A Model for the Development of the Rhizobial and Arbuscular Mycorrhizal Symbioses in Legumes and Its Use to Understand the Roles of Ethylene in the Establishment of these two Symbioses

We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermis–outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants

Endoplasmic microtubules configure the subapical cytoplasm and are required for fast growth of Medicago truncatula root hairs

To investigate the configuration and function of microtubules (MTs) in tip-growing Medicago truncatula root hairs, we used immunocytochemistry or in vivo decoration by a GFP linked to a MT-binding domain. The two approaches gave similar results and allowed the study of MTs during hair development. Cortical MTs (CMTs) are present in all developmental stages. During the transition from bulge to a tip-growing root hair, endoplasmic MTs (EMTs) appear at the tip of the young hair and remain there until growth arrest. EMTs are a specific feature of tip-growing hairs, forming a three-dimensional array throughout the subapical cytoplasmic dense region. During growth arrest, EMTs, together with the subapical cytoplasmic dense region, progressively disappear, whereas CMTs extend further toward the tip. In full-grown root hairs, CMTs, the only remaining population of MTs, converge at the tip and their density decreases over time. Upon treatment of growing hairs with 1 ?M oryzalin, EMTs disappear, but CMTs remain present. The subapical cytoplasmic dense region becomes very short, the distance nucleus tip increases, growth slows down, and the nucleus still follows the advancing tip, though at a much larger distance. Taxol has no effect on the cytoarchitecture of growing hairs; the subapical cytoplasmic dense region remains intact, the nucleus keeps its distance from the tip, but growth rate drops to the same extent as in hairs treated with 1 ?M oryzalin. The role of EMTs in growing root hairs is discussed

Configuration of the microtubule cytoskeleton in elongating fibers of flax (Linum usitatissimum L.)

There are three basic types of plant cell growth: isodiametric, unidirectional diffuse, and tip growth. During plant cell growth, microtubules are present in the cell cortex, appressed against the plasma membrane. It is well documented that these cortical microtubules determine the orientation of cell growth in unidirectional intercalary, growing cells, in which the microtubules are always found perpendicular to the axis of cell elongation. There are indications that bast fiber cells of flax have two types of cell growth within one single cell: unidirectional intercalary and tip growth. Since the ultimate length of fiber cells determines the quality of the flax fiber for industry, we study the growth of these cells. In order to determine whether tip growth occurs or not, we use the following tip growth indicators: the type of cell wall at the cell tip, vesicle accumulation at the cell tip, calcium gradient at the cell tip, configuration of the actin cytoskeleton, and configuration of the microtubule cytoskeleton. Here we report on the microtubule cytoskeleton. We started to study the microtubules in fixed flax fiber cells using immunocytochemistry. Both, sections, as well as enzymatically isolated fibers, were analyzed at increasing distances from the shoot apex. Young fibers from the subapical region exhibit microtubules that are positioned approximately perpendicular to the cell's long axis. After prolonged elongation they are found in a helical orientation. In the two tapering regions of the elongating fiber, the microtubules are more often in a perpendicular orientation than in the middle zone of the cell indicating that within a single fiber more than one growth rate might occur. After elongation ceased, all cortical MTs are positioned approximately parallel to the long axis of the fiber. Based on the changes of the configurations of MTs in growing fibers, it is concluded that flax fibers exhibit coordinated growth first, and then also begin to exhibit intrusive growth at both ends as well. Until now, no observations support tip growth

Configuration of the microtubule cytoskeleton in elongating fibers of flax (Linum usitatissimum L.)

There are three basic types of plant cell growth: isodiametric, unidirectional diffuse, and tip growth. During plant cell growth, microtubules are present in the cell cortex, appressed against the plasma membrane. It is well documented that these cortical microtubules determine the orientation of cell growth in unidirectional intercalary, growing cells, in which the microtubules are always found perpendicular to the axis of cell elongation. There are indications that bast fiber cells of flax have two types of cell growth within one single cell: unidirectional intercalary and tip growth. Since the ultimate length of fiber cells determines the quality of the flax fiber for industry, we study the growth of these cells. In order to determine whether tip growth occurs or not, we use the following tip growth indicators: the type of cell wall at the cell tip, vesicle accumulation at the cell tip, calcium gradient at the cell tip, configuration of the actin cytoskeleton, and configuration of the microtubule cytoskeleton. Here we report on the microtubule cytoskeleton. We started to study the microtubules in fixed flax fiber cells using immunocytochemistry. Both, sections, as well as enzymatically isolated fibers, were analyzed at increasing distances from the shoot apex. Young fibers from the subapical region exhibit microtubules that are positioned approximately perpendicular to the cell's long axis. After prolonged elongation they are found in a helical orientation. In the two tapering regions of the elongating fiber, the microtubules are more often in a perpendicular orientation than in the middle zone of the cell indicating that within a single fiber more than one growth rate might occur. After elongation ceased, all cortical MTs are positioned approximately parallel to the long axis of the fiber. Based on the changes of the configurations of MTs in growing fibers, it is concluded that flax fibers exhibit coordinated growth first, and then also begin to exhibit intrusive growth at both ends as well. Until now, no observations support tip growth