Beyond skin-deep: targeting the plant surface for crop improvement

Abstract

The above-ground plant surface is a well-adapted tissue layer that acts as an interface between the plant and its surrounding environment. As such, its primary role is to protect against desiccation and maintain the gaseous exchange required for photosynthesis. Further, this surface layer provides a barrier against pathogens and herbivory, while attracting pollinators and agents of seed dispersal. In the context of agriculture, the plant surface is strongly linked to post-harvest crop quality and yield. The epidermal layer contains several unique cell types adapted for these functions, while the non-lignified above-ground plant organs are covered by a hydrophobic cuticular membrane. This review aims to provide an overview of the latest understanding of the molecular mechanisms underlying crop cuticle and epidermal cell formation, with focus placed on genetic elements contributing towards quality, yield, drought tolerance, herbivory defence, pathogen resistance, pollinator attraction, and sterility, while highlighting the inter-relatedness of plant surface development and traits. Potential crop improvement strategies utilizing this knowledge are outlined in the context of the recent development of new breeding techniques.

Keywords: Biotechnology; crop improvement; cuticle; drought tolerance; epidermal layer; fruit quality; new breeding techniques (NBTs); stomata; trichome.

Fig. 1.

Simplified schematic cross-section of above-ground plant surface elements and the crop traits to which they contribute. Surface elements are highlighted by black boxes and include the L1 layer/identity, epidermal cells, trichomes, stomata, as well as the cuticle layer which is made up of three components: the cutin polymer (light green layer), cuticular waxes (dark green layer), and secondary metabolites (red dots). SAM, shoot apical meristem.

Fig. 2.

Contribution of surface-related genes to different crop quality traits. (A) The L1-related mis2 mutant developed rice grains that were irregularly shaped with an open-hull phenotype (Chun et al., 2020). GhMML4-silenced (C) cotton seeds showed reduced fibre densities compared with the wild type (B) (Wu et al., 2018). Decreased fruit spine density in CsMYB6-OE (E) cucumber compared with the wild type (D) (L. Zhao et al., 2020). (F) The pink-coloured fruit phenotype of IL1b resulting from the down-regulation of SlMYB12 and, subsequently, a lack of naringenin chalcone in the peel (Ballester et al., 2010). MdSHN3 is associated with apple russet formation with low and high expression levels driving regular (G) and impaired (H) cuticle accumulation, respectively (Lashbrooke et al., 2015b). (I) Enzymes involved in cuticle biosynthesis which have been linked to fruit dehydration (slgdsl1; Isaacson et al., 2009), fruit decay (slcyp86a69; Shi et al., 2013), as well as microcracks and fissures (SlDCR-RNAi; Lashbrooke et al., 2016) in tomato.

Fig. 3.

Network illustrating the interconnected nature of different surface elements (stomata, cuticle, conical cell, L1 layer, and trichome), driven by various crop genes, and their connection to beneficial traits (drought tolerance, crop fertility, disease resistance, pollinator attraction, herbivore defence, as well as crop quality and yield). Crop genes were selected based on their characterized involvement in the formation of two or more surface elements and/or their contributions towards two or more targeted traits.

Fig. 4.

Schematic illustration of genetic engineering techniques available for crop gene modification. (A) Transgenesis, involving the incorporation of genetic elements, derived from a sexually incompatible species, into a crop genome of interest. (B) Cisgenesis/intragenesis, involving the incorporation of genetic elements, derived from sexually compatible or native species, into a crop genome of interest. (C) DSB-mediated editing, with the assistance of target-specific nucleases (e.g. Cas engineered with a guide RNA), induces the NHEJ and HDR cell DNA repair mechanisms. For NHEJ, this leads to random insertions and deletions, while HDR, with the presence of a donor template, can insert or replace genes. DSB, double-strand break; NHEJ, non-homologous end joining; HDR, homology-directed repair. (D) Base editing requires a catalytically inactive nuclease (e.g. dCas engineered with a guide RNA) that is fused to a deaminase protein. The example provided illustrates the activity of a CBE which, in the presence of UGI, drives the deamination of cytosine to uracil. SSBs in the non-targeted strand stimulate DNA repair mechanisms which, following replication, leads to a C-to-T base substitution. CBE, cytosine base editor; UGI, uracil glycosylase inhibitor; SSB, single-strand break. (E) Prime editing requires a catalytically inactive nuclease (e.g. dCas engineered with a pegRNA, prime editing guide RNA) that is fused to a reverse transcriptase. Prime editors induce SSBs on the non-targeted strand to prime reverse transcription of the pegRNA template (containing the edit). During DNA repair and replication, the edited strand is integrated and copied into the complementary strand. Types of gene modification that can be achieved by each GE tool are highlighted by black boxes and include gain of function (GOF), loss of function (LOF), modulation of expression (MOE), and modulation of function (MOF).