On hormonal regulation of the dynamic apical hook development

Abstract

Summary To deal with the ever‐changing environment, sessile plants adapt diverse and plastic organ structures during postembryonic development. Among these, the apical hook forms shortly after seed germination of most dicots, and protects the delicate shoot meristem from mechanical damage during soil emergence. For decades, this structure has been taken as an excellent model for the investigation of the mechanisms underlying the differential growth of plant tissues. Here, we summarize recent advances in the investigation of the hormonal regulation of apical hook development, focusing on the convergence to auxin and a central regulator HOOKLESS1 (HLS1). We propose the revisitation of hook curvature kinematics at suborgan and single‐cell resolution, and further pursuance of the mechanistics of apical hook development through combinatorial approaches of automated imaging and multidimensional modeling.

Dark-Induced Hormonal Regulation of Plant Growth and Development

“…In darkness, COPI-SPA and CDD complex act together to degrade positive regulators of photomorphogenesis like HY5, LAF and HFR1 via UMP. However, these complexes also prevent degradation of PIFs (PIF3 and 4) by inhibiting a brassinosteroid (BR) signaling component BRASSINOSTEROID-INSENSITIVE 2 (BIN2) (Dong et al, 2014;Huang et al, 2016;Wang and Guo, 2018; Figure 1). The function of CSN complex is to derubylate and thus positively regulates the activity of CUL4 which is present on both COP1-SPA and CDD complexes (Chen et al, 2010;Dong et al, 2014).…”

“…An asymmetrical distribution of auxin (auxin concentration increases at concave side and inhibit cell expansion) causes differential cell expansion and division at both the sides of the hypocotyl. This leads to faster growth at the outer side compared to inner side, which culminates in the hook formation (Béziat and Kleine-Vehn, 2018;Gommers and Monte, 2018;Wang and Guo, 2018). Asymmetric distribution of auxin is facilitated by the influx and efflux carriers on the cell membrane.…”

“…PIN3 facilitates the auxin flow from vascular tissues to outer cortex and epidermis, whereas PIN4 and 7 support the auxin flow from vascular tissue to cortex and epidermis. In addition, ABCB transporter 1 and 19 which are localized at inner epidermal cells of the hook, steer the auxin transport to the convex side (Vandenbussche et al, 2010;Wu et al, 2010;Žádníkova et al, 2010Farquharson, 2017;Wang and Guo, 2018). The maintenance of differential auxin gradient involves the co-ordination of auxin synthesis, transport and signaling.…”

The curvature of cucumber fruits is associated with spatial variation in auxin accumulation and expression of a YUCCA biosynthesis gene

“…Auxin plays an important role in apical hook development in A. thaliana by inducing differential cell growth on both sides of the hook. Higher auxin concentration on the inner side of the hook compared with the outer side correlates to reduced cell elongation 14 . In addition, the auxin biosynthesis genes YUCCA1, TAA1/WEI8, and TAR2 are upregulated in the hook region, and the wei8, tar2, and yuc1/2/4/6 mutants of A. thaliana display improper apical hook formation 15,16 .…”

Ethylene Versus Salicylic Acid in Apical Hook Formation

“…Several plant hormones, including ethylene and salicylic acid (SA), affect apical hook formation (Wang and Guo, 2019). Ethylene activates genes required for apical hook formation, including HOOKLESS1 (HLS1), by inducing the accumulation of EIN3 and EIL1, two transcription factors that activate HLS1 and other genes that promote apical hook formation (An et al, 2012).…”