Plant architecture

Abstract

Plant architecture is species specific, indicating that it is under strict genetic control. Although it is also influenced by environmental conditions such as light, temperature, humidity and nutrient status, here we wish to focus only on the endogenous regulatory principles that control plant architecture. We summarise recent progress in the understanding of the basic patterning mechanisms involved in the regulation of leaf arrangement, the genetic regulation of meristem determinacy, i.e. the decision to stop or continue growth, and the control of branching during vegetative and generative development. Finally, we discuss the basis of leaf architecture and the role of cell division and cell growth in morphogenesis.

Fig. 1. Regulation of phyllotaxis. (A) Distichous phyllotaxis in Trisetum distichophyllum. Leaves diverge by 180° and alternate in two opposite rows. (B) Decussate phyllotaxis in Solenostemon scutellarioides. Pairs of opposite leaves are formed. Successive leaf pairs diverge by 90°. (C) Spiral phyllotaxis in Aloe polyphylla. Successive leaves are initiated with a divergence angle of 137°. Note that apparent spirals (parastichies; see Steeves and Sussex, 1989) are due to dense packing rather than the sequence of leaf formation. (D) The shoot apex of a tomato plant with the youngest leaf primordia in spiral succession (P₁, P₂, P₃ and the base of P₄) and the shoot apical meristem (M). (E) Model of phyllotactic regulation by an inhibitor (arrows) emanating from young primordia (P₁ and P₂). P₂ is surrounded by a weaker inhibitory field than P₁; thus I₁, is initiated closer to P₂. (F) Local administration of IAA (red paste) to the tip of an Arabidopsis pinformed1 mutant apex induces organ formation. (G–I) Model of auxin transport in phyllotaxis. Auxin is transported into the meristem, where it is absorbed by the pre-existing primordia, leading to accumulation of auxin and, consequently, organ formation at a certain minimal distance (I₁). If only the youngest primordium absorbs auxin, distichous phyllotaxis is established (G). If mainly P₁ but also, to a lesser extent, P₂ absorb auxin, spiral phyllotaxis results (H). If the size of the meristem allows for two auxin maxima to coexist, then pairs of opposite leaves are formed, resulting in decussate phyllotaxis (I). Subsequent pairs of leaves will diverge by 90°. (D) and (F) reprinted with permission from Reinhardt et al. (2000) © 2000 American Society of Plant Biologists.

Fig. 2. Regulation of branching, apical dominance and determinacy. (A) Organisation of a prototypical monopodial plant. The SAM (yellow) remains active during the entire life span of the plant. Depending on the developmental stage of the plant, axillary shoots (blue) form leaves or flowers; later, they are entirely transformed into flowers (top part). (B) Wild-type maize plant. (C) The maize mutant teosinte branched1 (tb1) [reprinted with permission from Doebley et al. (1997) © 1997 Macmillan Publishers Ltd]. (D) The flo mutant (left) versus wild type (right) [reprinted with permission from Coen et al. (1990) © 1997 Elsevier Science].

Fig. 3. Sympodial growth of the shoot apex in the Solanaceae. (A) Apex in the top view of a tobacco plant at the onset of flowering. The SAM (red) has undergone floral determination. Axillary meristems (blue) from the youngest leaves (green; removed) grow out in spiral succession. (B) Tomato apex as in (A). Note that the inflorescence meristem is subtended only by a rudimentary primordium (inset, arrow). (C–E) Schematic representation of sympodial development in tobacco (C), tomato (D) and petunia (E). (F) Sympodial unit of tobacco, consisting of one bract (green) in the axil of which the next sympodial unit is initiated (blue), whereas the apex terminates as a flower (red). The star denotes the position of the subtending leaf that was removed. (G) Sympodial unit of petunia as in (F). The new sympodial meristem is initiated in the axil of the younger bract (2). (H) Schematic representation of sympodial organisation in tomato, tobacco and petunia. Differences are interpreted as variations on a basic theme. The lower sympodial unit of tomato (tomato-sym) has four nodes with three leaves and two new sympodial meristems. In petunia, the sympodial unit has two nodes and one new sympodial meristem. In tobacco, it has only one node. The end-point of this progressive reduction is represented by the tomato inflorescence (tomato-inf), which consists of sympodial units with only one node that lacks a leaf. s, sepals.

Didier Reinhardt & Cris Kuhlemeier