Mechanisms of Pierce\u27s disease resistance in grapevine (Vitis vinifera L.): from xylem structure to whole plant function

Xylella fastidiosa (Xf) is a xylem-dwelling bacterium that causes Pierce’s disease (PD) in grapevines (Vitis vinifera L.), and disease in a range of other ecologically and economically important woody plants. To successfully colonize the xylem network, Xf cells accumulate on the vessel walls and form a biofilm. The biofilm contains cell wall-degrading enzymes, allowing the bacteria to breach the intervessel pit membranes. Thus, Xf can move from one vessel to another and colonize the xylem network. Degraded intervessel pit membranes and the production of tyloses in response to the presence of Xf likely contribute to significant declines in both hydraulic conductivity and resistance to drought-induced embolism spread. Indeed, Xf-infected grapevines typically display a range of symptoms that are often associated with water transport dysfunction.Despite the consensus that PD susceptibility is associated with Xf multiplication and systemic spread within the xylem network, there significant gaps in our understanding of the relationships between xylem structure and function that allow for Xf establishment and colonization still remain. For instance, although Xf can breach pit membranes to move from one vessel to another, the consequences of the breakdown of pit membranes within the context of embolism spread and hydraulic conductivity, and the subsequent implications for whole-plant physiological decline, remain inconclusive. Furthermore, the physical structure of the xylem network, i.e. the spatial distribution of xylem connections that might facilitate the spread of Xf, are largely unknown because of the complex, three-dimensional nature of the network. In my dissertation research, I explored the roles of the xylem structure and function related to the mechanisms of PD resistance. Throughout my work, I applied a holistic approach, coupling anatomical and physiological measurements across different grapevine genotypes with different levels of PD resistance -- from non-cultivated North American species to commercial European vinifera cultivars and their hybrids. In the first chapter, I tested the hypothesis where if the 3D structure of the xylem network connectivity plays a significant role in Xf spread, then PD resistant grapevine genotypes should have fewer total connections in the lateral and radial directions, which thereby limits the total number of pathways. Given that the Xf spread is essentially dependent on the intervessel connections, comparing the number and orientation of connections was a logical step in the fundamental understanding of this host-pathogen relationship. The chapter concludes, however, that there was limited evidence to support this hypothesis, and network connectivity does not appear to be strongly correlated with PD resistance and Xf spread. While network connectivity in the radial and lateral directions is somewhat variable within the genus Vitis, no clear trends emerged linking connectivity with resistance to PD. In the second chapter, I investigated the consequences of the extracellular cell wall-degrading enzyme released by Xf on pit membrane integrity and the downstream effects on water transport. The enzymatic breakdown of the pit membranes was relatively small, less than 10% of the pit aperture area, but enough to weaken pit membrane resistance to air-seeding by introducing pores into the membrane. Not only would larger pore diameters facilitate Xf movement, but they would subsequently increase the vulnerability of those vessels to drought-induced embolism spread. These factors would significantly affect the water transport capacity of infected grapevines and put them at greater risk to the effects of drought. In the third chapter, my objective was to determine the key physiological mechanisms that lead to mortality in the Xf infection process. This chapter reveals the mechanistic cascade of events that occur after Xf inoculation, with a coordinated decline in hydraulic conductivity, photosynthesis, and starch storage in PD susceptible grapevine genotypes. The results support the theory that hydraulic failure and carbon starvation underlie plant mortality resulting from PD. My dissertation explored the roles of the xylem structure and function on the PD mechanisms of resistance. Collectively, this work (1) identifies the variability in 3D xylem network traits in six different Vitis genotypes, representing the most complete analysis of its type for any plant group; (2) reveals that in young shoots the axial pathway appears to be the most important in determining the long-distance movement and systemic spread of Xf in the xylem network, (3) and provides a more robust, mechanistic understanding of the timing and sequence of events from initial Xf inoculation to ultimate death, as well as the variability in this mortality sequence in resistant and susceptible genotypes. As we do not have an effective remedy against the Xf bacterium, a more accurate understanding of how some grapevines resist to the infection process is one piece of this very important puzzle

Overemphasis on publications may disadvantage historically excluded groups in STEM before and during COVID-19: A North American survey-based study.

