On the Inside

Root Behavior in Intercropping Systems

The developmental plasticity of the root system is influenced by environmental factors, including the heterogeneous content of minerals such as phosphorus (P) and nitrogen in the soil. In addition to responding to environmental stimuli, however, plants also respond to other plants growing in close proximity. Several studies have shown that roots respond to plant neighbors in a very specific manner depending on the identity of the neighbors. Intercropping is a widely used agricultural system that contributes to more than 50% of the agriculture productivity in developing countries. So far, no studies concerning plant recognition in intercropping systems have been performed. Fang et al. (pp. 1277–1285) have examined what happens to the architecture of the root systems of maize (Zea mays) and soybean (Glycine max) in an intercropping system. They first examined the plant growth responses to P stress of two maize varieties intercropped with soybean in the field and then grew the same plant species in a transparent gel system in which they were able, by means of a three-dimensional scanning laser, to reconstruct the plant root system in three dimensions. These measurements revealed the dynamic in situ root behavior in a nondestructive manner in response to different P treatments. They found that nonkin recognition between maize and soybean plants varied between genotypes. When the maize variety GZ1 (but not maize NE1) was grown with soybean HX3, the roots on each plant tended to avoid each other and became shallower on stratified P supply. This response decreased with increasing distance between plants, suggesting that root interactions were important. This root segregation happened before individual roots came in direct contact with each other. The advantages of root avoidance in maize/soybean intercropping for maize growth under conditions of close planting distance may be a consequence of long-term domestication in an intercropping system under limited soil nutrient conditions and close planting distances.

Seed Color Change during Rice Domestication

The color of the immature seed hull of cultivated rice (Oryza sativa) turns into a straw-white color after full ripening. However, the seed hulls of the wild progenitors of cultivated rice are black at maturity. The black hull is always associated with the seed-shattering habit in wild rice. The black hull serves to camouflage the shattered wild rice seeds amid the dark mud in which they fall. In contrast, the nonshattering seed of cultivated rice is always associated with the straw-white-colored hull. Although black hull coloration is controlled by two or three complementary black hull (Bh) genes, the genetic changes underlying the transition from the black hull of the wild rice to the straw-white hull of cultivated rice remain unknown. Zhu et al. (pp. 1301–1311) report that they have cloned the Bh4 gene, a member of an amino acid transporter family. A 22-bp deletion within Bh4 caused a frame shift and truncated the BH4 protein that led to the straw-white hull phenotype in 94.9% of the straw-white-colored cultivated rice varieties that were screened. Although the prevalent mutation of Bh4 was the 22-bp deletion within the third exon, there were other mutations in this gene that led to the loss of black hull pigment, suggesting that different mutations occurred in the Bh4 gene during rice domestication and that this phenotypic trait might have had multiple origins.

Improving Plant-Derived Oils

The demand for vegetable oil is increasing both for human consumption and, more recently, for use as biofuels. One way to meet this demand without devoting additional land to agriculture is to increase the yield of oil per acre. Two contributions in this issue provide interesting examples of how genetic engineering is contributing to the qualitative and quantitative improvements in the quality of plant oils.

Maize kernel oil is composed primarily of triacylglycerol and is used as an energy source by the plant during seed germination. Most of the oil in a maize seed is found in the embryo, which is about 33% oil by weight. Breeding approaches to increase maize kernel oil have had some success, but high kernel oil concentration has been linked to a decrease in grain yield. To increase kernel oil content in maize seeds, Oakes et al. (pp. 1146–1157) introduced one of two types (as well as some truncated forms) of fungal diacylglycerol acyltransferase2 (DGAT2) genes from Umbelopsis ramanniana and Neurospora crassa into the maize genome using an embryo-enhanced promoter. DGAT catalyzes the final step of triacylglycerol biosynthesis by transferring an acyl group from acyl-CoA to the sn-3 position of 1,2-diacylglycerol. They report that the expression of the DGAT2 transgenes in maize resulted in small but statistically significant increases in kernel oil. No effect on hybrid grain yield from overexpression of these transgenes was observed. Importantly, oil increases were obtained from overexpression of the fungal DGATs in high-oil hybrids, indicating the possibility of further enhancing oil biosynthesis in high-oil germplasm. An unexpected seedling phenotype (the sticking together of primary leaves) was observed in some of the transgenic events and may provide insight into other roles of DGAT during development.

