis develops in the roots where AM fungi colonize the root cortex and form arbuscules within the cortical cells. Arbuscules are enveloped in a novel plant membrane and their establishment requires the coordinated cellular activities of both symbiotic partners. The arbuscule-cortical cell interface is the primary functional interface of the symbiosis and is of central importance in nutrient exchange. To determine the molecular events the underlie arbuscule development and function, it is first necessary to identify genes that may play a role in this process. Toward this goal we used the Affymetrix GeneChip ® Medicago Genome Array to document the M. truncatula transcript profiles associated with AM symbiosis, and then developed laser microdissection (LM) of M. truncatula root cortical cells to enable analyses of gene expression in individual cell types by RT-PCR.

SummaryThe identification of leaf wax genes involved in stress tolerance is expected to have great potential for crop improvement. Here we report the characterization of a novel AP2 domain-containing putative transcription factor gene from the model legume Medicago truncatula. The gene, designated WXP1, is able to activate wax production and confer drought tolerance in alfalfa (Medicago sativa), the most important forage legume species in the world and a close relative of M. truncatula. The predicted protein of WXP1 has 371 aa; it is one of the longest peptides of all the single AP2 domain proteins in M. truncatula. WXP1 is distinctly different from the most studied genes in the AP2/ERF transcription factor family such as AP2s, CBF/DREB1s, DREB2s, WIN1/ SHN1 and GL15. Transcript level of WXP1 is inducible by cold, abscisic acid and drought treatment mainly in shoot tissues in M. truncatula. Overexpression of WXP1 under the control of the CaMV35S promoter led to a significant increase in cuticular wax loading on leaves of transgenic alfalfa. Scanning electron microscopy revealed earlier accumulation of wax crystals on the adaxial surface of newly expanded leaves and higher densities of wax crystalline structures on both adaxial and abaxial surfaces of mature leaves. Gas chromatography-mass spectrometry analysis revealed that total leaf wax accumulation per surface area increased 29.6-37.7% in the transgenic lines, and the increase was mainly contributed by C30 primary alcohol. WXP1 overexpression induced a number of wax-related genes. Transgenic leaves showed reduced water loss and chlorophyll leaching. Transgenic alfalfa plants with increased cuticular waxes showed enhanced drought tolerance demonstrated by delayed wilting after watering was ceased and quicker and better recovery when the dehydrated plants were re-watered.

SummaryTo understand how auxin regulates root growth, we quantified cell division and elemental elongation, and examined actin organization in the primary root of Arabidopsis thaliana. In treatments for 48 h that inhibited root elongation rate by 50%, we find that auxins and auxin-transport inhibitors can be divided into two classes based on their effects on cell division, elongation and actin organization. Indole acetic acid (IAA), 1-naphthalene acetic acid (NAA) and tri-iodobenzoic acid (TIBA) inhibit root growth primarily through reducing the length of the growth zone rather than the maximal rate of elemental elongation and they do not reduce cell production rate. These three compounds have little effect on the extent of filamentous actin, as imaged in living cells or by chemical fixation and immuno-cytochemistry, but tend to increase actin bundling. In contrast, 2,4-dichlorophenoxy-acetic acid (2,4-D) and naphthylphthalamic acid (NPA) inhibit root growth primarily by reducing cell production rate. These compounds remove actin and slow down cytoplasmic streaming, but do not lead to mislocalization of the auxin-efflux proteins, PIN1 or PIN2. The effects of 2,4-D and NPA were mimicked by the actin inhibitor, latrunculin B. The effects of these compounds on actin were also elicited by a 2 h treatment at higher concentration but were not seen in two mutants, eir1-1 and aux1-7, with deficient auxin transport. Our results show that IAA regulates the size of the root elongation zone whereas 2,4-D affects cell production and actin-dependent processes; and, further, that elemental elongation and localization of PINs are appreciably independent of actin.

Roots are crucial for nutrient and water acquisition and can be targeted to enhance plant productivity under a broad range of growing conditions. A current challenge for plant breeding is the limited ability to phenotype and select for desirable root characteristics due to their underground location. Plant breeding efforts aimed at modifying root traits can result in novel, more stress-tolerant crops and increased yield by enhancing the capacity of the plant for soil exploration and, thus, water and nutrient acquisition. Available approaches for root phenotyping in laboratory, greenhouse and field encompass simple agar plates to labor-intensive root digging (i.e., shovelomics) and soil boring methods, the construction of underground root observation stations and sophisticated computer-assisted root imaging. Here, we summarize root architectural traits relevant to crop productivity, survey root phenotyping strategies and describe their advantages, limitations and practical value for crop and forage breeding programs.

A reverse genetic approach was used to investigate the functions of three members of the cellulose synthase superfamily in Arabidopsis (Arabidopsis thaliana), CELLULOSE SYNTHASE-LIKE D1 (CSLD1), CSLD2, and CSLD4. CSLD2 is required for normal root hair growth but has a different role from that previously described for CSLD3 (KOJAK). CSLD2 is required during a later stage of hair development than CSLD3, and CSLD2 mutants produce root hairs with a range of abnormalities, with many root hairs rupturing late in development. Remarkably, though, it was often the case that in CSLD2 mutants, tip growth would resume after rupturing of root hairs. In silico, semiquantitative reverse transcription-polymerase chain reaction, and promoter-reporter construct analyses indicated that the expression of both CSLD2 and CSLD3 is elevated at reduced temperatures, and the phenotypes of mutants homozygous for insertions in these genes were partially rescued by reduced temperature growth. However, this was not the case for a double mutant homozygous for insertions in both CSLD2 and CSLD3, suggesting that there may be partial redundancy in the functions of these genes. Mutants in CSLD1 and CSLD4 had a defect in male transmission, and plants heterozygous for insertions in CSLD1 or CSLD4 were defective in their ability to produce pollen tubes, although the number and morphology of pollen grains was normal. We propose that the CSLD family of putative glycosyltransferases synthesize a polysaccharide that has a specialized structural role in the cell walls of tip-growing cells.