Auxins and grass shoot architecture: how the most important hormone makes the most important plants

Abstract

Cereals are a group of grasses cultivated by humans for their grain. It is from these cereal grains that the majority of all calories consumed by humans are derived. The production of these grains is the result of the development of a series of hierarchical reproductive structures that form the distinct shoot architecture of the grasses. Being spatiotemporally complex, the coordination of grass shoot development is tightly controlled by a network of genes and signals, including the key phytohormone auxin. Hormonal manipulation has therefore been identified as a promising potential approach to increasing cereal crop yields and therefore ultimately global food security. Recent work translating the substantial body of auxin research from model plants into cereal crop species is revealing the contribution of auxin biosynthesis, transport, and signalling to the development of grass shoot architecture. This review discusses this still-maturing knowledge base and examines the possibility that changes in auxin biology could have been a causative agent in the evolution of differences in shoot architecture between key grass species, or could underpin the future selective breeding of cereal crops.

Keywords: Poaceae; Auxin; cereal; grain; grass; inflorescence; shoot architecture; shoot development; tillering.

Fig. 1.

Evolutionary comparison of the Poaceae. Species discussed in this review are in bold and accompanied by an inflorescence (spikelet) diagram, in which each floret is represented by a yellow oval, Hordeum vulgare (barley) (with infertile lateral spikelets shown in black), Triticum aestivum (wheat), Oryza sativa (rice), and Zea mays (maize). Other cereals that are not extensively studied or discussed in this review are included in grey. Defining features of inflorescence development are labelled in red.

Fig. 2.

Diagrams of the shoot architecture of the main Poaceae species discussed in this review. Each diagram represents an entire spike in black, and each yellow oval represents a floret, each of which can give rise to a single grain. (A) Maize tassel (male inflorescence): produces pairs of spikelets on short branches along its lateral branches and main spike; each spikelet then produces two florets. (B) Rice panicle: produces multiple orders of tillers; also, shown here with each spikelet producing a single fertile floret; significant support exists for a ‘three-floret spikelet’ hypothesis that postulates that two sterile lemmas on each inflorescence are in fact lateral florets. These florets are not included here due to their still hypothetical nature (Ren et al., 2020). (C) Wheat ear: produces mulitfloreted spikelets. (D) Barley ear (two-rowed): produces a unifloreted central spikelet, flanked by two lateral spikelets, which are typically sterile and represented by thin dark ovals. (E–H) Diagrams of shoot meristem development in the main Poaceae species discussed herein. (E) Maize tassel (male inflorescence). VM, vegetative meristem; IM, inflorescence meristem; BM, branch meristem; SBM, short branch meristem; pSM, pedicellate spikelet meristem; sSM, sessile spikelet meristem; FM, floret meristem. (F) Rice panicle. pBM, primary branch meristem; sBM, secondary branch meristem; SM: spikelet meristem. (G) Wheat ear. (H) Barley ear. TSM, triple spikelet meristem; CSM, central spikelet meristem; LSM, lateral spikelet meristem.