Publishing is a strong determinant of academic success and there is compelling evidence that identity may influence the academic writing experience and writing output. However, studies rarely quantitatively assess the effects of major life upheavals on trainee writing. The COVID-19 pandemic introduced unprecedented life disruptions that may have disproportionately impacted different demographics of trainees. We analyzed anonymous survey responses from 342 North American environmental biology graduate students and postdoctoral scholars (hereafter trainees) about scientific writing experiences to assess: (1) how identity interacts with scholarly publication totals and (2) how the COVID-19 pandemic influenced trainee perceptions of scholarly writing productivity and whether there were differences among identities. Interestingly, identity had a strong influence on publication totals, but it differed by career stage with graduate students and postdoctoral scholars often having opposite results. We found that trainees identifying as female and those with chronic health conditions or disabilities lag in publication output at some point during training. Additionally, although trainees felt they had more time during the pandemic to write, they reported less productivity and motivation. Trainees who identified as female; Black, Indigenous, or as a Person of Color [BIPOC]; and as first-generation college graduates were much more likely to indicate that the pandemic affected their writing. Disparities in the pandemic's impact on writing were most pronounced for BIPOC respondents; a striking 85% of BIPOC trainees reported that the pandemic affected their writing habits, and overwhelmingly felt unproductive and unmotivated to write. Our results suggest that the disproportionate impact of the pandemic on writing output may only heighten the negative effects commonly reported amongst historically excluded trainees. Based on our findings, we encourage the academy to consider how an overemphasis on publication output during hiring may affect historically excluded groups in STEM-especially in a post-COVID-19 era

Word cloud representation of respondents’ subfield of environmental biology.

Bigger words indicate higher occurrence in the survey responses. (PDF)</p

Postdoctoral scholar model results for Bayesian multiple regression of how years in training and identity affect publication output.

Years spent as a graduate student and postdoctoral scholar are continuous, and all other variables are factorial and coded as 1 = trainee identifies or 0 = trainee does not identify as first generation, female, BIPOC, having a chronic condition, or having English as a second language (ESL). Variables with 80% or higher probability of being on the same side of zero as the estimate (PD sign match) are bolded. 95% CRI = 95% credible interval, ESS = effective sample size. (PDF)</p

Fig 3 -

Binomial multiple regression models suggest identity of graduate students and postdoctoral scholars influenced yes/no responses of A) graduate students (n = 229) and B) postdoctoral scholars (n = 79) to the question “Has COVID-19 impacted your writing habits?”. All estimates are in logit scale for ease of comparison. More positive values indicate a higher probability of answering “yes.” Points are the parameter estimate medians, thick lines are 50% credible intervals (CRIs), and thin lines are 95% CRIs. Graduate and postdoc yrs indicate the number of years in training (as a continuous variable). First gen = first in family to graduate from college; female = female; BIPOC = Black, Indigenous, and/or a person of color; chronic condition = chronic health condition or disability; and ESL = English is the second language of respondents coded categorically (yes/no).</p

A full copy of Qualtrics survey used to collect our data.

This copy shows exact question phrasing and survey structure. (PDF)</p

The impact of the COVID-19 pandemic on trainee writing habits and perceptions of productivity and motivation.

A) When answering the question: “Has the COVID-19 pandemic impacted your writing habits?” the majority of respondents said yes. We then asked follow-up questions on perceived effects of the pandemic on time for writing, productivity, and motivation. B) While many respondents reported having more or much more time for writing, C) most respondents reported that they felt less or much less productive during the pandemic. D) Similarly, the majority of respondents reported feeling less or much less motivated.</p

S2 Fig -

Smoothed density of A) first-authored and B) co-authored publications of respondents separated by career stage. Career stage grad represents graduate students, while postdoc represents postdoctoral scholars. (PDF)</p

Binomial multiple regression on yes/no responses among postdoctoral scholars to the question “<i>Has COVID-19 impacted your writing habits</i>?”.

All estimates are in logit scale for ease of comparison. Variables with 85% or higher probability of being on the same side of zero as the estimate (PD sign match) are bolded. (PDF)</p