Regulation of stem initiation and its role in plant architecture

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Development of a vertical shoot axis capable of bearing organs above the ground was one of the key steps in the evolution of land plants. In spite of its ancient and central role in shaping plants, formation of the stem remains one of the least understood processes in plant development. This process also has practical importance: mutations that reduce stem growth have been widely used to improve crop yield but also have undesired side effects on plant growth, for example during seed germination. A better understanding of how genes control stem growth is required to develop more precise genetic tools to increase plant productivity by modifying plant height and shape.One way to develop new tools to modify plant height is to study how genes modulate stem growth at different stages of the plant's life. This modulation is seen markedly in plants with a rosette habit, such as Arabidopsis, beet, radish, lettuce and cabbage, in which stem elongation is initially inhibited, but later activated during flowering. This transition is triggered by environmental conditions such as day length and temperature, and initiates growth of the stem in a specific region of the shoot apex, called the rib zone (RZ). Our laboratory has been studying how the RZ functions in the in the reference plant Arabidopsis. We found that a gene called ATH1 has a key role in controlling when the stem is formed: ATH1 is initially active in the RZ to prevent stem growth but is inhibited by flowering signals to initiate the stem. However, it is not known how flowering signals control ATH1 and how ATH1 inhibits stem growth. Our initial results indicate that ATH1 controls stem initiation by acting as a "gatekeeper" for two hormone signals that are known to promote stem growth: gibberellin and brassinosteroid. To test this idea, we propose to follow how ATH1 affects genes involved in signalling by these hormones. In addition, we will test whether these genes mediate the effects of ATH1 on stem growth. We also aim to understand how flowering signals connect to ATH1 to co-ordinate flower development and stem elongation. We will initially focus on regulatory sequences within the ATH1 gene, which suggest direct links to known regulators of flowering. We will introduce mutations in these sequences and look for plants in which stem growth is uncoupled from flowering. Depending on the regulatory sequences involved, we will then test whether they mediate the input of specific regulatory proteins that regulate the transition to flowering.Another motivation for studying regulatory sequences in ATH1 is that they may play a role in existing diversity in stem growth in vegetable Brassicas, and could be used to create useful, new variation in plant height and shape. Previous work showed that variation in stem height in Brassica crops is associated with differences in the genomic region containing ATH1, and that crops with a rosette habit (cabbage, kale) have specific differences in ATH1 in comparison to those with elongated stems. To test whether these differences are responsible for variation in stem growth, we will generate plants that carry ATH1 from long-or short-stemmed Brassica, but are otherwise genetically identical. If existing variation in Brassica ATH1 cannot explain differences in stem height, we will use the findings from Arabidopsis to generate novel regulatory mutations to uncouple stem growth from flowering in Brassica. Overall, this work will reveal the genetic mechanism controlling stem initiation, will shed light on how a variety of plant shapes have been selected during crop breeding, and will produce novel genetic tools to control the height and shape of plants.