Rice bundle sheath cell shape is regulated by the timing of light exposure during leaf development

Plant leaves contain multiple cell types which achieve distinct characteristics whilst still coordinating development within the leaf. The bundle sheath possesses larger individual cells and lower chloroplast content than the adjacent mesophyll, but how this morphology is achieved remains unknown. To identify regulatory mechanisms determining bundle sheath cell morphology we tested the effects of perturbing environmental (light) and endogenous signals (hormones) during leaf development of Oryza sativa (rice). Total chloroplast area in bundle sheath cells was found to increase with cell size as in the mesophyll but did not maintain a ‘set‐point’ relationship, with the longest bundle sheath cells demonstrating the lowest chloroplast content. Application of exogenous cytokinin and gibberellin significantly altered the relationship between cell size and chloroplast biosynthesis in the bundle sheath, increasing chloroplast content of the longest cells. Delayed exposure to light reduced the mean length of bundle sheath cells but increased corresponding leaf length, whereas premature light reduced final leaf length but did not affect bundle sheath cells. This suggests that the plant hormones cytokinin and gibberellin are regulators of the bundle sheath cell‐chloroplast relationship and that final bundle sheath length may potentially be affected by light‐mediated control of exit from the cell cycle

The role of gibberellin in the reproductive development of Arabidopsis thaliana

The plant hormone gibberellin (GA) promotes several processes during Arabidopsis reproductive development, including the transition to flowering, floral organ growth and fertility. GA functions during stamen development to promote degradation of the tapetum cell layer through programmed cell death (PCD) and in post-anthesis pollen development. Bioactive GA is synthesised through a multi-step pathway, in which the last two biosynthetic steps are expressed as conserved multigene families. One of these, the GA 20-oxidases (GA20ox) consists of five paralogues in Arabidopsis, though physiological functions have only been ascribed to two (AtGA20ox1 and -2). Through a reverse genetics approach, this project demonstrates that AtGA20ox1, -2 and -3 account for almost all GA20ox activity in Arabidopsis, with very little evidence of any functions for AtGA20ox4 or -5. Unlike AtGA20ox1, -2, -3 and -4, AtGA20ox5 possesses only partial GA20ox activity, performing the first two out of three sequential catalytic conversions in vitro. Partial functional redundancy occurs between AtGA20ox1, -2 and -3 across Arabidopsis development, although AtGA20ox1 and -2 dominate. Mapping of floral AtGA20ox expression through qPCR and the creation of transgenic GUS reporter lines found that the relationship between these three paralogues is complex, and not explicable through the simple hypothesis of co-expression in the same tissues. During anther development, the reported expression of AtGA20ox1, -2, -3 and -4 is mainly restricted to the tapetum cell layer, and loss of AtGA20ox1, -2 and -3 results in an anther developmental arrest in which the tapetum does not degrade. This project demonstrates that stamen development is dependent on an optimum level of GA, with GA-deficiency restricting filament elongation to prevent pollination and GA-overdose negatively affecting anther development. DELLA repression of GA signalling is necessary for successful pollen development, with two of the five DELLA paralogues, RGA and GAI, critical to this process in the Columbia ecotype

The early inflorescence of Arabidopsis thaliana demonstrates positional effects in floral organ growth and meristem patterning.

Linear modelling approaches detected significant gradients in organ growth and patterning across early flowers of the Arabidopsis inflorescence and uncovered evidence of new roles for gibberellin in floral development. Most flowering plants, including the genetic model Arabidopsis thaliana, produce multiple flowers in sequence from a reproductive shoot apex to form a flower spike (inflorescence). The development of individual flowers on an Arabidopsis inflorescence has typically been considered as highly stereotypical and uniform, but this assumption is contradicted by the existence of mutants with phenotypes visible in early flowers only. This phenomenon is demonstrated by mutants partially impaired in the biosynthesis of the phytohormone gibberellin (GA), in which floral organ growth is retarded in the first flowers to be produced but has recovered spontaneously by the 10th flower. We presently lack systematic data from multiple flowers across the Arabidopsis inflorescence to explain such changes. Using mutants of the GA 20-OXIDASE (GA20ox) GA biosynthesis gene family to manipulate endogenous GA levels, we investigated the dynamics of changing floral organ growth across the early Arabidopsis inflorescence (flowers 1-10). Modelling of floral organ lengths identified a significant, GA-independent gradient of increasing stamen length relative to the pistil in the wild-type inflorescence that was separable from other, GA-dependent effects. It was also found that the first flowers exhibited unstable organ patterning in contrast to later flowers and that this instability was prolonged by exogenous GA treatment. These findings indicate that the development of individual flowers is influenced by hitherto unknown factors acting across the inflorescence and also suggest novel functions for GA in floral patterning

The Time domain

Slide designed by Jason Hilton, Rob MacKenzie, Andrew Plackett, Bruno Barcante Ladvocat Cintra, Sabrine Dhaouadi and Chantal Jackson for the Trees in time 2023 Annual Meeting of the Birmingham institute of Forest Research (BIFoR). Palaeontologial timescales incorporate the concepts of time-averaging developed by S.M. Kidwell and colleagues

DELLA activity is required for successful pollen development in the Columbia ecotype of Arabidopsis.

Excessive gibberellin (GA) signalling, mediated through the DELLA proteins, has a negative impact on plant fertility. Loss of DELLA activity in the monocot rice (Oryza sativa) causes complete male sterility, but not in the dicot model Arabidopsis (Arabidopsis thaliana) ecotype Landsberg erecta (Ler), in which DELLA function has been studied most extensively, leading to the assumption that DELLA activity is not essential for Arabidopsis pollen development. A novel DELLA fertility phenotype was identified in the Columbia (Col-0) ecotype that necessitates re-evaluation of the general conclusions drawn from Ler. Fertility phenotypes were compared between the Col-0 and Ler ecotypes under conditions of chemical and genetic GA overdose, including mutants in both ecotypes lacking the DELLA paralogues REPRESSOR OF ga1-3 (RGA) and GA INSENSITIVE (GAI). Ler displays a less severe fertility phenotype than Col-0 under GA treatment. Col-0 rga gai mutants, in contrast with the equivalent Ler phenotype, were entirely male sterile, caused by post-meiotic defects in pollen development, which were rescued by the reintroduction of DELLA into either the tapetum or developing pollen. We conclude that DELLA activity is essential for Arabidopsis pollen development. Differences between the fertility responses of Col-0 and Ler might be caused by differences in downstream signalling pathways or altered DELLA expression