A second contribution reports some fascinating effects that nonsymbiotic plant hemoglobins have upon the oil quantity and quality of Arabidopsis (Arabidopsis thaliana) seeds. Hemoglobins are an ancient class of oxygen-binding proteins, ubiquitous in nature and expressed in organisms as different as animals, bacteria, fungi, and plants. The first plant hemoglobins to be discovered were leghemoglobins, which are highly abundant proteins in root nodules of legume plants infected with symbiotic nitrogen-fixing bacteria. Nonsymbiotic plant hemoglobins have been found in various plant tissues and plant species, especially in crop plants. Different classes of nonsymbiotic plant hemoglobins have been identified and divided into class 1 (Hb1) and class 2 (Hb2) based on phylogenetic characteristics, gene expression pattern, and oxygen-binding properties. Class 1 and class 2 hemoglobins markedly differ in their oxygen-binding affinity, with the respective K_m being 1 to 2 nm for Hb1 and 150 nm for Hb2. Expression of Hb1 is very low under normal conditions but is strongly induced by hypoxic stress. Under these stress conditions, overexpression of Hb1 leads to a decrease in nitric oxide, maintenance of the cellular energy status and growth, and increased survival of the plants. Relatively little is known concerning the function of Hb2 in plants. Vigeolas et al. (pp. 1435–1444) have generated transgenic Arabidopsis plants that overexpress endogenous class 1 or class 2 hemoglobins specifically in developing seeds under control of a seed-specific promoter. They report that overexpressing Arabidopsis hemoglobin-2 (AHb2), but not AHb1, led to an increase in the lipid content of the seeds that was mainly attributable to an increase in the level of polyunsaturated fatty acids. These changes were accompanied by an increase in the adenylate energy state and the Suc content of the seeds. Under low external oxygen, AHb2 overexpression maintained an up to 5-fold higher energy state and prevented fermentation. This is consistent with the idea that AHb2 overexpression results in improved oxygen availability within developing seeds. Thus, the results indicate a specific role of class 2 hemoglobins in promoting the accumulation of polyunsaturated fatty acids, which have important functions in the response of the plant to cold stress and in human health.

Target of Rapamycin Controls rRNA Expression and Plant Development

Target of Rapamycin (TOR) encodes a large (280 kD) Ser/Thr protein kinase and plays a central role as a regulator of cell growth, cell death, nutrition, starvation, and hormone and stress responses in a wide diversity of eukaryotic cells. One of the key functions of TOR is its involvement in ribosome biogenesis, a process that requires the coordinated production of several ribosomal components, including four different rRNAs and about 130 ribosomal proteins. TOR has been implicated to play an important regulatory role in the expression of several of these components. TOR is structurally and functionally conserved in eukaryotic species, but very little is known about TOR signaling and TOR's functional domains in plants. Insensitivity to rapamycin has been a major impediment to making progress concerning the biological effects of TOR activity in plants. The Arabidopsis genome contains a single copy of TOR, and loss-of-function mutations cause embryo lethality in Arabidopsis. Ren et al. (pp. 1367–1382) have characterized TOR functional domains required for plant growth and development and show that the kinase domain plays critical roles in development, nuclear localization, and rRNA expression in Arabidopsis. They report that the kinase domain is essential for the role of TOR in embryogenesis and 45S rRNA expression. Genetic complementation showed that the TOR kinase domain alone in the tor-10/tor-10 mutant background can rescue early embryo lethality and restore normal development. Overexpression of full-length TOR or kinase domain in Arabidopsis displayed developmental abnormalities such as delayed flowering, delayed senescence, and tuberous roots.

Multigene Family of Plant Myosins

The recent sequencing of several complete genomes of green algae, mosses, dicots, and monocots has provided the opportunity for a much deeper insight into myosin evolution and classification in the Viridiplantae. Plants possess two myosin classes, VIII and XI. The myosins XI have been implicated in organelle transport, F-actin organization, and cell and plant growth. It has also been reported that flowering plants generally possess larger families of myosin genes than lower plants. For instance, Arabidopsis has 13 class XI myosin motors compared with only two in moss Physcomitrella patens. Because of the large size of myosin gene families, knowledge of these molecular motors remains sketchy. The first genome-wide characterization of the myosins XI in Arabidopsis yielded a surprising outcome: None of the 13 myosin gene single knockouts had more than very subtly altered phenotypes under optimal growth conditions, suggesting that the functions of myosins in plants are redundant. Using deep transcriptome sequencing and bioinformatics, Peremyslov et al. (pp. 1191–1204) have systematically investigated the genome-wide patterns of splicing, expression, and phylogeny of the plant myosin genes in two model plants, Arabidopsis and Brachypodium distachyon. Among their many findings is that there are two predominant patterns of the myosin gene expression, namely pollen/stamen-specific and ubiquitous expression throughout the plant. They also report that some myosin genes are alternatively spliced, and some are expressed in a circadian manner. A novel, myosin XI-K-derived gene of Arabidopsis that is expressed preferentially in the emerging vascular tissue is also described. The authors have also constructed a detailed phylogenetic tree of plant myosins that sheds new light on myosin evolution and classification. These phylogenetic reconstructions indicate that the last common ancestor of the angiosperms possessed two myosins VIII and five myosins XI, many of which underwent additional lineage-specific duplications.