The tips have been covered so light cannot reach them. Auxin is in the same concentration on both sides of the shoots, so they grow evenly and longer on both sides. One side of the tips are in more light than the other side.
Auxin is in a greater concentration on the shaded side, causing the cells there to grow longer than the cells on the lit side. Does the hobbyist at home know what auxin is?
By understanding the science behind the art, home gardeners can generate a positive impact on their home landscapes. There are 4 basic questions that must be asked about auxin. They are: 1. What is auxin? What are the major functions of auxin? What are the effects of auxin on plants? Other scientists, including Boyen-Jensen, Paal, and Went, independently used the same experimental system to show that the bending was promoted by a mobile signal that was hydrophilic in nature, and this signal was finally identified as IAA [ 1 ].
These early discoveries have spurred the development of a lively and active research field that has made remarkable discoveries in the past decades. It appears that auxin affects almost all developmental steps in plants from early embryogenesis to fruit ripening and controls organogenesis at the meristems, which define plant architecture.
Nowadays, research focuses on understanding how such a small molecule can be ubiquitous and at the same time have context-dependent function. No, you do not find the same amount of auxin in all the tissues of a plant.
In fact, the uneven auxin distribution is a key factor for proper development. IAA concentrations can differ by an order of magnitude between shoot and root and appear highest in meristems located at the tip of the roots and the shoots. Even though many cell types seem able to produce auxin [ 2 ], the capacity in young leaves is comparatively high. This freshly made auxin is then transported from source organs such as young leaves to sink organs such as meristems where auxin accumulates.
In those organs, levels of auxin differ between cell types [ 3 ]. The precise positioning of these auxin channels orient auxin flux and create heterogeneity for IAA distribution. High auxin levels are especially present in the center of the root meristem, also known as the quiescent center where stem cells are embedded.
From this zone auxin concentration tends to decrease. Hence, a gradient forms that is thought to be crucial for establishing a proper developmental pattern and maintain stem cell niches. Homeostasis of auxin is also regulated by auxin conjugation.
IAA is then coupled to an amino acid and stored or degraded. Any modification of its homeostasis will lead to dramatic phenotypic changes, as exemplified by mutants that overproduce auxin e. These phenotypes highlight the importance of controling auxin distribution in plant development. However, how the cell senses IAA and responds to auxin stimulus are also critical questions. Auxin homeostasis is essential for proper development. Drawings of a wild-type seedling and an adult plant center of the figure in green and some characterized mutants for auxin homeostasis in boxes.
Mutants with affected auxin biosynthesis are in light brown boxes. Mutants for influx aux1 [ 28 ] and efflux auxin transporters pin1 [ 7 ] are shown in the light red boxes. The central role of auxin in plant development makes the quest towards understanding the mechanisms underlying its action a fascinating and challenging one. Many studies in the past decades have led to a comprehensive insight into how auxin is perceived and how cells respond to auxin.
It is well established that auxin can trigger very fast non-transcriptional responses, such as activation of the plasma membrane proton pump and ion channels, as well as the reorientation of microtubules [ 8 ].
On the other hand, it has become clear that many of the developmental responses to auxin are mediated by changes in the expression of thousands of genes [ 9 ]. These non-transcriptional and transcriptional responses may be interconnected and a major future challenge will be to define how cells merge these two pathways.
Since the late 80s and the use of genetics in the model plant Arabidopsis , impressive progress has been made in the understanding of transcriptional auxin signaling and many components have been identified. Surprisingly, it appears that only three dedicated molecular components are required to reconstruct a minimal nuclear auxin pathway NAP in yeast [ 10 ]. The first of these are the DNA-binding auxin response factors ARFs , which are in charge of regulating auxin-dependent genes. This simple system seems to account for much of the regulation in auxin-dependent gene expression Fig.
The nuclear auxin pathway NAP is the machinery that controls auxin gene expression. These co-receptors will subsequently be degraded, allowing the ARF to modulate auxin-related gene expression. These do not seem to be part of the NAP and their precise role in auxin signaling is still a matter of debate [ 11 ]. This question is perhaps the most intriguing and pressing one in current auxin research. Several new studies have provided insights that help address this question. Even if the NAP appears rather simple, it is in fact highly feedback-regulated and encapsulates much diversity, such that a complex array of outputs can be generated [ 9 ].
Besides this differential expression, biochemical studies, as well as reconstruction experiments in a synthetic yeast system, also demonstrated that these different actors of the NAP have different intrinsic properties, for example, in interaction affinities among the three core components [ 9 ]. These specific interaction capacities bring an incredible complexity to the NAP and seem to be a cornerstone that gives dynamic range and specificity to the system.
They are not interchangeable and mutations lead to distinct phenotypes, indicating that they are important factors in determining pathway specificity [ 13 ]. Indeed, the set of DNA binding motifs identified for ARFs has recently been expanded using genome-wide analysis and in vitro approaches [ 14 — 16 ]. Even though ARF proteins have similar intrinsic DNA binding specificities, it recently became apparent that differences in DNA binding properties may arise from the ARFs binding DNA as dimers with distinct preference for spacing between two adjacent binding sites [ 14 , 15 ].
The tips have been removed. No light reaches the tips. More light reaches one side of the tips. No auxin is produced. Equal concentration of auxin on both sides. Greater concentration of auxin on the shaded side.
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