Phyllotaxis.

Leaves and flowers are arranged in regular patterns around the stem of a plant, a phenomenon known as phyllotaxis. Different arrangements occur, such as distichous, decussate or spiral (Figure 1). Most prevalent in nature are spirals in which the average divergence angles between successive organs are close to 137.5°, the so-called 'golden angle'. It is this exact number that has given phyllotaxis its special flavor as a quantitative developmental problem, and over the centuries, it has enjoyed the attention of scientists far beyond botany. In the 1830s mathematicians described the spirals as they related to the Fibonacci numbers, and in the 1860s improved microscopes made it possible for botanists to observe the initiation of leaf and flower primordia in a diversity of plants. This descriptive work led to the conclusion that new organ primordia form in the first available space between existing primordia, a conclusion still valid today. But how does it work? Ideas from the early 20 century suggested that an inhibitor produced by existing primordia diffuses towards the shoot apical meristem: where the concentration of the inhibitor falls below a threshold value, an organ is initiated. Other models dating back to the 1870s have tried to explain phyllotactic patterning by applying the laws of mechanics. Such models went through a long period of marginal interest, but have experienced a remarkable renaissance over the past 20 years. In this Primer I will give a broad overview of phyllotaxis, its emergence from the shoot apical meristem, how auxin and its transporter function as a 'pattern generator', and the role of tissue mechanics and computational modeling.